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This chapter describes how to configure the various IP routing protocols. For a complete description of the commands listed in this chapter, refer to the "IP Routing Protocols Commands" chapter of the Router Products Command Reference publication. For information on configuring the IP protocol, refer to the "Configuring IP" chapter of this manual. For historical background and a technical overview of IP routing protocols, see the Internetworking Technology Overview publication.
Cisco's implementation of each of the IP routing protocols is discussed in detail at the beginning of the individual protocol sections throughout this chapter.
IP routing protocols are divided into two classes: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). The IGPs and EGPs that Cisco supports are listed in the following sections.
Interior protocols are used for routing networks that are under a common network administration. All IP interior gateway protocols must have a list of networks specified before routing activities can begin. A routing process listens to updates from other routers on these networks and broadcasts its own routing information on those same networks. The interior routing protocols supported are as follows:
Exterior protocols are used to exchange routing information between networks that do not share a common administration. IP exterior gateway protocols require three sets of information before routing can begin:
The supported exterior routing protocols are as follows:
Our routers also support two router discovery protocols, Gateway Discovery Protocol (GDP) and ICMP Router Discovery Protocol (IRDP), which allow hosts to locate routers.
GDP was developed by Cisco and is not an industry standard. Unsupported, example GDP clients can be obtained upon request from Cisco. Our IRDP implementation fully conforms to the router discovery protocol outlined in RFC 1256.
We support the operation of multiple routing protocols in a single router in order to connect networks that use different routing protocols. It is possible, for example, to run RIP on one subnetted network, IGRP on another subnetted network, and to exchange routing information between them in a controlled fashion. The routing protocols available were not designed to interoperate with one another, so each protocol collects different types of information and reacts to topology changes in its own way. For example, RIP uses a hop count metric and IGRP uses a five-element vector of metric information. In the case where routing information is being exchanged between different networks that use different routing protocols, there are many configuration options to enable to filter the exchange of routing information.
Our routers can handle simultaneous operation of up to 30 dynamic IP routing processes.The combination of routing processes on a router can consist of the following protocols (with the limits noted):
With any of the IP routing protocols, you need to create the routing process, associate networks with the routing process, and customize the routing protocol for your particular network.
You will need to perform some combination of the following tasks to configure IP routing protocols:
Each task is described in a following section. Configuration examples can be found at the end of this chapter (page 16-42).
Choosing a routing protocol is a complex task. When choosing a routing protocol, consider (at least) the following:
The following sections describe the configuration tasks associated with each routing protocol that we support. This publication does not provide in-depth information on how to choose routing protocols; you must choose routing protocols that best suit your needs. For detailed information on the technology behind the major routing protocols, see the Internetworking Technology Overview manual or other internetworking publications.
The Interior Gateway Routing Protocol (IGRP) is a dynamic distance-vector routing protocol designed by Cisco Systems in the mid-1980s for routing in an autonomous system containing large, arbitrarily complex networks with diverse bandwidth and delay characteristics.
IGRP uses a combination of user-configurable metrics including internetwork delay, bandwidth, reliability, and load.
IGRP also advertises three types of routes: interior, system, and exterior, as shown in Figure 1-1. Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.
System routes are routes to networks within an autonomous system. The router derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers. System routes do not include subnet information.
Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The router chooses a gateway of last resort from the list of exterior routes that IGRP provides. The router uses the gateway (router) of last resort if it does not have a better route for a packet and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.
By default, a router running IGRP sends an update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within three update periods (270 seconds). After seven update periods (630 seconds), the router removes the route from the routing table.
IGRP uses flash update and poison reverse updates to speed up the convergence of the routing algorithm. Flash update is the sending of an update sooner than the standard periodic update interval of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger routing loops caused by increases in routing metrics. The poison reverse updates are sent to remove a route and place it in holddown, which keeps new routing information from being used for a certain period of time.
To configure IGRP, it is only mandatory to create the IGRP routing process. The other tasks described are optional.
Each task is described in a following section.
The tasks you must perform to create the IGRP routing process are summarized in the following table. Perform them in the order listed.
IGRP sends updates to the interfaces in the specified network(s). If an interface's network is not specified, it will not be advertised in any IGRP update.
Because IGRP is normally a broadcast protocol, in order for IGRP routing updates to reach point-to-point or nonbroadcast networks, you must configure the router to permit this exchange of routing information.
To permit information exchange, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Define a neighboring router with which to exchange point-to-point routing information. | neighbor ip-address |
To control the set of interfaces that you want to exchange routing updates with, you can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. See the discussion on filtering in the section in this chapter titled "Filter Routing Information."
IGRP can simultaneously use an asymmetric set of paths for a given destination. This feature is known as unequal-cost load balancing. Unequal-cost load balancing allows traffic to be distributed among multiple (up to four) unequal-cost paths to provide greater overall throughput and reliability. Alternate path variance (that is, the difference in desirability between the primary and alternate paths) is used to determine the feasibility of a potential route. An alternate route is feasible if the next router in the path is closer to the destination (has a lower metric value) than the current router and if the metric for the entire alternate path is within the variance. Only paths that are feasible can be used for load balancing and included in the routing table. These conditions limit the number of cases in which load balancing can occur, but ensure that the dynamics of the network will remain stable.
The following general rules apply to IGRP unequal-cost load balancing:
If these conditions are met, the route is deemed feasible and can be added to the routing table.
By default, the amount of variance is set to one (equal-cost load balancing). You can define how much worse an alternate path can be before that path is disallowed by performing the following task in router configuration mode:
| Task | Command |
|---|---|
| Define the variance associated with a particular path. | variance multiplier |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring IGRP feasible successor.
By default, if IGRP or enhanced IGRP has multiple routes of unequal cost to the same destination, the router will distribute traffic among the different routes by giving each route a share of the traffic in inverse proportion to its metric. If you want to have faster convergence to alternate routes but you do not want to send traffic across inferior routes in the normal case, you might prefer to have no traffic flow along routes with higher metrics.
To control how traffic is distributed among multiple routes of unequal cost, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Distribute traffic proportionately to the ratios of metrics, or by the minimum-cost route. | traffic-share {balanced | min} |
You have the option of altering the default behavior of IGRP routing and metric computations. This allows, for example, tuning system behavior to allow for transmissions via satellite. Although IGRP metric defaults were carefully selected to provide excellent operation in most networks, you can adjust the IGRP metric. Adjusting IGRP metric weights can dramatically affect network performance, however, so ensure you make all metric adjustments carefully.
To adjust the IGRP metric weights, perform the following task in router configuration mode. Due to the complexity of this task, we recommend that you only perform it with guidance from an experienced system designer.
| Task | Command |
|---|---|
| Adjust the IGRP metric. | metric weights TOS K1 K2 K3 K4 K5 |
By default, the IGRP composite metric is a 24-bit quantity that is a sum of the segment delays and the lowest segment bandwidth (scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network of mixed media (FDDI, Ethernet, and serial lines running from 9600 bps to T1 rates), the route with the lowest metric reflects the most desirable path to a destination.
When a router learns that a network is at a greater distance than was previously known, or is now down, the route to that network is placed into holddown. During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric will be ignored. This mechanism is often used to help avoid routing loops in the network, but has the effect of increasing the topology convergence time. Since Release 8.2(5) of our software, IGRP has had other mechanisms available to avoid these routing loops, so the holddown period can be disabled. All routers that are in the same IGRP AS must be running Release 8.2(5) or later to disable holddowns.
To disable holddowns with IGRP, perform the following task in router configuration mode. All routers running IGRP on your network must be consistent in their use of this command.
| Task | Command |
|---|---|
| Disable the IGRP holddown period. | no metric holddown |
The router enforces a maximum diameter to the IGRP network. Routes whose hop counts exceed this diameter will not be advertised. The default maximum diameter is 100 hops. The maximum diameter is 255 hops.
To configure the maximum diameter, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure the maximum network diameter. | metric maximum-hops hops |
Open Shortest Path First (OSPF) is an IGP developed by the OSPF working group of the Internet Engineering Task Force (IETF). Designed expressly for IP networks, OSPF supports IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending/receiving packets.
Cisco's implementation conforms to the OSPF Version 2 specifications detailed in the Internet RFC 1247. The list that follows outlines key features supported in Cisco's OSPF implementation.
OSPF typically requires coordination among many internal routers, area border routers (routers connected to multiple areas), and AS boundary routers. At a minimum, OSPF-based routers can be configured with all default parameter values, no authentication, and interfaces assigned to areas. If you intend to customize your environment, you must ensure coordinated configurations of all routers.
Configuring an OSPF routing process involves the general tasks that follow. Enabling OSPF is mandatory; the other tasks are optional but might be required for your application.
For all other tasks listed, see the following sections.
As with other routing protocols, enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range of IP addresses. Perform these tasks in global configuration mode as shown in the following table.
Our OSPF implementation allows you to alter certain interface-specific OSPF parameters, as needed. You can:
You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on your network have compatible values.
In interface configuration mode, specify any of the following interface parameters as needed for your network:
OSPF classifies different media into three types of networks by default:
You can configure your network as either a broadcast or a nonbroadcast multiaccess network.
X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. See the x25 map and frame-relay map command descriptions in the Router Products Command Reference publication for more detail.
You have the choice of configuring your OSPF network type to either broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure nonbroadcast multiaccess networks, such as X.25, Frame Relay, and SMDS, as broadcast networks. This feature saves you from having to configure neighbors, as described in the section following this one.
To configure your OSPF network type, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the OSPF network type for a specified interface. | ip ospf network {broadcast | non-broadcast} |
Because there may be many routers attached to an OSPF network, a designated router is selected for the network. It is necessary to use special configuration parameters in the designated router selection if broadcast capability is not configured.
These parameters need only be configured in those routers that are themselves eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).
To configure routers that interconnect to nonbroadcast networks, perform the following task in router configuration mode. You can specify the following neighbor parameters, as required:
| Task | Command |
|---|---|
| Configure routers interconnecting to nonbroadcast networks. | neighbor ip-address [priority 8-bit-number] [poll-interval number-of-seconds] |
Our OSPF software allows you to configure several area parameters. These area parameters, shown in the following table, include authentication, defining stub areas, and assigning specific costs to the default summary route. Authentication allows password-based protection against unauthorized access to an area. Stub areas are areas into which information on external routes is not sent. Instead, there is a default external route generated by the area border router into the stub area for destinations outside the autonomous system (AS).
In router configuration mode, specify any of the following area parameters as needed for your network:
Route summarization is the consolidation of advertised addresses. This feature causes a single summary route to be advertised to other areas by an area border router. In OSPF, an area border router will advertise networks in one area into another area. If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the area border router to advertise a summary route that covers all the individual networks within the area that fall into the specified range.
To specify an address range, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify an address range for which a single route will be advertised. | area area-id range address mask |
In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone is purposefully partitioned, you can establish a virtual link. The two endpoints of a virtual link are area border routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other area border router), and the nonbackbone area that the two routers have in common (called the transit area). Note that virtual links cannot be configured through stub areas.
To establish a virtual link, perform the following task in router configuration mode:
Use the show ip ospf virtual-links EXEC command to display virtual link information. Use the show ip ospf EXEC command to display the router ID of an OSPF router.
You can force an AS boundary router to generate a default route into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the router automatically becomes an AS boundary router. However, an AS boundary router does not, by default, generate a default route into the OSPF routing domain.
To force the AS boundary router to generate a default route, perform the following task in router configuration mode:
See also the discussion of redistribution of routes in the "Configure Routing Protocol-Independent Features" section later in this chapter.
You can configure OSPF to look up Domain Name System (DNS) names for use in all OSPF show command displays. This feature makes it easier to identify a router, because it is displayed by name rather than by its router ID or neighbor ID.
To configure DNS name lookup, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Configure DNS name lookup. | ip ospf-name-lookup |
OPSF uses the largest IP address configured on the router's interfaces as its Router ID. If the interface associated with this IP address is ever brought down, or if the address is removed, the OSPF process must recalculate a new Router ID and resend all of its routing information out its interfaces.
If a loopback interface is configured with an IP address, the router will use this IP address as its Router ID, even if other interfaces have larger IP addresses. Since loopback interfaces never go down, greater stability in the routing table is achieved.
OSPF automatically prefers a Loopback interface over any other kind and it chooses the first Loopback interface found. If no Loopback interfaces are present, the highest IP address in the router is chosen. You cannot tell OSPF to use any particular interface.
To configure an IP address on a loopback interface, perform the following tasks in global configuration mode:
| Task | Command |
|---|---|
| Step 1 Enter global configuration mode. | See Table 2-1. |
| Step 2 Create a loopback interface, which places you in interface configuration mode. | interface loopback 0 |
| Step 3 Assign an IP address to this interface. | ip address IP-address network-mask |
Because simplex interfaces between two routers on an Ethernet represent only one network segment, for OSPF you have to configure the transmitting interface to be a passive interface. This prevents OSPF from sending hello packets for the transmitting interface. Both routers are able to see each other via the hello packet generated for the receiving interface.
To configure OSPF on simplex Ethernet interfaces, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Suppress the sending of hello packets through the specified interface. | passive-interface interface |
The Routing Information Protocol (RIP) is a relatively old but still commonly used IGP created for use in small, homogeneous networks. It is a classical distance-vector routing protocol.
The Routing Information Protocol (RIP) uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. Each router sends routing information updates every 30 seconds; this process is termed advertising. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by the nonupdating router as being unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the nonupdating router.
The measure, or metric, that RIP uses to rate the value of different routes is the hop count. The hop count is the number of routers that can be traversed in a route. A directly connected network has a metric of zero; an unreachable network has a metric of 16. This small range of metrics makes RIP unsuitable as a routing protocol for large networks. If the router has a default network path, RIP advertises a route that links the router to the pseudo-network 0.0.0.0. The network 0.0.0.0 does not exist; RIP treats 0.0.0.0 as a network to implement the default routing feature. Our routers will advertise the default network if a default was learned by RIP, or if the router has a gateway of last resort and RIP is configured with a default metric.
RIP sends updates to the interfaces in the specified network(s). If an interface's network is not specified, it will not be advertised in any RIP update.
For information about filtering RIP information, see the "Filter Routing Information" section later in this chapter. RIP is documented in RFC 1058.
To configure RIP, perform the following tasks in the order shown:
It is possible to run IGRP and RIP concurrently. The IGRP information will override the RIP information be default because of IGRP's administrative distance.
However, running IGRP and RIP concurrently does not work well when the network topology changes. Because IGRP and RIP have different update timers and because they require different amounts of time to propagate routing updates, one part of the network will end up believing IGRP routes and another part will end up believing RIP routes. This will result in routing loops. Even though these loops do not exist for very long, the TTL will quickly reach zero, and ICMP will sent a "TTL exceeded" message. This message will cause most applications to stop attempting network connections.
Because RIP is normally a broadcast protocol, in order for RIP routing updates to reach point-to-point or nonbroadcast networks, you must configure the router to permit this exchange of routing information.
You configure the router to permit this exchange of routing information by performing the following task in router configuration mode:
| Task | Command |
|---|---|
| Define a neighboring router with which to exchange point-to-point routing information. | neighbor ip-address |
To control the set of interfaces that you want to exchange routing updates with, you can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. See the discussion on filtering in the section in this chapter titled "Filter Routing Information."
IS-IS, which stands for Intermediate System to Intermediate System, is an International Organization for Standardization (ISO) dynamic routing specification. IS-IS is described in ISO 10589. Cisco's implementation of IS-IS allows you to configure IS-IS as an IP routing protocol on your router.
Configuring an IS-IS routing process involves the general tasks listed below. The only required task is that you do the following:
The following tasks are optional, although you might be required to perform them depending upon your specific application:
The following sections describe the required task first, followed by the optional tasks for configuring IS-IS.
As with other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to specific networks. You can specify only one IS-IS process per router. Only one IS-IS process is allowed whether you run it in integrated mode, ISO CLNS only, or IP only.
Network Entity Titles, or NETs, define the area addresses for the IS-IS area. Multiple NETs per router are allowed, up to a maximum of three. Refer to the "Configuring ISO CLNS" chapter for a more detailed discussion of NETs.
Perform the following tasks to enable IS-IS on the router:
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring IS-IS as an IP routing protocol.
Our IS-IS implementation allows you to alter certain interface-specific IS-IS parameters. You can:
You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on the network have compatible values.
You can configure a cost for a specified interface. The only metric that is supported by the router and that you can configure is the default-metric, which you can configure for Level 1 and/or Level 2 routing. The other metrics currently are not supported.
To configure the metric for the specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the metric (or cost) for the specified interface. | isis metric default-metric [delay-metric [expense-metric [error-metric]]] {level-1 | level-2} |
You can specify the length of time, in seconds, between Hello packets that the router sends on the interface.
To specify the length of time between Hello packets for the specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Specify the length of time, in seconds, between Hello packets the router sends on the specified interface. | isis hello-interval seconds {level-1 | level-2} |
The Hello interval can be configured independently for Level 1 and Level 2, except on serial
point-to-point interfaces. (Because there is only a single type of Hello packet sent on serial links, it is independent of Level 1 or Level 2.) Specify an optional level for X.25, SMDS, and Frame Relay multiaccess networks.
Complete Sequence Number PDUs (CSNPs) are sent by the designated router to maintain database synchronization. You can configure the IS-IS CSNP interval for the interface.
To configure the CSNP interval for the specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the IS-IS CSNP interval for the specified interface. | isis csnp-interval seconds {level-1 | level-2} |
This feature does not apply to serial point-to-point interfaces. It applies to WAN connections if the WAN is viewed as a multiaccess meshed network.
You can configure the number of seconds between retransmission of IS-IS link state PDUs (LSPs) for point-to-point links.
To set the retransmission level, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the number of seconds between retransmission of IS-IS link state PDUs (LSPs) for point-to-point links. | isis retransmit-interval seconds |
The value you specify should be an integer greater than the expected round-trip delay between any two routers on the attached network. The setting of this parameter should be conservative, or needless retransmission will result. The value should be larger for serial lines and virtual links.
You can configure the priority to use for designated router election. Priorities can be configured for Level 1 and Level 2 individually.
To specify the designated router election, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the priority to use for designated router election. | isis priority value {level-1 | level-2} |
You can specify adjacency levels on a specified interface. This parameter is also referred to as the interface circuit type.
To specify the interface circuit type, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the type of adjacency desired for neighbors on the specified interface (the interface circuit type). | isis circuit-type {level-1 | level-1-2 | level-2-only} |
You can assign different passwords for different routing levels. Specifying Level 1 or Level 2 configures the password for only Level 1 or Level 2 routing, respectively. If you do not specify a level, the default is Level 1. By default, authentication is disabled.
To configure a password for the specified level, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the authentication password for a specified interface. | isis password password {level-1 | level-2} |
You can configure the following miscellaneous, optional IS-IS parameters:
You can force a default route into an IS-IS routing domain. Whenever you specifically configure redistribution of routes into an IS-IS routing domain, the router does not, by default, generate a default route into the IS-IS routing domain. The following feature allows you to force the boundary router do this.
To generate a default route, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Force a default route into the IS-IS routing domain. | default-information originate [route-map map-name] |
See also the discussion of redistribution of routes in the "Configure Routing Protocol-Independent Features" section later in this chapter.
You can configure the router to act as a Level 1 (intra-area) router, as both a Level 1 router and a Level 2 (interarea) router, or as an interarea router only.
To specify router level support, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure the level at which the router should operate. | is-type {level-1 | level-1-2 | level-2-only} |
You can assign passwords to areas and domains.
The area authentication password is inserted in Level 1 (station router level) LSPs, CSNPs, and Partial Sequence Number PDUs (PSNPs). The routing domain authentication password is inserted in Level 2 (the area router level) LSP, CSNP, and PSNPs.
To configure either area or domain authentication passwords, perform the following tasks in router configuration mode:
| Task | Command |
|---|---|
| Configure the area authentication password. | area-password password |
| Configure the routing domain authentication password. | domain-password password |
You can create aggregate addresses that are represented in the routing table by a summary address. This process is called route summarization. One summary address can include multiple groups of addresses for a given level. Routes learned from other routing protocols also can be summarized. The metric used to advertise the summary is the smallest metric of all the more specific routes.
To create a summary of addresses for a given level, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Create a summary of addresses for a given level. | summary-address address mask {level-1 | level-1-2 | level-2} |
The Border Gateway Protocol (BGP), as defined in RFCs 1163 and 1267, allows you to set up an interdomain routing system that automatically guarantees the loop-free exchange of routing information between autonomous systems (ASs).
In BGP, each route consists of a network number, a list of ASs that information has passed through (called the AS path), and a list of other path attributes. The BGP implementation supports all path attributes defined in RFC 1163 and 1267. Cisco supports BGP versions 2 and 3. This section describes some details of that implementation.
You can configure the value for the INTER_AS metric attribute. The same metric value will be sent with all BGP updates originating from the router. The default is to not include an INTER_AS METRIC in BGP updates.
A third-party next-hop router address is used in the NEXT_HOP attribute, regardless of the AS of that third-party router. The router automatically calculates the value for this attribute.
Transitive, optional path attributes are passed along to other BGP-speaking routers. The current BGP implementation does not generate such attributes.
The BGP process selects a single AS path to use and to pass along to other BGP-speaking routers. Cisco's BGP implementation has a reasonable set of factory defaults that can be overridden by administrative weights. The algorithm for path selection is as follows:
Perform the following mandatory tasks to configure BGP on your router:
The following BGP tasks are optional:
Each task is described in a following section.
To enable BGP routing, establish a BGP routing process on the router and specify those networks within the router's autonomous system (AS) to be advertised. Perform this task in global configuration mode. There is a limit of 200 networks that can be advertised from one AS.
Like other exterior gateway protocols (EGPs), BGP must completely understand the relationships it is to have with its neighbors. BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS; external neighbors are in different ASs. Normally, external neighbors are adjacent to each other and share a subnet, while internal neighbors may be anywhere in the same AS.
To configure BGP neighbors, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify a BGP neighbor. | neighbor ip-address remote-as access-list-number |
You also can configure neighbor templates which use a word argument rather than an IP address to configure BGP neighbors. This is an advanced feature requiring a well-thought network architecture. Do not use this feature without thoroughly understanding its application.
Perform the following tasks in router configuration mode to configure BGP neighbor templates:
Once you have defined two routers to be BGP neighbors, they will form a BGP connection and exchange routing information. If you subsequently change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you need to reset BGP connections for the configuration change to take effect. Perform either of the following tasks in EXEC mode to reset BGP connections:
| Task | Command |
| Reset a particular BGP connection. | clear ip bgp address |
| Reset all BGP connections. | clear ip bgp * |
If you want to restrict the routing information that the router learns or advertises, you can filter BGP routing updates to and from particular neighbors. To do this, define an access list and apply it to the updates. Distribute-list filters are applied to network numbers and not AS paths.
To filter BGP routing updates, perform the following task in router configuration mode:
| Task | Command |
| Filter BGP routing updates to/from neighbors as specified in an access list. | neighbor ip-address distribute-list access-list-number {in | out} |
In addition to filtering routing updates to and from particular neighbors, you can specify an access list filter on both incoming and outbound BGP AS paths. Each filter is an access list based on regular expressions. To do this, define an AS path access list and apply it to updates to and from particular neighbors. See the "Regular Expressions" appendix in the Router Products Command Reference publication for more information on forming regular expressions.
The following table shows the tasks to configure BGP path filtering:
You can configure the router to disable next-hop processing on BGP updates. This is useful in non-meshed networks such as Frame Relay or X.25 where BGP neighbors may not have direct access to all other neighbors on the same IP subnet.
To disable next-hop processing, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable next-hop processing on BGP updates. | neighbor ip-address next-hop-self |
An administrative weight is a number that you can assign to a path so that you can control the path selection process. A weight can be a number from -65536 to 65535. Paths that the router originates have weight 32768 by default, other paths have weight zero. If you have particular neighbors that you wish to prefer for most of your traffic, you can assign a weight to all paths learned from a neighbor. Perform the following task in router configuration mode to configure BGP administrative weights:
| Task | Command |
| Specify a weight for all paths from a neighbor. | neighbor ip-address weight weight |
In addition, you can assign weights based on AS path access lists. A given weight is added to the weight of the path if the AS path is accepted by the access list. Any number of weight filters are allowed.
Perform the following tasks to assign weights based on AS path access lists:
If your autonomous system (AS) will be passing traffic through it from another AS to a third AS, it is very important that your AS be consistent about the routes that it advertises. For example, if your BGP were to advertise a route before all routers in your network had learned about the route through your IGP, your AS could receive traffic that some routers cannot yet route. To prevent this from happening, BGP must wait until the IGP has propagated routing information across your AS. This causes BGP to be synchronized with the IGP. Synchronization is enabled by default.
In some cases, you do not need synchronization. If you will not be passing traffic from a different AS through your AS, or if all routers in your AS will be running BGP, you can disable synchronization. Disabling this feature can allow you to carry fewer routes in your IGP, increase the number of paths that BGP can select, and allow BGP to converge more quickly, however you must run BGP on all routers in your AS and there must be a full IBGP connectivity mesh between these routers. To disable synchronization, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable synchronization between BGP and an IGP. | no synchronization |
In general, you will not want to redistribute most BGP routes into your IGP. A common design is to redistribute one or two routes and to make them exterior routes in IGRP or have your BGP speakers generate a default route for your AS.
In most circumstances, you also will not want to redistribute your IGP into BGP. Just list the networks in your AS with network router configuration commands and your networks will be advertised. Networks that are listed this way are referred to as local networks and have a BGP origin attribute of "IGP." They must appear in the main IP routing table and can have any source; for example, they can be directly connected or learned via an IGP. The BGP routing process periodically scans the main IP routing table to detect the presence or absence of local networks, updating the BGP routing table as appropriate.
If you do perform redistribution into BGP, you must be very careful about the routes that can be in your IGP, especially if the routes were redistributed from BGP into the IGP elsewhere. This creates a situation where BGP is potentially injecting information into the IGP and then sending such information back into BGP and vice versa.
Networks that are redistributed into BGP from the EGP protocol will be given the BGP origin attribute "EGP." Other networks that are redistributed into BGP will have the BGP origin attribute of "incomplete." The origin attribute in our implementation is only used in the path selection process.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of synchronization.
You can adjust several miscellaneous BGP parameters, as indicated in the following subsections.
If you would like to provide BGP routing information to a large number of neighbors, you can configure BGP to accept neighbors based on an access list. If a neighbor attempts to initiate a BGP connection, its address must be accepted by the access list for the connection to be accepted. If you do this, the router will not attempt to initiate a BGP connection to these neighbors, so the neighbors must be explicitly configured to initiate the BGP connection. If no access list is specified, all connections are accepted.
If a neighbor is running a different version of BGP, you should configure the version of BGP that the neighbor is speaking.
External BGP peers normally must reside on a directly connected network. Sometimes it is useful to relax this restriction in order to test BGP; do so by specifying the neighbor ebgp-multihop command
For internal BGP, you may want to allow your BGP connections to stay up regardless of which interfaces are available on the router. To do this, you first configure a loopback interface and assign it an IP address. Next, configure the BGP update source to be the loopback interface. Finally, configure your neighbor to use the address on the loopback interface. Configure any of the following neighbor options in router configuration mode:
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for examples of configuring BGP neighbor options.
You can indicate which networks are reachable using a backdoor route that the border router should use. A backdoor network is treated as a local network, except that it is not advertised. To configure backdoor routes, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Indicate reachable networks through backdoor routes. | network address backdoor |
Administrative distance is a measure of the ability of a routing protocol to provide optimal routes. BGP uses three different administrative distances--external, internal, and local. Routes learned through external BGP are given the external distance, routes learned with internal BGP are given the internal distance, and routes that are part of this AS are given the local distance. To assign a BGP administrative distance, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Assign a BGP administrative distance. | distance bgp external-distance internal-distance local-distance |
Changing the administrative distance of BGP routes is considered dangerous and generally is not recommended. The external distance should be lower than any other dynamic routing protocol, and the internal and local distances should be higher than any other dynamic routing protocol.
BGP uses certain timers to control periodic activities such as the sending of keepalive messages and the interval after not receiving a keepalive message after which the router declares a peer dead. You can adjust these timers. When a connection is started, BGP will negotiate the hold time with the neighbor. The smaller of the two hold times will be chosen. The keepalive timer is then set based on the negotiated holdtime and the configured keepalive time. To adjust BGP timers, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust BGP timers. | timers bgp keepalive holdtime |
BGP uses the INTER_AS METRIC as a hint to external neighbors about preferred paths. If you have a router that traffic should avoid, you can configure that router with a higher INTER_AS METRIC. Doing this sets the INTER_AS METRIC on all paths that the router advertises. Perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Set an INTER_AS METRIC. | default-metric metric |
The Exterior Gateway Protocol (EGP), specified in RFC 904, is an older EGP used for communicating with certain routers in the Defense Data Network (DDN) that the U.S. Department of Defense designates as core routers. EGP also was used extensively when attaching to the National Science Foundation Network (NSFnet) and other large backbone networks.
An exterior router uses EGP to advertise its knowledge of routes to networks within its autonomous system. It sends these advertisements to the core routers, which then readvertise their collected routing information to the exterior router. A neighbor or peer router is any router with which the router communicates using EGP.
Cisco's implementation of EGP supports three primary functions, as specified in RFC 904:
The following tasks enable EGP routing on your router. The first two tasks are mandatory; the others are optional.
Each task is described in a following section.
To enable EGP routing, you must specify an AS number, generate an EGP routing process, and indicate the networks for which the EGP process will operate.
Perform these required tasks in the order given as shown in the following table:
A router using EGP cannot dynamically determine its neighbor or peer routers. You must therefore provide a list of neighbor routers.
To specify an EGP neighbor, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify an EGP neighbor. | neighbor ip-address |
The EGP timers consist of a hello timer and a poll time interval timer. The hello timer determines the frequency in seconds with which the router sends hello messages to its peer. The poll time is how frequently to exchange updates. Our implementation of EGP allows these timers to be adjusted by the user.
To adjust EGP timers, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust EGP timers. | timers egp hello polltime |
EGP supports a third-party mechanism in which EGP tells an EGP peer that another router (the third party) on the shared network is the appropriate router for some set of destinations.
To specify third-party routers in updates, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify a third-party through which certain destinations can be achieved. | neighbor ip-address third-party third-party-ip-address [internal | external] |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring third-party EGP support.
You might want to provide backup in the event of site failure by having a second router belonging to a different AS act as a backup to the EGP router for your AS. To differentiate between the primary and secondary EGP routers, the two routers will advertise network routes with differing EGP distances or metrics. A network with a low metric is generally favored over a network with a high metric.
Networks declared as local are always announced with a metric of zero. Networks that are redistributed will be announced with a metric specified by the user. If no metric is specified, redistributed routes will be advertised with a metric of three. All redistributed networks will be advertised with the same metric. The redistributed networks can be learned from static or dynamic routes. See also the "Redistribute Routing Information" section later in this chapter.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring backup routers.
You also can designate network 0.0.0.0 as a default route. If the next hop for the default route can be advertised as a third party, it will be included as a third party.
To enable the use of default EGP routes, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure EGP to generate a default route. | default-information originate |
Normally, an EGP process expects to communicate with neighbors from a single AS. Because all neighbors are in the same AS, the EGP process assumes that these neighbors all have consistent internal information. Therefore, if the EGP process is informed about a route from one of its neighbors, it will not send it out to other neighbors.
With core EGP, the assumption is that all neighbors are from different ASs, and all have inconsistent information. In this case, the EGP process distributes routes from one neighbor to all others (but not back to the originator). This allows the EGP process to be a central clearinghouse for information with a single, central manager of routing information (sometimes called a core gateway). To this end, one core gateway process can be configured for each router.
To define a core gateway process, perform the following steps in the order in which they appear:
The EGP process defined in this way can act as a peer with any autonomous system (AS), and information is interchanged freely between ASs.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring an EGP core gateway.
The Gateway Discovery Protocol (GDP), designed by Cisco to address customer needs, allows hosts to dynamically detect the arrival of new routers, as well as determine when a router goes down. You must have host software to take advantage of this protocol.
For ease of implementation on a variety of host software, GDP is based on the User Datagram Protocol (UDP). The UDP source and destination ports of GDP datagrams are both set to 1997 (decimal).
There are two types of GDP messages: report and query. On broadcast media, report message packets are periodically sent to the IP broadcast address announcing that the router is present and functioning. By listening for these report packets, a host can detect a vanishing or appearing router. If a host issues a query packet to the broadcast address, the routers each respond with a report sent to the host's IP address. On nonbroadcast media, routers send report message packets only in response to query message packets. The protocol provides a mechanism for limiting the rate at which query messages are sent on nonbroadcast media.
Figure 1-2 shows the format of the GDP report message packet format. A GDP query message packet has a similar format, except that the count field is always zero and no address information is present.
The fields in the Report and Query messages are as follows:
Numerous actions can be taken by the host software listening to GDP packets. One possibility is to flush the host's ARP cache whenever a router appears or disappears. A more complex possibility is to update a host routing table based on the coming and going of routers. The particular course of action taken depends on the host software and your network requirements.
To enable GDP routing and other optional GDP tasks as required for your network, perform the following tasks in interface configuration mode:
Like GDP, the ICMP Router Discovery Protocol (IRDP) allows hosts to locate routers. When operating as a client, router discovery packets are generated, and when operating as a host, router discovery packets are received.
The only required task for configuring IRDP routing on a specified interface is listed in the following table. Perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Enable IRDP processing on an interface. | ip irdp |
When you enable IRDP processing, the default parameters will apply. You can optionally change any of these IRDP parameters, as listed in the following table. Perform the following tasks in interface configuration mode:
A router can proxy-advertise other machines that use IRDP; however, this is not recommended because it is possible to advertise nonexistent machines or machines that are down.
Previous sections addressed configurations of specific routing protocols. This section describes the following protocol-independent tasks:
Each task is described in a following section.
OSPF, static routes, and IS-IS support variable-length subnet masks (VLSMs). With VLSMs, you can use different masks for the same network number on different interfaces, which allows you to conserve IP addresses and more efficiently use available address space. However, using VLSMs also presents address assignment challenges for the network administrator and ongoing administrative challenges.
Refer to RFC 1219 for detailed information about VLSMs and how to correctly assign addresses.
The best way to implement VLSMs is to keep your existing numbering plan in place and gradually migrate some networks to VLSMs to recover address space. See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of using VLSMs.
Static routes are user-defined routes that cause packets moving between a source and a destination to take a specified path. Static routes can be important if the router cannot build a route to a particular destination. They are also useful for specifying a gateway of last resort to which all unroutable packets will be sent.
To configure static routes, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Establish a static route. | ip route network [mask ] {address | interface} [distance] |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring static routes.
The router remembers static routes until you remove them (using the no form of the ip route global configuration command). However, you can override static routes with dynamic routing information through prudent assignment of administrative distance values. Each dynamic routing protocol has a default administrative distance, as listed in Table 1-1. If you would like a static route to be overridden by information from a dynamic routing protocol, simply ensure that the administrative distance of the static route is higher than that of the dynamic protocol.
Static routes that point to an interface will be advertised via RIP, IGRP, and other dynamic routing protocols, regardless of whether redistribute static commands were specified for those routing protocols. This is because static routes that point to an interface are considered in the routing table to be connected and hence lose their static nature. However, if you define a static route to an interface that is not one of the networks defined in a network command, no dynamic routing protocols will advertise the route unless a redistribute static command is specified for these protocols.
When an interface goes down, all static routes through that interface are removed from the IP routing table. Also, when the router can no longer find a valid next hop for the address specified as the forwarding router's address in a static route, the static route is removed from the IP routing table.
| Route Source | Default Distance |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| External BGP | 20 |
| IGRP | 100 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EGP | 140 |
| Internal BGP | 200 |
| Unknown | 255 |
A router may not be able to determine the routes to all other networks. To provide complete routing capability, the common practice is to use some routers as "smart routers" and give the remaining routers default routes to the smart router. (Smart routers have routing table information for the entire internetwork.) These default routes can be passed along dynamically or can be configured into the individual routers.
Most dynamic interior routing protocols include a mechanism for causing a smart router to generate dynamic default information that is then passed along to other routers.
If a router has a directly connected interface onto the specified default network, the dynamic routing protocols running on that router will generate or source a default route. In the case of RIP, it will advertise the pseudonetwork 0.0.0.0. In the case of IGRP, the network itself is advertised and flagged as an exterior route.
A router that is generating the default for a network also may need a default of its own. One way of doing this is to specify a static route to the network 0.0.0.0 through the appropriate router.
To define a static route to a network as the static default route, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Specify a default network. | ip default-network network-number |
When default information is being passed along through a dynamic routing protocol, no further configuration is required. The system will periodically scan its routing table to choose the optimal default network as its default route. In the case of RIP, it will be only one choice, network 0.0.0.0. In the case of IGRP, there may be several networks that can be candidates for the system default. The router uses both administrative distance and metric information to determine the default route (gateway of last resort). The selected default route appears in the gateway of last resort display of the show ip route EXEC command.
If dynamic default information is not being passed to the router, candidates for the default route can be specified with the ip default-network command. In this usage, ip default-network takes a nonconnected network as an argument. If this network appears in the routing table from any source (dynamic or static), it is flagged as a candidate default route and is a possible choice as the default route for the router.
If the router has no interface on the default network but does have a route to it, it will consider this network as a candidate default path. The route candidates will be examined and the best one will be chosen based on administrative distance and metric. The gateway to the best default path will become the gateway of last resort for the router.
In addition to running multiple routing protocols simultaneously, the router can redistribute information from one routing protocol to another. For example, you can instruct the router to readvertise IGRP-derived routes using the RIP protocol, or to readvertise static routes using the IGRP protocol. This applies to all of the IP-based routing protocols.
You also can conditionally control the redistribution of routes between routing domains by defining a method known as route maps between the two domains.
The following two tables list tasks associated with route redistribution.
Perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Define any route maps needed to control redistribution. | route-map map-tag {permit | deny} sequence-number |
Perform the following tasks in router configuration mode:
The metrics of one routing protocol do not necessarily translate into the metrics of another. For example, the RIP metric is a hop count and the IGRP metric is a combination of five quantities. In such situations, an artificial metric is assigned to the redistributed route. Because of this unavoidable tampering with dynamic information, carelessly exchanging routing information between different routing protocols can create routing loops, which can seriously degrade network operation.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for examples of configuring redistribution and route maps.
This section describes supported automatic metric translations between the routing protocols. These descriptions assume that you have not defined a default redistribution metric that replaces metric conversions.
You can filter routing protocol information by performing the following tasks:
Use the following tables to help you perform these tasks.
To prevent other routers on a local network from learning about routes dynamically, you can keep routing update messages from being sent through a router interface. This feature applies to all IP-based routing protocols except BGP and EGP.
OSPF and IS-IS behaviors are somewhat different. In OSPF, the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface. In IS-IS, the specified IP addresses are advertised without actually running IS-IS on those interfaces.
To prevent routing updates through a specified interface, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Suppress the sending of routing updates through the specified router interface. | passive-interface interface |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for examples of configuring passive interfaces.
To prevent other routers from learning one or more routes, you can suppress routes from being advertised in routing updates. The interface-name argument of this feature does not apply to OSPF. When used for OSPF, this feature applies only to external routes.
To suppress routes from being advertised in routing updates, perform the following task in router configuration mode:
You might want to avoid processing certain routes listed in incoming updates. This feature does not apply to OSPF or IS-IS.
Perform this task in router configuration mode:
| Task | Command |
|---|---|
| Suppress routes listed in updates from being processed. | distribute-list access-list-number in [interface-name] |
To provide a local mechanism for increasing the value of routing metrics, you can apply an offset to routing metrics. This feature applies to IGRP and RIP.
To apply an offset to routing metrics, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Apply an offset to routing metrics. | offset-list {in | out} offset [access-list-number] |
An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. In a large network, some routing protocols and some routers can be more reliable than others as sources of routing information. Also, when multiple routing processes are running in the same router for IP, it is possible for the same route to be advertised by more than one routing process. By specifying administrative distance values, you enable the router to intelligently discriminate between sources of routing information. The router will always pick the route whose routing protocol has the lowest administrative distance.
To filter sources of routing information, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Filter on routing information sources. | distance weight [address-mask [access-list-number]] [ip] |
For example, consider a router using IGRP and RIP. Suppose you trust the IGRP-derived routing information more than the RIP-derived routing information. In this example, because the default IGRP administrative distance is lower than the default RIP administrative distance, the router uses the IGRP-derived information and ignores the RIP-derived information. However, if you lose the source of the IGRP-derived information (to a power shutdown in another building, for example), the router uses the RIP-derived information until the IGRP-derived information reappears.
Assigning administrative distances is a problem unique to each network and is done in response to the greatest perceived threats to the network. Even when general guidelines exist, the network manager must ultimately determine a reasonable matrix of administrative distances for the network as a whole. Table 1-2 shows the default administrative distance for various sources of routing information.
| Route Source | Default Distance |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| External BGP | 20 |
| IGRP | 100 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EGP | 140 |
| Internal BGP | 200 |
| Unknown | 255 |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for examples of setting administrative distances.
Routing protocols use a variety of timers that determine such variables as the frequency of routing updates, the length of time before a route becomes invalid, and other parameters. You can adjust these timers to tune routing protocol performance to better suit your internetwork needs.
For IGRP and RIP, you can make the following timer adjustments:
EGP and BGP have their own timers commands, although some EGP timers may be set with this command. See the EGP and BGP sections, respectively.
It also is possible to tune the IP routing support in the software to enable faster convergence of the various IP routing algorithms and hence, quicker fallback to redundant routers. The total effect is to minimize disruptions to end users of the network in situations where quick recovery is essential.
The following two tables list tasks associated with adjusting routing protocol timers and the keepalive interval.
Perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust routing protocol timers. | timers basic update invalid holddown flush [sleeptime] |
Perform the following the following task in interface configuration mode:
You also can configure the keepalive interval, the frequency at which the router sends messages to itself (Ethernet and Token Ring) or to the other end (hdlc-serial, ppp-serial) to ensure that a network interface is alive. The interval in some previous software versions was 10 seconds; it is now adjustable in one-second increments down to one second. An interface is declared down after three update intervals have passed without receiving a keepalive packet.
When adjusting the keepalive timer for a very low bandwidth serial interface, large packets can delay the smaller keepalive packets long enough to cause the line protocol to go down. You might need to experiment to determine the best value.
Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks, such as Frame Relay and SMDS, situations can arise for which this behavior is less than ideal. For these situations, you may wish to disable split horizon. This applies to IGRP and RIP.
To enable or disable split horizon, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Enable split horizon. | ip split-horizon |
| Disable split horizon. | no ip split-horizon |
Split horizon for Frame Relay and SMDS encapsulation is disabled by default. Split horizon is not disabled by default for interfaces using any of the X.25 encapsulations. For all other encapsulations, split horizon is enabled by default.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of using split horizon.
You can remove all contents of a particular cache, table, or database. You also can display specific router statistics. The following sections describe each of these tasks.
You can remove all contents of a particular cache, table, or database. Clearing a cache, table, or database can become necessary when the contents of the particular structure have become or are suspected to be invalid.
The following table lists the tasks associated with clearing caches, tables, and databases for IP routing protocols. Perform these tasks in EXEC mode:
You can display specific router statistics such as the contents of IP routing tables, caches, and databases. Information provided can be used to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path your router's packets are taking through the network.
To display various router statistics, perform the following tasks in EXEC mode:
This section contains configuration examples of various IP routing protocol features. The topics addressed are as follows:
OSPF, static routes, and IS-IS support variable-length subnet masks (VLSMs). With VLSMs, you can use different masks for the same network number on different interfaces, which allows you to conserve IP addresses and more efficiently use available address space.
In the following example, a 14-bit subnet mask is used, leaving two bits of address space reserved for serial line host addresses. There is sufficient host address space for two host endpoints on a point-to-point serial link.
interface ethernet 0
ip address 131.107.1.1 255.255.255.0
! 8 bits of host address space reserved for ethernets
interface serial 0
ip address 131.107.254.1 255.255.255.252
! 2 bits of address space reserved for serial lines
! Router is configured for OSPF and assigned AS 107
router ospf 107
! Specifies network directly connected to the router
network 131.107.0.0 0.0.255.255 area 0.0.0.0
In the following example, packets for network 10.0.0.0 from Router B, where the static route is installed, will be routed through 131.108.3.4 if a route with an administrative distance less than 110 is not available. Figure 1-3 illustrates this point. The route learned by a protocol with an administrative distance of less than 110 may cause Router B to send traffic destined for network 10.0.0.0 via the alternate path--through Router D.
ip route 10.0.0.0 255.0.0.0 131.108.3.4 110
The following example shows how you would configure three routers to run IS-IS as an IP routing protocol. Figure 1-4 illustrates the example configuration.
router isis
net 49.0001.0000.0000.000a.00
interface e 0
ip router isis
interface s 0
ip router isis
router isis
net 49.0001.0000.0000.000b.00
interface e 0
ip router isis
interface e 1
ip router isis
interface s 0
ip router isis
router isis
net 49.0001.0000.0000.000c.00
interface e 1
ip router isis
interface e 2
ip router isis
In the example that follows, three static routes are specified, two of which are to be advertised. Do this by specifying the redistribute static router configuration command, then specifying an access list that allows only those two networks to be passed to the IGRP process. Any redistributed static routes should be sourced by a single router to minimize the likelihood of creating a routing loop.
ip route 192.1.2.0 255.255.255.0 192.31.7.65
ip route 193.62.5.0 255.255.255.0 192.31.7.65
ip route 131.108.0.0 255.255.255.0 192.31.7.65
access-list 3 permit 192.1.2.0
access-list 3 permit 193.62.5.0
!
router igrp 109
network 192.31.7.0
default-metric 10000 100 255 1 1500
redistribute static
distribute-list 3 out static
Each IGRP routing process can provide routing information to only one autonomous system; the router must run a separate IGRP process and maintain a separate routing database for each autonomous system it services. However, you can transfer routing information between these routing databases.
Suppose the router has one IGRP routing process for network 15.0.0.0 in autonomous system 71 and another for network 192.31.7.0 in autonomous system 109, as the following commands specify:
router igrp 71
network 15.0.0.0
router igrp 109
network 192.31.7.0
To transfer a route to 192.31.7.0 into autonomous system 71 (without passing any other information about autonomous system 109), use the command in the following example:
router igrp 71
redistribute igrp 109
distribute-list 3 out igrp 109
access-list 3 permit 192.31.7.0
Consider a wide area network at a university that uses RIP as an interior routing protocol. Assume that the university wants to connect its wide area network to a regional network, 128.1.0.0, which uses IGRP as the routing protocol. The goal in this case is to advertise the networks in the university network to the routers on the regional network. The commands for the interconnecting router are listed in the example that follows.
router igrp 109
network 128.1.0.0
redistribute rip
default-metric 10000 100 255 1 1500
distribute-list 10 out rip
In this example, the router global configuration command starts an IGRP routing process. The network router configuration command specifies that network 128.1.0.0 (the regional network) is to receive IGRP routing information. The redistribute router configuration command specifies that RIP-derived routing information be advertised in the routing updates. The default-metric router configuration command assigns an IGRP metric to all RIP-derived routes.
The distribute-list router configuration command instructs the router to use access list 10 (not defined in this example) to limit the entries in each outgoing update. The access list prevents unauthorized advertising of university routes to the regional network.
OSPF typically requires coordination among many internal routers, area border routers, and AS boundary routers. At a minimum, OSPF-based routers can be configured with all default parameter values, with no authentication, and with interfaces assigned to areas.
Three examples follow:
The following example illustrates a simple OSPF configuration that enables OSPF routing process 9000, attaches Ethernet 0 to area 0.0.0.0, and redistributes RIP into OSPF, and OSPF into RIP.
interface Ethernet0
ip address 130.93.1.1 255.255.255.0
ip ospf cost 1
!
interface Ethernet 1
ip address 130.94.1.1 255.255.255.0
!
router ospf 9000
network 130.93.0.0 0.0.255.255 area 0.0.0.0
redistribute rip metric 1 subnets
!
router rip
network 130.94.0.0
redistribute ospf 9000
default-metric 1
The following example illustrates the assignment of four area IDs to four IP address ranges. In the example, OSPF routing process 109 is initialized, and four OSPF areas are defined: 10.9.50.0, 2, 3, and 0. Areas 10.9.50.0, 2, and 3 mask specific address ranges, while Area 0 enables OSPF for all other networks.
router ospf 109
network 131.108.20.0 0.0.0.255 area 10.9.50.0
network 131.108.0.0 0.0.255.255 area 2
network 131.109.10.0 0.0.0.255 area 3
network 0.0.0.0 255.255.255.255 area 0
!
! Interface Ethernet0 is in area 10.9.50.0:
interface Ethernet 0
ip address 131.108.20.5 255.255.255.0
!
! Interface Ethernet1 is in area 2:
interface Ethernet 1
ip address 131.108.1.5 255.255.255.0
!
! Interface Ethernet2 is in area 2:
interface Ethernet 2
ip address 131.108.2.5 255.255.255.0
!
! Interface Ethernet3 is in area 3:
interface Ethernet 3
ip address 131.109.10.5 255.255.255.0
!
! Interface Ethernet4 is in area 0:
interface Ethernet 4
ip address 131.109.1.1 255.255.255.0
!
! Interface Ethernet5 is in area 0:
interface Ethernet 5
ip address 10.1.0.1 255.255.0.0
Each network router configuration command is evaluated sequentially, so the specific order of these commands in the configuration is important. The router sequentially evaluates the address/wildcard-mask pair for each interface as follows:
Step 1 The wildcard-mask is logically ORed with the interface IP address.
Step 2 The wildcard-mask is logically ORed with address in the network command.
Step 3 The router compares the two resulting values.
Step 4 If they match, OSPF is enabled on the associated interface and this interface is attached to the OSPF area specified.
Consider the first network command. Area ID 10.9.50.0 is configured for the interface on which subnet 131.108.20.0 is located. Assume that a match is determined for interface Ethernet 0. Interface Ethernet 0 is attached to Area 10.9.50.0 only.
The second network command is evaluated next. For Area 2, the same process is then applied to all interfaces (except interface Ethernet 0). Assume that a match is determined for interface Ethernet 1. OSPF is then enabled for that interface and Ethernet 1 is attached to Area 2.
This process of attaching interfaces to OSPF areas continues for all network commands. Note that the last network command in this example is a special case. With this command all available interfaces (not explicitly attached to another area) are attached to Area 0.
The following example outlines a configuration for several routers within a single OSPF autonomous system. Figure 1-5 provides a general network map that illustrates this example configuration.
In this configuration, five routers are configured in OSPF autonomous system AS 109:
AS 109 is connected to the outside world via the BGP link to the external peer at IP address 11.0.0.6.
interface Ethernet 1
ip address 131.108.1.1 255.255.255.0
router ospf 109
network 131.108.0.0 0.0.255.255 area 1
interface Ethernet 2
ip address 131.108.1.2 255.255.255.0
router ospf 109
network 131.108.0.0 0.0.255.255 area 1
interface Ethernet 3
ip address 131.108.1.3 255.255.255.0
interface Serial 0
ip address 131.108.2.3 255.255.255.0
router ospf 109
network 131.108.1.0 0.0.0.255 area 1
network 131.108.2.0 0.0.0.255 area 0
interface Ethernet 4
ip address 10.0.0.4 255.0.0.0
interface Serial 1
ip address 131.108.2.4 255.255.255.0
router ospf 109
network 131.108.2.0 0.0.0.255 area 0
network 10.0.0.0 0.255.255.255 area 0
interface Ethernet 5
ip address 10.0.0.5 255.0.0.0
interface Serial 2
ip address 11.0.0.5 255.0.0.0
router ospf 109
network 10.0.0.0 0.255.255.255 area 0
redistribute bgp 109 metric 1 metric-type 1
router bgp 109
network 131.108.0.0
network 10.0.0.0
neighbor 11.0.0.6 remote-as 110
The following example configuration accomplishes several tasks in setting up an area border router. These tasks can be split into two general categories:
The specific tasks outlined in this configuration are detailed briefly in the following descriptions. Figure 1-6 illustrates the network address ranges and area assignments for the interfaces.
The basic configuration tasks in this example are as follows:
Configuration tasks associated with redistribution are as follows:
The following is an example OSPF configuration:
interface Ethernet0
ip address 192.42.110.201 255.255.255.0
ip ospf authentication-key abcdefgh
ip ospf cost 10
!
interface Ethernet1
ip address 131.119.251.201 255.255.255.0
ip ospf authentication-key ijklmnop
ip ospf cost 20
ip ospf retransmit-interval 10
ip ospf transmit-delay 2
ip ospf priority 4
!
interface Ethernet2
ip address 131.119.254.201 255.255.255.0
ip ospf authentication-key abcdefgh
ip ospf cost 10
!
interface Ethernet3
ip address 36.56.0.201 255.255.0.0
ip ospf authentication-key ijklmnop
ip ospf cost 20
ip ospf dead-interval 80
!
OSPF is on network 131.119:
router ospf 201
network 36.0.0.0 0.255.255.255 area 36.0.0.0
network 192.42.110.0 0.0.0.255 area 192.42.110.0
network 131.119.0.0 0.0.255.255 area 0
area 0 authentication
area 36.0.0.0 stub
area 36.0.0.0 authentication
area 36.0.0.0 default-cost 20
area 192.42.110.0 authentication
area 36.0.0.0 range 36.0.0.0 255.0.0.0
area 192.42.110.0 range 192.42.110.0 255.255.255.0
area 0 range 131.119.251.0 255.255.255.0
area 0 range 131.119.254.0 255.255.255.0
redistribute igrp 200 metric-type 2 metric 1 tag 200 subnets
redistribute rip metric-type 2 metric 1 tag 200
!
IGRP AS 200 is on 131.119.0.0:
router igrp 200
network 131.119.0.0
!
! RIP for 192.42.110
!
router rip
network 192.42.110.0
redistribute igrp 200 metric 1
redistribute ospf 201 metric 1
The following examples illustrate configurations for advertising and redistributing BGP routes. The first example details the configuration for two neighboring routers that run IGRP within their respective ASs and that are configured to advertise their respective BGP routes between each other. The second example illustrates route redistribution of BGP into IGRP and IGRP into BGP.
This example provides the required configuration for two routers (R1 and R2) that are intended to advertise BGP routes to each other and to redistribute BGP into IGRP.
! Assumes AS 1 has network number 131.108.0.0
router bgp 1
network 131.108.0.0
neighbor 192.5.10.1 remote-as 2
!
router igrp 1
network 131.108.0.0
network 192.5.10.0
redistribute bgp 1
! Note that IGRP is not redistributed into BGP
!
router bgp 2
network 150.136.0.0
neighbor 192.5.10.2 remote-as 1
!
router igrp 2
network 150.136.0.0
network 192.5.10.0
redistribute bgp 2
The most complex redistribution case is one in which mutual redistribution is required between an IGP (in this case IGRP) and BGP.
Suppose that EGP is running on a router somewhere else in AS 1, and that the EGP routes are injected into IGRP routing process 1. You must filter to ensure that the proper routes are advertised. The example configuration for router R1 illustrates use of access filters and a distribution list to filter routes advertised to BGP neighbors. This example also illustrates configuration commands for redistribution between BGP and IGRP.
router bgp 1
network 131.108.0.0
neighbor 192.5.10.1 remote-as 2
neighbor 192.5.10.15 remote-as 1
! 131.108.2.1 an internal peer
neighbor 192.5.10.24 remote-as 3
! 131.108.3.1 is an external peer
redistribute igrp 1
distribute-list 1 out igrp 1
!
! All networks that should be
! advertised from R1 are
! controlled with access lists:
!
access-list 1 permit 131.108.0.0
access-list 1 permit 150.136.0.0
access-list 1 permit 128.125.0.0
!
router igrp 1
network 131.108.0.0
network 192.5.10.0
redistribute bgp 1
The following example shows a router in autonomous system 109 using both the RIP and the IGRP routing protocols. The example advertises IGRP-derived routes using the RIP protocol and assigns the IGRP-derived routes a RIP metric of 10.
router rip
default-metric 10
redistribute igrp 109
Figure 1-7 shows this type of redistribution.
The examples in this section illustrate the use of redistribution in various cases, with and without route maps. Examples from both the IP and CLNS routing protocols are given.
The following redistributes all OSPF routes into IGRP:
router igrp 109
redistribute ospf 110
The following redistributes RIP routes with a hop count equal to 1 into OSPF. These routes will be redistributed into OSPF as external link state advertisements with a metric of 5, metric type of Type 1, and a tag equal to 1.
router ospf 109
redistribute rip route-map rip-to-ospf
route-map rip-to-ospf permit
match metric 1
set metric 5
set metric-type type1
set tag 1
The following redistributes OSPF learned routes with tag 7 as a RIP metric of 15.
router rip
redistribute ospf 109 route-map 5
route-map 5 permit
match tag 7
set metric 15
The following redistributes OSPF intra-area and interarea routes with next-hop routers on interface Serial 0 into BGP with an INTER_AS metric of 5.
router bgp 109
redistribute ospf 109 route-map 10
route-map 10 permit
match route-type internal
match interface serial 0
set metric 5
The following redistributes two types of routes into the integrated IS-IS routing table (supporting both IP and CLNS). The first are OSPF external IP routes with tag 5; these are inserted into Level 2 IS-IS LSPs with a metric of 5. The second are ISO-IGRP derived CLNS prefix routes that match CLNS access list 2000. These will be redistributed into IS-IS as Level 2 LSPs with a metric of 30.
router isis
redistribute ospf 109 route-map 2
redistribute iso-igrp nsfnet route-map 3
route-map 2 permit
match route-type external
match tag 5
set metric 5
set level level-2
route-map 3 permit
match address 2000
set metric 30
With the following configuration, OSPF external routes with tags 1, 2, 3, and 5 are redistributed into RIP with metrics of 1, 1, 5, and 5, respectively. The OSPF routes with a tag of 4 are not redistributed.
router rip
redistribute ospf 109 route-map 1
route-map 1 permit
match tag 1 2
set metric 1
route-map 1 permit
match tag 3
set metric 5
route-map 1 deny
match tag 4
route map 1 permit
match tag 5
set metric 5
The following configuration sets the condition that if there is an OSPF route to network 140.222.0.0, generate the default network 0.0.0.0 into RIP with a metric of 1.
router rip
redistribute ospf 109 route-map default
route-map default permit
match ip address 1
set ip destination 2
set metric 1
access-list 1 permit 140.222.0.0 0.0.255.255
access-list 2 permit 0.0.0.0 0.0.0.0
Given the following configuration, a RIP learned route for network 160.89.0.0 and an ISO-IGRP learned route with prefix 49.0001.0002 will be redistributed into an IS-IS Level 2 LSP with a metric of 5.
router isis
redistribute rip route-map 1
redistribute iso-igrp remote route-map 1
route-map 1 permit
match ip address 1
match clns address 2
set metric 5
set level level-2
access-list 1 permit 160.89.0.0 0.0.255.255
clns filter-set 2 permit 49.0001.0002...
The following configuration illustrates how a route map is referenced by the default-information router configuration command. This is called conditional default origination. OSPF will originate the default route (network 0.0.0.0) with a Type 2 metric of 5 if 140.222.0.0, with network 0.0.0.0 in the routing table.
route-map ospf-default permit
match ip address 1
set metric 5
set metric-type type-2
access-list 1 140.222.0.0 0.0.255.255
router ospf 109
default-information originate route-map ospf-default
In Figure 1-8, the assigned metrics meet the conditions required for a feasible successor relationship, so the paths in this example can be included in routing tables and used for load balancing.
The feasibility test would work as follows:
Assume that Router C1 already has a route to Network A with metric m and has just received an update about Network A from C2. The best metric at C2 is p. The metric that C1 would use through C2 is n.
If the following two conditions are met, the route to network A through C2 will be included in C1's routing table:
The configuration for Router C1 would be as follows:
router igrp 109
variance 10
A maximum of four paths can be in the routing table for a single destination. If there are more than four feasible paths, the four best feasible paths are used.
In Figure 1-9, with synchronization on, Router B will not advertise network 10.0.0.0 to Router A until an IGRP route for network 10.0.0.0 exists. If you specify the no synchronization router configuration command, Router B will advertise network 10.0.0.0 as soon as possible. However, since routing information still must be sent to interior peers, you must configure a full internal BGP mesh.
The following example specifies that the router at the address 131.108.1.2 is a neighbor in AS number 109.
neighbor 131.108.1.2 remote-as 109
In the following example, a BGP router is assigned to AS 109, and two networks are listed as originating in the AS. Then the addresses of three remote routers (and their ASs) are listed. The router being configured will share information about networks 131.108.0.0 and 192.31.7.0 with the neighbor routers. The first router listed is in the same Class B network, but in a different AS; the second neighbor router configuration command illustrates specification of an internal neighbor (with the same AS number) at address 131.108.234.2; and the last neighbor command specifies a neighbor on a different network.
router bgp 109
network 131.108.0.0
network 192.31.7.0
neighbor 131.108.200.1 remote-as 167
neighbor 131.108.234.2 remote-as 109
neighbor 150.136.64.19 remote-as 99
In Figure 1-10, Router A is being configured. The internal BGP neighbor is not directly linked to Router A. External neighbors (in AS 167 and AS99) must be linked directly to Router A.
In the following example, the router is configured to allow connections from any router that has an IP address in access list 1; that is, any router with a 192.31.7.x address. Neighbors not explicitly specified as neighbors can connect to the router, but the router will not attempt to connect to them if the connection is broken. Continuing with the preceding sample configuration, the configuration is as follows:
router bgp 109
network 131.108.0.0
network 192.31.7.0
neighbor 131.108.200.1 remote-as 167
neighbor 131.108.234.2 remote-as 109
neighbor 150.136.64.19 remote-as 99
neighbor any 1
access-list 1 permit 192.31.7.0 0.0.0.255
In this example configuration, the router is in AS 110 communicating with an EGP neighbor in AS 109 with address 131.108.6.5. Network 131.108.0.0 is advertised as originating within AS 110. The configuration specifies that two routers, 131.108.6.99 and 131.108.6.100, should be advertised as third-party sources of routing information for those networks that are accessible through those routers. The global configuration commands also specify that those networks should be flagged as internal to AS 110.
autonomous-system 110
router egp 109
network 131.108.0.0
neighbor 131.108.6.5
neighbor 131.108.6.5 third-party 131.108.6.99 internal
neighbor 131.108.6.5 third-party 131.108.6.100 internal
The following example configuration illustrates a router that is in AS 110 communicating with an EGP neighbor in AS 109 with address 131.108.6.5. Network 131.108.0.0 is advertised with a distance of 1, and networks learned by RIP are being advertised with a distance of 5. Access list 3 filters which RIP-derived networks are allowed in outgoing EGP updates.
autonomous-system 110
router egp 109
network 131.108.0.0
neighbor 131.108.6.5
redistribute rip
default-metric 5
distribute-list 3 out rip
The following example illustrates how an EGP core gateway can be configured.
Figure 1-11 illustrates an environment with three routers (designated C1, C2, and C3) attached to a common X.25 network. The routers are intended to route information using EGP. With the following configuration (on the router designated Core), C1, C2, and C3 cannot route traffic directly to each other via the X.25 network:
access-list 1 permit 10.0.0.0 0.255.255.255
! global access list assignment
router egp 0
neighbor any 1
This configuration specifies that an EGP process on any router on network 10.0.0.0 can act as a peer with the Core router. All traffic in this configuration will flow through the Core router.
Third-party advertisements allow traffic to bypass the Core router and go directly to the router that advertised reachability to the Core.
access-list 2 permit 10.0.0.0 0.255.255.255
! global access list assignment
router egp 0
neighbor any 2
neighbor any third-party 10.1.1.1
The following example illustrates a typical configuration for an EGP router process. The router is in autonomous system 109 and is peering with routers in AS 164, as shown in Figure 1-12. It will advertise the networks 131.108.0.0 and 192.31.7.0 to the router in AS 164, 10.2.0.2. The information sent and received from peer routers can be filtered in various ways, including blocking information from certain routers and suppressing the advertisement of specific routes.
autonomous-system 109
router egp 164
network 131.108.0.0
network 192.31.7.0
neighbor 10.2.0.2
The following example sends IGRP updates to all interfaces on network 131.108.0.0 except interface Ethernet 1. Figure 1-13 shows this configuration.
router igrp 109
network 131.108.0.0
passive-interface ethernet 1
As in the first example, IGRP updates are sent to all interfaces on network 131.108.0.0 except interface Ethernet 1 in the following example. However, in this case a neighbor router configuration command is included. This command permits the sending of routing updates to specific neighbors. One copy of the routing update is generated per neighbor.
router igrp 109
network 131.108.0.0
passive-interface ethernet 1
neighbor 131.108.20.4
In OSPF, hello packets are not sent on an interface that is specified as passive. Hence, the router will not be able to discover any neighbors, and none of the OSPF neighbors will be able to see the router on that network. In effect, this interface will appear as a stub network to the OSPF domain. This is useful if you want to import routes associated with a connected network into the OSPF domain without any OSPF activity on that interface.
The passive-interface router configuration command typically is used when the wildcard specification on the network router configuration command configures more interfaces than is desirable. The following configuration causes OSPF to run on all subnets of 131.108.0.0:
interface Ethernet 0
ip address 131.108.1.1 255.255.255.0
interface Ethernet 1
ip address 131.108.2.1 255.255.255.0
interface Ethernet 2
ip address 131.108.3.1 255.255.255.0
router ospf 109
network 131.108.0.0 0.0.255.255 area 0
If you do not want OSPF to run on 131.108.3.0, enter the following commands:
router ospf 109
network 131.108.0.0 0.0.255.255 area 0
passive-interface Ethernet 2
In the following example, the router igrp global configuration command sets up IGRP routing in AS number 109. The network router configuration commands specify IGRP routing on networks 192.31.7.0 and 128.88.0.0. The first distance router configuration command sets the default administrative distance to 255, which instructs the router to ignore all routing updates from routers for which an explicit distance has not been set. The second distance command sets the administrative distance to 90 for all routers on the Class C network 192.31.7.0. The third distance command sets the administrative distance to 120 for the router with the address 128.88.1.3.
router igrp 109
network 192.31.7.0
network 128.88.0.0
distance 255
distance 90 192.31.7.0 0.0.0.255
distance 120 128.88.1.3 0.0.0.0
The following example assigns the router with the address 192.31.7.18 an administrative distance of 100, and all other routers on subnet 192.31.7.0 an administrative distance of 200.
distance 100 192.31.7.18 0.0.0.0
distance 200 192.31.7.0 0.0.0.255
However, if you reverse the order of these commands, all routers on subnet 192.31.7.0 are assigned an administrative distance of 200, including the router at address 192.31.7.18.
distance 200 192.31.7.0 0.0.0.255
distance 100 192.31.7.18 0.0.0.0
Assigning administrative distances is a problem unique to each network and is done in response to the greatest perceived threats to the connected network. Even when general guidelines exist, the network manager must ultimately determine a reasonable matrix of administrative distances for the network as a whole.
In the following example, the distance value for IP routes learned is 90. Preference is given to these IP routes rather than routes with the default administrative distance value of 110.
router isis
distance 90 ip
Two examples of configuring split horizon are provided.
The following sample configuration illustrates a simple example of disabling split horizon on a serial link. In this example, the serial link is connected to an X.25 network.
interface serial 0
encapsulation x25
no ip split-horizon
In the next example, Figure 1-14 illustrates a typical situation in which the no ip split-horizon interface configuration command would be useful. This figure depicts two IP subnets that are both accessible via a serial interface on Router C (connected to Frame Relay network). In this example, the serial interface on Router C accommodates one of the subnets via the assignment of a secondary IP address.
The Ethernet interfaces for Router A, Router B, and Router C (connected to IP networks 12.13.50.0, 10.20.40.0, and 20.155.120.0) all have split horizon enabled by default, while the serial interfaces connected to networks 128.125.1.0 and 131.108.1.0 all have split horizon disabled by default. The partial interface configuration specifications for each router that follow Figure 1-14 illustrate that the ip split-horizon command is not explicitly configured under normal conditions for any of the interfaces.
In this example, split horizon must be disabled in order for network 128.125.0.0 to be advertised into network 131.108.0.0, and vice versa. These subnets overlap at Router C, interface S0. If split horizon were enabled on serial interface S0, it would not advertise a route back into the Frame Relay network for either of these networks.
interface ethernet 1
ip address 12.13.50.1
!
interface serial 1
ip address 128.125.1.2
encapsulation frame-relay
interface ethernet 2
ip address 20.155.120.1
!
interface serial 2
ip address 131.108.1.2
encapsulation frame-relay
interface ethernet 0
ip address 10.20.40.1
!
interface serial 0
ip address 128.124.1.1
ip address 131.108.1.1 secondary
encapsulation frame-relay
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