|
|
This chapter describes how to configure Internet Protocol (IP) enhanced IGRP on routers and interfaces configured for IP. For a complete description of the commands mentioned in this chapter, refer to the "IP Enhanced IGRP Commands" chapter in this publication. For a description of other IP configuration commands, refer to the Router Products Configuration Guide and Router Products Command Reference publications. For historical background and a technical overview of IP routing protocols, see the Internetworking Technology Overview publication.
IP enhanced IGRP provides the following features:
To configure IP enhanced IGRP, complete the tasks in the following sections. At a minimum, you must enable IP enhanced IGRP. The remaining tasks are optional.
See the end of this chapter for configuration examples.
To create an IP enhanced IGRP routing process, perform the following tasks:
IP enhanced IGRP sends updates to the interfaces in the specified network(s). If you do not specify an interface's network, it will not be advertised in any IP enhanced IGRP update.
If you have routers on your network that are configured for IGRP and you want to make a transition to routing enhanced IGRP, you need to designate transition routers that have both IGRP and enhanced IGRP configured. In these cases, perform the tasks as noted in the previous section, "Create the IP enhanced IGRP Routing Process," and also read the section on configuring IGRP in the Router Products Configuration Guide. You must use the same autonomous system number in order for routes to be redistributed automatically.
To configure IP enhanced IGRP-specific parameters, perform one or more of the following tasks:
IP enhanced 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 up to four unequal-cost paths to provide greater overall throughput and reliability. Alternate path variance (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 IP enhanced 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). To change the variance to define how much worse an alternate path can be before that path is disallowed, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Define the variance associated with a particular path. | variance multiplier |
See the "IP Enhanced IGRP Configuration Examples" section at the end of this chapter for an example of configuring an IP enhanced IGRP feasible successor.
You can adjust the default behavior of IP enhanced IGRP routing and metric computations. For example, this allows you to tune system behavior to allow for satellite transmission. Although IP enhanced IGRP metric defaults have been carefully selected to provide excellent operation in most networks, you can adjust the IP enhanced IGRP metric. Adjusting IP enhanced IGRP metric weights can dramatically affect network performance, so be careful if you adjust them.
To adjust the IP enhanced IGRP metric weights, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust the IP enhanced IGRP metric. | metric weights tos k1 k2 k3 k4 k5 |
By default, the IP enhanced IGRP composite metric is a 32-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.
You can configure IP enhanced IGRP to perform automatic summarization of subnet routes into network-level routes. For example, you can configure subnet 131.108.1.0 to be advertised as 131.108.0.0 over interfaces that have subnets of 192.31.7.0 configured. Automatic summarization is performed when there are two or more network router configuration commands configured for the IP enhanced IGRP process. By default, this feature is enabled.
To disable automatic summarization, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable automatic summarization. | no auto-summary |
Route summarization works in conjunction with the ip summary-address eigrp interface configuration command, in which additional summarization can be performed. If auto-summary is in effect, there usually is no need to configure network level summaries using the ip summary-address eigrp command.
You can configure a summary aggregate address for a specified interface. If there are any more-specific routes in the routing table, IP enhanced IGRP will advertise the summary address out the interface with a metric equal to the minimum of all more-specific routes.
To configure a summary aggregate address, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure a summary aggregate address. | ip summary-address eigrp autonomous-system-number address mask |
To configure protocol-independent parameters, perform one or more of the following tasks:
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 IP enhanced IGRP-derived routes using the RIP protocol, or to readvertise static routes using the IP enhanced IGRP protocol. This capability applies to all the IP-based routing protocols.
You may also conditionally control the redistribution of routes between routing domains by defining a method known as route maps between the two domains.
To redistribute routes from one protocol into another, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Redistribute routes from one routing protocol into another. | redistribute protocol autonomous-system-number [route-map map-tag] |
To define route maps, 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 |
By default, the redistribution of default information between IP enhanced IGRP processes is enabled. To disable the redistribution, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable the redistribution of default information between IP enhanced IGRP processes. | no default-information allowed {in | out} |
See the "IP Enhanced IGRP Configuration Examples" section at the end of this chapter for examples of configuring redistribution and route maps.
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 IP enhanced 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.
To set metrics for redistributed routes, perform the first task when redistributing from IP enhanced IGRP, and perform the second task when redistributing into IP enhanced IGRP. Each task is done in router configuration mode.
You can filter routing protocol information by performing the following tasks:
Use the information in the following sections to 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.
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 a router interface. | passive-interface type unit |
To control which routers learn about routes, you can control the advertising of routes in routing updates. To do this, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Control the advertising of routes in routing updates. | distribute-list access-list-number out [interface-name | routing-process | autonomous-system-number] |
To control the processing of routes listed in incoming updates, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Control which incoming route updates are processes. | 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. To do so, 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, the same route may be advertised by more than one routing process. Specifying administrative distance values enables the router to discriminate between sources of routing information. The router always picks the route whose routing protocol has the lowest administrative distance.
There are no general guidelines for assigning administrative distances, because each network has its own requirements. You must determine a reasonable matrix of administrative distances for the network as a whole. Table 3-1 shows the default administrative distance for various routing information sources.
| Route Source | Default Distance |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| Enhanced IGRP summary route | 5 |
| External BGP | 20 |
| Internal enhanced IGRP | 90 |
| IGRP | 100 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EGP | 140 |
| External enhanced IGRP | 170 |
| Internal BGP | 200 |
| Unknown | 255 |
For example, consider a router using IP enhanced IGRP and RIP. Suppose you trust the IP enhanced IGRP-derived routing information more than the RIP-derived routing information. Because the default IP enhanced IGRP administrative distance is lower than that for RIP, the router uses the IP enhanced IGRP-derived information and ignores the RIP-derived information. However, if you lose the source of the IP enhanced IGRP-derived information (for example, because of a power shutdown), the router uses the RIP-derived information until the IP enhanced IGRP-derived information reappears.
To filter sources of routing information, perform the following tasks in router configuration mode:
| Task | Command |
|---|---|
| Filter on routing information sources. | distance eigrp internal-distance external-distance |
You can adjust the interval between hello packets and the hold time.
Routers periodically send hello packets to each other to dynamically learn of other routers on their directly attached networks. The routers use this information to discover who their neighbors are and to learn when their neighbors become unreachable or inoperative. By default, hello packets are sent every 5 seconds.
You can configure the hold time on a specified interface for the IP enhanced IGRP routing process designated by the autonomous system number. The hold time is advertised in hello packets and indicates to neighbors the length of time they should consider the sender valid. The default hold time is three times the hello interval, or 15 seconds.
To change the interval between hello packets, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the hello interval for an IP enhanced IGRP routing process. | ip hello-interval eigrp autonomous-system-number seconds |
On very congested and large networks, 15 seconds may not be sufficient time for all routers to receive hello packets from their neighbors. In this case, you may want to increase the hold time.
To change the hold time, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the hold time for an IP enhanced IGRP routing process. | ip hold-time eigrp autonomous-system-number seconds |
Split horizon controls the sending of IP enhanced IGRP update and query packets. When split horizon is enabled on an interface, these packets are not sent for destinations for which this interface is the next hop. This reduces the possibility of routing loops.
By default, split horizon is enabled by default on all interfaces.
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.
To disable split horizon, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Disable split horizon. | no ip split-horizon eigrp autonomous-system-number |
See the "IP Enhanced IGRP Configuration Examples" section at the end of this chapter for an example of using split horizon.
You can display router statistics such as the contents of IP routing tables, caches, and databases. You can use the information displayed to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path that your router's packets are taking through the network.
To display various router statistics, perform one or more of the following tasks at the EXEC prompt:
The following sections provide IP enhanced IGRP configuration examples:
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 3-1 illustrates this point. The route learned by a protocol with an administrative distance 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
In the following example, three static routes are specified, two of which are to be advertised. You can advertise them by specifying the redistribute static router configuration command, then specifying an access list that allows only those two networks to be passed to the IP enhanced 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 131.108.0.0 255.255.0.0 192.31.7.65
access-list 3 permit 192.1.2.0
access-list 3 permit 193.62.5.0
!
router eigrp 109
network 192.31.7.0
redistribute static route-map static-to-eigrp
route-map static-to-eigrp
match ip address 3
set metric 10000 100 255 1 1500
Each IP enhanced IGRP routing process can provide routing information to only one autonomous system; the router must run a separate IP enhanced 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 IP enhanced 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 eigrp 71
network 15.0.0.0
router eigrp 109
network 192.31.7.0
To transfer a route from 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 eigrp 71
redistribute eigrp 109 route-map 109-to-71
route-map 109-to-71 permit
match ip address 3
set metric 10000 100 1 255 1500
access-list 3 permit 192.31.7.0
The following example is an alternative way to transfer a route to 192.31.7.0 into autonomous system 71. Unlike the previous configuration, this one does not allow you to arbitrarily set the metric.
router eigrp 71
redistribute eigrp 109
distribute-list 3 out eigrp 109
access-list 3 permit 192.31.7.0
This sections provides two examples of RIP and IP enhanced IGRP redistribution, a simple one and a complex one.
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 IP enhanced 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 eigrp 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 IP enhanced IGRP routing process. The network router configuration command specifies that network 128.1.0.0 (the regional network) is to send and receive IP enhanced 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 IP enhanced 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.
The most complex redistribution case is one in which mutual redistribution is required between an IGP (in this case IP enhanced IGRP) and BGP.
Suppose that BGP is running on a router somewhere else in AS 1, and that the BGP routes are injected into IP enhanced IGRP routing process 1. You must use filters 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 IP enhanced IGRP.
! Configuration for router R1:
router bgp 1
network 131.108.0.0
neighbor 192.5.10.1 remote-as 2
neighbor 192.5.10.15 remote-as 1
neighbor 192.5.10.24 remote-as 3
redistribute eigrp 1
distribute-list 1 out eigrp 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 eigrp 1
network 131.108.0.0
network 192.5.10.0
redistribute bgp 1
The following example shows a router in autonomous system 1 using both the OSPF and the IP enhanced IGRP routing protocols. The example advertises OSPF-derived routes using the IP enhanced IGRP protocol and assigns the OSPF-derived routes an IP enhanced IGRP metric of 1000 100 255 1 1500.
router eigrp 1
network 131.108.0.0
redistribute ospf 1
default-metric 1000 100 255 1 1500
router ospf 1
network 160.89.0.0 0.0.255.255 area 0.0.0.0
The following example redistributes all OSPF routes into IP enhanced IGRP:
router eigrp 109
redistribute ospf 110
default-metric 1000 100 255 1 1500
The following example configures route summarization on the interface and also configures the auto-summary feature. This configuration causes IP enhanced IGRP to summarize network 10.0.0.0 out interface Ethernet 0 only. In addition, this example disables auto summarization.
interface Ethernet 0
ip summary-address eigrp 1 10.0.0.0 255.0.0.0
!
router eigrp 1
network 131.108.0.0
no auto-summary
In Figure 3-2, 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 eigrp 109
variance 10
A maximum of four paths for a single destination can be present in the routing table. If there are more than four feasible paths, the four best feasible paths are used.
Figure 3-3 illustrates a configuration in which disabling split horizon would be useful. In this configuration, two IP subnets are both accessible via a serial interface on Router C (connected to Frame Relay network). The serial interface on Router C accommodates one of the subnets via the assignment of a secondary IP address. 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
no ip split-horizon eigrp
!
interface serial 0
ip address 128.124.1.1
ip address 131.108.1.1 secondary
encapsulation frame-relay
|
|