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Multi-homed BGP configurations can bias the exit point advertised by the eBGP peer. This is called the multi-exit discriminator, and it may be used to provide a fixed value—the lowest is preferred—or it may be based on the IGP metric, which is typically provided by OSPF. Note that this value does not propagate beyond the link.
Administrators may also use route maps to modify and influence the routing tables. Route maps operate on a match-and-set model where conditions may be checked before the router applies the set. For example, the administrator may wish to modify only routes from network 172.16.0.0. In this configuration, the route map would match 172.16.0.0 and then set the modified value. The administrator may wish to use this function to adjust the metric.
The following BGP configuration is provided as a sample of some of the commands used. In reality, BGP configurations can be very simple; however, most installations to the Internet require additional parameters that can cause difficulty. Notice how the specific IP address of each neighbor is provided in the configuration and that the update-source for AS 65342 is defined as Loopback0. The route-map Filter has also been applied.
router bgp 65470
no synchronization
bgp dampening
network 10.9.14.0 mask 255.255.255.0
neighbor 192.168.19.33 remote-as 65391
neighbor 192.168.19.33 soft-reconfiguration inbound
neighbor 192.168.19.33 route-map Filter out
neighbor 172.16.55.10 remote-as 65342
neighbor 172.16.55.10 update-source Loopback0
route-map Filter permit 10
match ip address 192
The BGP routing protocol selects routes based on information obtained from the Adjunct-RIB-In table. There are actually three tables according to the specifications, as shown in Table 4.6. RIB stands for Routing Information Base.
| Table | Function |
|---|---|
| Adjunct-RIB-In | Learned from inbound update messages. Contains routes that are unprocessed from inbound peer advertisements. |
| Adjunct-RIB-Out | Contains routes that the local BGP speaker will advertise to peers. |
| Local-RIB | Contains local routing information that the BGP speaker obtained from applying local policies to Adjunct-RIB-In routing information. |
| While these databases are presented as separate entities, they are not necessarily so. |
There are three route-selection decision-process phases. These are described in Table 4.7.
| Selection Phase | Function |
|---|---|
| Phase 1 | Calculates the preference for each route received and advertises routes that have the highest preference. |
| Phase 2 | Selects the best route for each destination and places that route into the appropriate Local-RIB. |
| Phase 3 | Disseminates routes in the Local-RIB to each neighbor AS peer. |
Typically a route will have a best path that the router can use, but it is possible to have a tie. In this scenario, the lowest multi-exit discriminator (MED) value is used to break the tie. If the MED is not provided, the route with the lowest interior distance cost will be used. BGP speakers with the lowest BGP identifier—the IP address—will win ties as well. This is another use of the loopback address in BGP installations.
Like OSPF, IS-IS (Intermediate System-to-Intermediate System) is an interoperable, link-state, standards-based routing protocol that is supported by various vendors. However, it also can be difficult to configure due to topology restrictions, many of which are shared with OSPF. The sole metric—bandwidth—is also viewed as a limitation to the protocol and may account for its low acceptance in the market.
The benefits of IS-IS include fast convergence and support for VLSM. Hellos are sent at regular intervals and routing updates are sent only in response to a topology change—and then only include the changes themselves.
One of the concepts of IS-IS is that it is an interior routing protocol, like OSPF, RIP, and IGRP. Interior routing protocols are generally considered to be inappropriate for use between administrative entities—BGP being the de facto standard for these connections. As noted previously, BGP is both an internal and external (iBGP and eBGP) protocol, depending on the AS configuration.
 | The exterior routing protocol, ES-IS, is used for exterior routing. |
IS-IS makes use of a two-area structure, with area defined as layers. Layer 1 is used for intra-domain routing, whereas Layer 2 is used for inter-domain routing—Layer 2 linking two routing domains (areas) in the IS-IS syntax. Hierarchies are established as Layer 1 routers need only find a Layer 2 router for forwarding—similar to a border router in OSPF.
Metrics in IS-IS, by default, are comprised of a single path value—the maximum value of which is 1024. Individual links are limited to a maximum setting of 64. IS-IS also provides a limited quality-of-service function in its CLNP header, which can account for other link costs. CLNP stands for Connectionless Network Protocol, which was originally developed for the routing of DECnet/OSI packets. The protocol has been modified to support IP. At the present time, there is little reason to select IS-IS—EIGRP and OSPF dominate the marketplace. The Cisco Web site provides additional information on the protocol, should you wish to study it further.
This chapter addressed the IP routing protocols and processes as they relate to network design. These protocols include the following:
The chapter also identified some of the reasons IP routing might be better handled by one protocol than another. Incorporated into that decision were a number of criteria, including the following:
In addition, the chapter addressed the proprietary IGRP routing protocol and presented features and options that the designer might wish to consider when deploying this routing protocol. Some of these issues included convergence and efficiency.
The presentation on OSPF discussed several of the advantages offered by this protocol, including its wide availability. In addition, designers should feel comfortable with a number of the implementation techniques used for successful OSPF designs, including the following:
The process by which convergence occurs was also described.
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