Review Questions

1: How and why is IBGP used for interdomain routing?

2: Is it possible to have two BGP sessions between routers for redundancy?

3: How many BGP sessions, routes, and updates/per second can a router handle?

Answers:

1: How and why is IBGP used for interdomain routing?

A: Some early deployments of BGP redistributed routes learned via EBGP into the IGP (OSPF/IS-IS), and carried the routes via both the IBGP and IGP. A border router would not readvertise routes learned via IBGP until the same routes were visible in the IGP—that is, until IBGP and the IGP were synchronized. However, commonly used IGPs do not scale particularly well to tens of thousands of routes, so this approach is now rarely used. Instead, the IGP carries just enough information (generally, all the links internal to the autonomous system) to provide a route to all BGP next hops. Synchronization is disabled via the BGP subcommand no bgp synchronization.

2: Is it possible to have two BGP sessions between routers for redundancy?

A: Provided that two different IP addresses are used for the sessions, yes. Note that a router will accommodate only a single remote-AS configuration line for any neighbor IP address. However, this might not be an effective method for achieving redundancy. A more successful approach is to peer using loopback addresses. If the session is EBGP, you will need to add the neighbor <remote-ip> ebg-multihop BGP neighbor subcommand.

3: How many BGP sessions, routes, and updates/per second can a router handle?

A: There is no simple answer to this question. Cisco continually tunes the number of configurable sessions, memory utilization, and update processing speed to handle increasing customer requirements. Performance will depend on the number and complexity of attributes associated with each BGP route. At the time of this writing, applications requiring the configuration of hundreds of peers, hundreds of thousands of BGP paths, and hundreds to thousands of updates per second are emerging.

Bear in mind, however, that performance in these areas will be strongly influenced by the amount of memory—and particularly the Cisco platform—you are using. High-end platforms, such as the 7500 series with RSP2 or RSP4 processors, or GSRs, are required for the most demanding ISP backbone applications. For simple BGP connectivity to an ISP, however, lower-end access routers will suffice, particularly if there is no requirement for receiving full Internet routes.

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