Category: BGP

Designing and operating large BGP networks has always been a challenge, as you have to deploy BGP on all core routers and design a hierarchy of internal BGP routers to get around the full-mesh limitation. When MPLS was introduced, it gave us means of deploying BGP only on the network edges, with the core routers carrying just the information about the BGP next hops.

As I know some of you run large networks, could you help me understand what you're using (without giving away too much information, of course):

We all know that BGP is meant to converge slowly… well, the MPLS/VPN service providers tend to disagree, as their users are not used to minute-long convergence times. One of the major components of slow BGP convergence is the time it takes a router to discover that a neighbor has disappeared. Traditionally, the BGP keepalive packets were sent every minute and it took up to three minutes to discover that a neighbor is down.

Of course you could fine-tune those times with the neighbor timers configuration command, but the reduced timers resulted in increased TCP traffic and consequently increased CPU load, which could reach tens of percents if the timers were set to a few seconds and the router had lots of BGP neighbors.

The primary mission of Border Gateway Protocol (BGP) (transport of Internet routing between ISPs) has influenced the implementation of BGP default route origination, advertisements and propagation. This article describes the functionality and caveats of BGP default routing. All router configurations and printouts are taken from a sample network illustrated in the following diagram:

Martin Kluge sent me an interesting BGP question: he has two upstream links and runs BGP on both. Since his router is low on RAM, he cannot accept full routing, so he’s just announcing his IP prefix and using static default routing toward upstream ISPs.

In Cisco IOS release 12.3T (integrated in 12.4), we've got an interesting (and quite understated) BGP feature: BGP peer-groups are no longer a performance feature (previously, IOS used them to reduce the time needed to compute outbound BGP updates). IOS now performs automatic grouping of BGP neighbors in dynamic update peer-groups that receive identical BGP updates based on per-neighbor outbound parameters.