Minimizing Flapping
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to
network A flaps. The router in autonomous system 2 (in which route dampening is enabled) assigns network
A a penalty of 1000 and moves it to history state. The router in autonomous system 2 continues to advertise
the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty
exceeds a configurable suppression limit, the router stops advertising the route to network A, regardless of
how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once
again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.
No penalty is applied to a BGP peer reset when route dampening is enabled, even though the reset withdraws
the route.
Note
BGP Routing Domain Confederation
One way to reduce the iBGP mesh is to divide an autonomous system into multiple subautonomous systems
and group them into a single confederation. To the outside world, the confederation looks like a single
autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other
autonomous systems in the same confederation. Although the peers in different autonomous systems have
eBGP sessions, they exchange routing information as if they were iBGP peers. Specifically, the next hop,
MED, and local preference information is preserved. This feature allows you to retain a single IGP for all of
the autonomous systems.
BGP Route Reflectors
BGP requires that all iBGP speakers be fully meshed. However, this requirement does not scale well when
there are many iBGP speakers. Instead of configuring a confederation, you can reduce the iBGP mesh by
using a route reflector configuration.
Figure 6: Three Fully Meshed iBGP Speakers, on page 48 illustrates a simple iBGP configuration with three
iBGP speakers (routers A, B, and C). Without route reflectors, when Router A receives a route from an external
neighbor, it must advertise it to both routers B and C. Routers B and C do not readvertise the iBGP learned
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
47
Implementing BGP
BGP Routing Domain Confederation
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