Cisco ASR 9000 Series Routing Configuration Guide

to be incongruent. Both of the autonomous systems must be configured for internal multiprotocol BGP
(IMBGP) in the figure.
A multicast routing protocol, such as PIM, uses the multicast BGP database to perform Reverse Path Forwarding
(RPF) lookups for multicast-capable sources. Thus, packets can be sent and accepted on the multicast topology
but not on the unicast topology.
Figure 5: Multicast BGP Environment
Route Dampening
Route dampening is a BGP feature that minimizes the propagation of flapping routes across an internetwork.
A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then
unavailable, and so on.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous
system 2, and autonomous system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes
unavailable). Under circumstances without route dampening, the eBGP neighbor of autonomous system 1 to
autonomous system 2 sends a withdraw message to autonomous system 2. The border router in autonomous
system 2, in turn, propagates the withdrawal message to autonomous system 3. When the route to network A
reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to
autonomous system 3. If the route to network A repeatedly becomes unavailable, then available, many
withdrawal and advertisement messages are sent. Route flapping is a problem in an internetwork connected
to the Internet, because a route flap in the Internet backbone usually involves many routes.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
46
Implementing BGP
Route Dampening

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English