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 5.1.x
OL-30423-03 45
Implementing BGP
Route Dampening
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