Route Dampening - Cisco Catalyst 9500 series Configuration Manual

Route Dampening

With route reflectors, all IBGP speakers need not be fully meshed because another method is used to pass
learned routes to neighbors. When you configure an internal BGP peer to be a route reflector, it is responsible
for passing IBGP learned routes to a set of IBGP neighbors. The internal peers of the route reflector are divided
into two groups: client peers and nonclient peers (all the other routers in the autonomous system). A route
reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The
nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The
clients in the cluster do not communicate with IBGP speakers outside their cluster.
When the route reflector receives an advertised route, it takes one of these actions, depending on the neighbor:
• A route from an external BGP speaker is advertised to all clients and nonclient peers.
• A route from a nonclient peer is advertised to all clients.
• A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be fully
Usually a cluster of clients have a single route reflector, and the cluster is identified by the route reflector
router ID. To increase redundancy and to avoid a single point of failure, a cluster might have more than one
route reflector. In this case, all route reflectors in the cluster must be configured with the same 4-byte cluster
ID so that a route reflector can recognize updates from route reflectors in the same cluster. All the route
reflectors serving a cluster should be fully meshed and should have identical sets of client and nonclient peers.
Route Dampening
Route flap dampening is a BGP feature designed to minimize 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. When route dampening is enabled, a numeric penalty value is assigned
to a route when it flaps. When a route's accumulated penalties reach a configurable limit, BGP suppresses
advertisements of the route, even if the route is running. The reuse limit is a configurable value that is compared
with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up is advertised again.
Dampening is not applied to routes that are learned by IBGP. This policy prevents the IBGP peers from having
a higher penalty for routes external to the AS.
More BGP Information
For detailed descriptions of BGP configuration, see the "Configuring BGP" chapter in the "IP Routing Protocols"
part of the Cisco IOS IP Configuration Guide, Release 12.4. For details about specific commands, see the
Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.
How to Configure BGP
Default BGP Configuration
The table given below shows the basic default BGP configuration. For the defaults for all characteristics, see
the specific commands in the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols,
Release 12.4.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
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meshed.
Configuring IP Unicast Routing