3Com MSR 50 Series Configuration Manual page 836

836 C 56: BGP C
HAPTER ONFIGURATION
c
BGP route dampening uses a penalty value to judge the stability of a route. The
bigger the value, the less stable the route. Each time a route flap occurs (the state
change of a route from active to inactive is a route flap), BGP adds a penalty value
(1000, which is a fixed number and cannot be changed) to the route. When the
penalty value of the route exceeds the suppress value, the route is suppressed,
that is, it is neither added into the routing table, nor advertised to other BGP
peers.
The penalty value of the suppressed route will reduce to half of the suppress value
after a period of time. This period is called Half-life. When the value decreases to
the reusable threshold value, the route is added into the routing table and
advertised to other BGP peers in update packets.
Figure 242 BGP route dampening
Penalty
Value
Suppress Time
Half-life
Peer group
A peer group is a collection of peers with the same attributes. When a peer joins
the peer group, the peer obtains the same configuration as the peer group. If
configuration of the peer group is changed, configuration of group members is
also changed.
There are many peers in a large BGP network. Some of these peers may be
configured with identical commands. The peer group feature simplifies
configuration of this kind.
When a peer is added into a peer group, the peer enjoys the same route update
policy as the peer group, improving route distribution efficiency.
CAUTION: If an option is configured both for a peer and for the peer group, the
latest configuration takes effect.
Community
A peer group makes peers in it enjoy the same policy, while a community makes a
group of BGP routers in several ASs enjoy the same policy. Community is a path
attribute and advertised between BGP peers, without being limited by AS.
A BGP router can modify the community attribute for a route before sending it to
other peers.
Suppress
Threshold
Reusable
Threshold
Time