3Com S7906E Configuration Manual page 781

In most cases, BGP is used in complex networks, where route changes are very frequent. To solve the
problem caused by route flaps, BGP route dampening is used to suppress unstable routes.
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, 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 from being added into the routing table or being advertised to
other BGP peers.
The penalty value of the suppressed route will decrease to a 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.
Figure 1-12 BGP route dampening
Peer group
You can organize BGP peers with the same attributes into a group to simplify configurations on them.
When a peer joins the peer group, the peer obtains the same configuration as the peer group. If the
configuration of the peer group is changed, the configuration of group members is changed accordingly.
When a peer is added into a peer group, the peer enjoys the same route update policy as the peer
group to improve route distribution efficiency.
If an option is configured for both a peer and its peer group, the last 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.
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