Extreme Networks ExtremeWare XOS 10.1, Руководство пользователя

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212 ExtremeWare XOS 10.1 Concepts Guide
Exterior Gateway Routing Protocols
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Adding Neighbors to a BGP Peer Group
To create a new neighbor and add it to a BGP peer group, use the following command:
create bgp neighbor <remoteaddr> peer-group <peer-group-name> {multi-hop}
The new neighbor is created as part of the peer group and inherits all of the existing parameters of the
peer group. The peer group must have remote AS configured.
To add an existing neighbor to a peer group, use the following command:
configure bgp neighbor [all | <remoteaddr>] peer-group [<peer-group-name> | none]
{acquire-all}
If you do not specify acquire-all, only the mandatory parameters are inherited from the peer group. If
you specify acquire-all, all of the parameters of the peer group are inherited. This command disables the
neighbor before adding it to the peer group.
To remove a neighbor from a peer group, use the
peer-group none
option.
When you remove a neighbor from a peer group, it retains the parameter settings of the group. The
parameter values are not reset to those the neighbor had before it inherited the peer group values.
BGP Route Flap 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 a route becomes unavailable, a Withdrawal message
is sent to other connected routers, which in turn propagate the Withdrawal message to other routers. As
the route becomes available again, an Advertisement message is sent and propagated throughout the
network. As a route repeatedly changes from available to unavailable, large numbers of messages
propagate throughout the network. This is a problem in an internetwork connected to the Internet
because a route flap in the Internet backbone usually involves many routes.
Minimizing the Route Flap
The route flap dampening feature minimizes the flapping problem as follows. Suppose that the route to
network 172.25.0.0 flaps. The router (in which route dampening is enabled) assigns network 172.25.0.0 a
penalty of 1000 and moves it to a “history” state in which the penalty value is monitored. The router
continues to advertise the status of the route to neighbors. The penalties are cumulative. When the route
flaps so often that the penalty exceeds a configurable suppress limit, the router stops advertising the
route to network 172.25.0.0, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network 172.25.0.0 is decayed until the reuse limit is reached, upon which the
route is once again advertised. At half of the reuse limit, the dampening information for the route to
network 172.25.0.0 is removed.
The penalty is decayed by reducing the penalty value by one-half at the end of a configurable time
period, called the half-life. Routes that flap many times may reach a maximum penalty level, or ceiling,
after which no additional penalty is added. The ceiling value is not directly configurable, but the
configuration parameter used in practice is the maximum route suppression time. No matter how often
a route has flapped, once it stops flapping, it will again be advertised after the maximum route
suppression time.