down. Every time the link goes down, the upstream peer withdraws the routes from all
its neighbors. When the link comes back up again, the peer advertises those routes
globally. When the link problem appears again, the peer withdraws the routes again. This
process continues until the underlying problem is fixed.
The router stores a penalty value with each route. Each time the route flaps, the router
increases the penalty by 1000. If the penalty for a route reaches a configured suppress
value, the router suppresses the route. That is, the router does not include the route as
a forwarding entry and does not advertise the route to BGP peers.
The penalty decrements by 50 percent for each half-life interval that passes. The half-life
interval resets when the route flaps and the penalty increments. The route remains
suppressed until the penalty falls below the configured reuse threshold, at which point
the router once again advertises the route. You can specify a max-suppress-time for route
suppression; after this interval passes, the router once again advertises the route.
BGP creates a dampening parameter block for each unique set of dampening
parameters—such as suppress threshold and reuse threshold—used by BGP. For example,
if you have a route map that sets the dampening parameters to one set of values for
some routes and to another set of values for the remaining routes, BGP uses and stores
two dampening parameter blocks, one for each set.
Global Route Flap Dampening
Use the bgp dampening command if you want to enable route flap dampening with the
same values on all BGP routes, or on all routes matching the specified route map. If you
specify a route map, the router dampens only routes that are permitted by the route map.
For example:
host1(config-router)#bgp dampening 8 600 2500 30 route-map 1
bgp dampening
• Use to enable BGP route flap dampening on all BGP routes or routes matching a
specified route map.
• You can specify a complete set of values that determine how routes are dampened.
If you choose to do so, you must specify the entire set:
• half-life— When this period expires, the penalty assigned to a route is decreased by
half
• reuse— When the penalty for a flapping route falls below this limit, the route is
unsuppressed (added back to the BGP table and used for forwarding)
• suppress—When a route’s penalty exceeds this limit, the route is suppressed
• max-suppress-time—When the period a route has been suppressed exceeds this
limit, the route becomes unsuppressed
• If you specify the preceding set of dampening values, you can optionally specify a
half-life-unreachable period to apply to unreachable routes. If you do not specify this
value, the same half-life period is used for both reachable and unreachable routes.
• Dampening applies only to routes learned by means of EBGP.
Copyright © 2010, Juniper Networks, Inc.100
JunosE 11.2.x BGP and MPLS Configuration Guide
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