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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
C613-50032-01 REV D AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 1.23
BGP and BGP4+ Route Dampening
Route dampening is a BGP and BGP4+ feature designed to minimize the propagation of
flapping routes across a network. A route is considered to be flapping when it is
repeatedly available, then unavailable, then available, then unavailable, etc.
Consider a network with three BGP autonomous systems: autonomous system 1,
autonomous system 2, and autonomous system 3. If the route to network A in
autonomous system 1 flaps, and is unavailable without route dampening, the eBGP
neighbor of autonomous system 2 sends a withdraw message to autonomous system 2.
The router in autonomous system 2 sends the withdraw message to autonomous system
3. When a route to network A appears, autonomous system 1 sends an advertisement
message to autonomous system 2, which sends it to autonomous system 3.
If the route to network A repeatedly becomes unavailable, then available, many
withdrawal and advertisement messages are sent. This is a problem in a network
connected to the Internet since route flapping in the Internet often involves many routes.
The route dampening feature minimizes the flapping problem as follows. If the route to
network A flaps, then the router in autonomous system 2, with route dampening enabled,
assigns network A a penalty of 1000 and moves it to the history state.
The router in autonomous system 2 continues to advertise the status of the route to
neighbors. When the route flaps so the penalty exceeds the limit, the router stops
advertising the route to network A, so the route becomes dampened.
The penalty placed on network A is reduced until the limit is reached, when the route is
advertised. At half the limit, dampening information for a route to network A is removed.
The below terms are used for BGP and BGP4+ route dampening:
■ Flap—A route is available, then unavailable, or vice versa.
■ History state—After a route flaps once, it is assigned a penalty and put into history
state, meaning the router does not have the best path, based on historical
information.
■ Penalty—Each time a route flaps, the router configured for route dampening in
another autonomous system assigns the route a penalty of 1000. Penalties are
cumulative. The penalty for the route is stored in the BGP routing table until the
penalty exceeds the suppress limit. At that point, the route state changes from history
to damp.
■ Damp state—In this state, the route has flapped so often that the router will not
advertise this route to BGP neighbors.
■ Suppress limit—A route is suppressed when its penalty exceeds this limit. The
default value is 2000.
■ Half-life—Once the route has been assigned a penalty, the penalty is decreased by
half after the half-life period (which is 15 minutes by default). The process of reducing
the penalty happens every 5 seconds.