BGP Route Dampening

At a Glance: designed to reduce router processing load caused by unstable routes; prevents sustained routing oscillations without affecting well-behaved routes; RFC 2439: BGP Route Flap Dampening; minimizes BGP updates by suppressing unstable routes.

Route-Dampening Operation:

• EBGP route flaps = 1000 penalty points
• IBGP route flaps not dampened;
• penalty not user-configurable;
• penalty decays via exponential decay algorithm
• if penalty > suppress limit, route is dampened
• if penalty < reuse limit, dampened route is propagated
• if penalty < 1/2 reuse limit, flap history forgotten
• route is never dampened more than the maximum suppress time limit
• unreachable route with flap history is in history state (still in BGP table to retain flap history)
• penalty is applied to an individual path, not the prefix

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Configuring Route Dampening:

router(config-router)# bgp dampening [half-life reuse suppress max-suppress-time] [route-map route-map-name]
half-life	time for penalty to halve (default 15 minutes)
reuse	penalty points when dampened route is reused (default 750)
suppress	penalty points when route is suppressed (default 2000)
max-suppress-time	maximum time a route is suppressed (default 1hour; maximum 255 minutes)

router(config-route-map)# set dampening half-life reuse suppress max-suppress-time

- used for less aggressive dampening of routes towards root DNS servers - dampening of smaller prefixes more aggressively - selective dampening based on BGP neighbors and route-map match criteria

Other Commands:

clear ip bgp ip_addr flap-statistics [{regexp regexp} | {filter-list listname} | {ipaddr mask} ]

clear ip bgp dampening [ipaddr mask]

show ip bgp dampened-paths

show ip bgp flap-statistics [{regexp regexp}|{filter-list listname}|{ipaddr mask [longer-prefix]}]

debug ip bgp dampening