Wednesday, May 25, 2011

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

Configuring Route Dampening:
bgp dampening [half-life reuse suppress max-suppress-time] [route-map route-map-name]
half-lifetime for penalty to halve (default 15 minutes)
reusepenalty points when dampened route is reused (default 750)
suppresspenalty points when route is suppressed (default 2000)
max-suppress-timemaximum time a route is suppressed (default 1hour; maximum 255 minutes)

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

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