Route Flap Damping (RFD) is a mechanism supported by 7450, 7750, and 7950 routers, as well as other BGP routers, that was designed to help improve the stability of Internet routing by mitigating the impact of route flaps. Route flaps describe a situation where a router alternately advertises a route as reachable and then unreachable or as reachable through one path and then another path in rapid succession. Route flaps can result from hardware errors, software errors, configuration errors, unreliable links, and so on. However not all perceived route flaps represent a true problem; when a best path is withdrawn the next-best path may not be immediately known and may trigger a number of intermediate best path selections (and corresponding advertisements) before it is found. These intermediate best path selections may travel at different speeds through different routers because of the effect of the min-route-advertisement interval (MRAI) and other factors. RFD does not handle this type of situation particularly well and for this and other reasons many Internet service providers do not use RFD.
In SR OS route flap damping is configurable; by default, it is disabled. It can be enabled on EBGP and confed-EBGP sessions by including the damping command in their group or neighbor configuration. The damping command has no effect on IBGP sessions. When a route of any type (any AFI/SAFI) is received on a non-IBGP session that has damping enabled.
If the route changes from reachable to unreachable because of a withdrawal by the peer then damping history is created for the route (if it does not already exist) and in that history the Figure of Merit (FOM), an accumulated penalty value, is incremented by 1024.
If a reachable route is updated by the peer with new path attribute values then the FOM is incremented by 1024.
In SR OS the FOM has a hard upper limit of 21540 (not configurable).
The FOM value is decayed exponentially as described in RFC 2439. The half-life of the decay is 15 minutes by default, however a BGP import policy can be used to apply a non-default damping profile to the route, and the half-life in the non-default damping profile can have any value between 1 and 45 minutes.
The FOM value at the last time of update can be displayed using the show router bgp damping detail command. The time of last update can be up to 640 seconds ago; SR OS does not calculate the current FOM every time the show command is entered.
When the FOM reaches the suppress limit, which is 3000 by default, but can be changed to any value between 1 and 20000 in a non-default damping profile, the route is suppressed, meaning it is not used locally and not advertised to peers. The route remains suppressed until either the FOM exponentially decays to a value less than or equal to the reuse threshold or the max-suppress time is reached. By default, the reuse threshold is 750 and the max-suppress time is 60 minutes, but these can be changed in a non-default damping profile: reuse can have a value between 1 and 20000 and max-suppress can have a value between 1 and 720 minutes.