SprintLink and Global MPLS BGP Dampening Policy

Sprint utilizes BGP dampening in the network to limit the impact of problems to a localized area in the network. Dampening prevents a customer's network from causing instabilities in the backbone and in other customer networks.

Route flap dampening was introduced to Cisco BGP starting with version 11.0 and is the primary mechanism for minimizing the instability caused by flapping. The following terms are used when describing dampening:

Term & Meaning

  • Dampen

    To suppress the routing announcements from customer networks.
  • Filter

    All customer routing announcements are passed through filters which limit the announcements in some way. The normal filter is applied to limit the addresses that can be announced; however, when a customer network is changing rapidly such that the routing announcements are being added and removed frequently, BGP will implement the dampening filter which will prevent propagation of the changes into the backbone until the network is stable.
  • Half-life time

    Once the route has been assigned a penalty, the penalty is decreased by half after the half-life period. Currently, a half-life time of 15 minutes is used in the network.
  • Maximum suppress time

    This is the maximum number of dampen-period doublings allowed (the dampen-period is increased by the half-life time with each flap after the third one). Currently, a maximum suppress time of 30 minutes, or 2 doublings, is used in the network.
  • Penalty

    A numeric value that is assigned to a route when it flaps. Currently, a penalty of 1000 is set for each flap.
  • Reuse limit

    A configurable numeric value that is compared with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up will no longer be suppressed. Currently, a reuse limit of 1500 is used in the network.
  • Route flap

    The withdrawal and subsequent announcement of a route over an eBGP session.
  • Suppressed

    A route that is not used or filtered even though it has been announced.
  • Suppress limit

    A numeric value that is compared with the penalty. If the penalty is greater than the suppress limit, the route is suppressed. Currently, a suppress limit of 3000 is used in the network.

The Route Dampening Process:

A route that is flapping receives a penalty of 1000 for each flap. When the accumulated penalty reaches a configurable limit, BGP suppresses advertisements of the route even if the route is announced. The accumulated penalty is decremented by the half-life time. When the accumulated penalty is less than the reuse limit, the route is advertised again if it has not been withdrawn.

For example, a route with a penalty of 3000 will take the half-life time of 15 minutes to cut the penalty in half and reach the reuse penalty of 1500, at which point the route will be advertised again. Similarly, a route with a penalty of 6000 will take 15 minutes to cut the penalty in half and reach a penalty of 3000. It will then an additional 15 minutes to cut the penalty in half again and reach the reuse penalty of 1500. The total decay time from 6000 to 1500 (i.e., reuse) with a half-life time of 15 will be 30 minutes. Please see RFC 2439 for more details.

If you are seeing a dampened route and would like to know what the penalties are in the router that is dampening the route, please contact NOC_Support@sprint.net.