BGP: Route Filtering and Summarization

Four tools can be used to filter BGP routes:

  • Distribution Lists
  • Prefix Lists
  • AS_PATH filter lists
  • Route Maps

The aggregate-address command can be used to filter component subnet of a summary route.

  • All can filter in or out updates per neighbor or per peer group
  • peer group configurations require cisco IOS software to process the routing policy against the update only once, rather than once per neighbor
  • the filters cannot be applied to a single neighbor that is part of a peer group, the filter can only be applied to the entier group or the neighbor must be configured outside the peer group
  • each tools matching logic examines the contents of the BGP update message which includes the BGP PAs and NLRI
  • if a filters config has changed a clear command is required for the changed filter to take effect
  • the clear command can use the soft reconfiguration option to implement changes without requiring the BGP peers to be brought down and back up.
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