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.