Describe Routing Filters and What They Accomplish

Posted on 11:05 AM by Bharathvn

Route filters are used in several routing protocols. Most common are the OSPF and BGP implementations.

OSPF uses route filters, or route maps, to restrict summary routes and prevent routes from being imported into the route table. Most route maps use match clauses to match prefixes that they wish to accept or deny.

BGP routers use route filters to enforce policy. There are three filter types that can be applied to match updates exchanged between BGP speakers:

❑ Path filters: Using the AS-PATH attribute, if the update matches the filter criteria, the update is accepted or denied.
❑ Prefix filters: Using the prefix in the NRLI, if the update matches the filter criteria, the update is accepted or denied.
❑ Route maps: As with the interior routing protocols, route maps can have more actions associated with the match criteria. Routes can be accepted or denied, but attributes can be changed as well. There is also work being done in the field of exchanging the route filters between BGP speakers.

For more information on OSPF and BGP route filtering, check out Routing TCP/IP, Volume 1 (2nd Edition) (CCIE Professional Development) by Jeff Doyle and Jennifer Carroll (Cisco Press. 2006). For more information on BGP outbound route filtering, check out “Outbound Route Filtering Capability for BGP-4 - draft-ietf-idr-route-filter-16.txt,” by E. Chen and Y. Rekhter.