BGP - Outbound Route Filtering
Prefix-based Outbound Route Filtering (ORF) is a mechanism used by BGP routers to send and receive ORF capabilities for the purpose of minimizing the number of BGP updates sent between BGP peers, and thus can help reduce the amount of system resources required for generating and processing routing updates by filtering out unwanted routing updates at the source.
For example, BGP ORF can be used to reduce the amount of processing required on a router that is not accepting full routes from a service provider network.
BGP ORF enables one BGP peer to send a prefix list to its neighbor to be used to filter outbound routes. In other words, one BGP peer tells the other BGP peer what routes not to send, thus reducing the number of BGP updates, and the size of local BGP tables.
The BGP prefix-based outbound route filtering is enabled through the advertisement of ORF capabilities to peer routers. The advertisement of the ORF capability indicates that a BGP peer will accept a prefix list from a neighbor and apply the prefix list to locally configured ORFs (if any exist). When this capability is enabled, the BGP speaker can install the inbound prefix list filter to the remote peer as an outbound filter, which reduces unwanted routing updates.