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| 9.8 |  | BGP Route Filtering and Policy Routing |
| 9.8.1 |  | BGP route filtering |

Route filtering empowers a BGP speaker to choose what routes to exchange with any of its BGP peers. Route filtering is the cornerstone of policy routing. For example, an AS can identify inbound traffic that it is willing to accept by filtering its outbound advertisements. Conversely, an AS can control what routes its outbound traffic uses by specifying the routes to accept from EBGP neighbors.

Policies that are even more precise can be defined by way of route filters. For example, BGP routes passing through a filter can have their attributes manipulated to affect the best path decision process.

Both the inbound and outbound filtering concepts can be applied between peers and between routing protocols running on a single router. At the peer level, inbound filtering indicates that the BGP speaker is filtering routing updates coming from other peers. The outbound filtering limits the routing updates advertised from this BGP speaker towards other peers.

At the protocol level, inbound filtering limits the routing updates being redistributed into a protocol. Outbound filtering limits the routing updates being injected from this protocol. With respect to BGP, for example, inbound filtering limits the updates being redistributed from other protocols into BGP. Outbound filtering limits the updates being redistributed from BGP into an IGP.

The following sections examine the use of filters to implement routing policy and the use of distribute lists and prefix lists to filter routing updates.

| Web Links BGP Prefix-Based Outbound Route Filtering http://www.cisco.com/univercd/cc/td/doc/product/ software/ios120/120newft/120limit/ 120st/120st11/bgpbporf.htm