9.2
Route Filtering and Manipulation Process (Policies)
9.2.1
Route filtering and attribute manipulation
The concept of route filtering is straightforward: A BGP speaker can choose what routes to send and what routes to receive from any of its BGP peers. Route filtering is essential in defining routing policies-for example, a network administrator can identify the inbound traffic they are willing to accept from other ASs by specifying the list of routes that should be advertised over EBGP. Conversely, an AS can control what routes its outbound traffic uses by specifying the routes it accepts from its neighbors.

Filtering can also used to limit routing updates flowing from one routing protocol to another. Routes permitted through a filter can have their attributes manipulated to affect the BGP best path decision process.

Inbound and Outbound Filtering

Both the inbound and outbound filtering concepts can be applied between peers and between routing protocols running on a single router; The Figure to the left illustrates this.

At the peer level, inbound filtering indicates that the BGP speaker is filtering routing updates coming from other peers, and outbound filtering limits the routing updates advertised from this BGP speaker towards other peers. Filtering behavior is the same whether the BGP peers are external (EBGP) or internal (IBGP). At the protocol level, inbound filtering limits the routing updates being injected 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 such as IGP and static into BGP. Outbound filtering limits the updates being redistributed from BGP into IGP.