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.