User's Manual

BGP: Concepts and Terminology
Page 4 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
BGP: Concepts and Terminology
Before moving on to look at the filtering processes, it is important to first have some
understanding of certain aspects of how BGP works. The following sections describe:
 BGP peers
 BGP updates
 Update attributes
BGP peers
Definition Within the BGP protocol, the exchange of routing information is carried out between pairs
of routers. Two routers create a TCP connection with each other, and exchange routing
information as specific data packets within that TCP session. The routers at the ends of the
TCP connection are referred to as BGP peers. Any given router can form peering
relationships with multiple routers.
Usually a BGP router with an ISP will form peer relationships with BGP routers at other ISPs
or clients with which it has entered into data transporting agreements.
The process of BGP filtering usually comes down to a matter of specifying the routes that will
be sent to, or received from, each of a router's peers.
BGP updates
Definition Once a router has established a BGP connection with a peer, it will start to exchange routing
information with that peer. A BGP update message is the packet that is used to transfer the
routing information.
The routing information contained within an update message consists of:
 a set of attribute values (see the next section for a description of the possible attributes)
and
 a list of one or more prefixes. A prefix is the network portion of an IP address, in dotted
decimal notation, optionally followed by a "/" character and a decimal number from 0 to
32. Each prefix contained within an update message represents a network that can be
reached through the IP address given in the NextHop attribute contained in the same
update message.
Note: There is only one NextHop attribute in an update message, so all the routes in the
update message have the same next hop.