366 CHAPTER 30: BGP CONFIGURATION
■ Eliminating route loops completely by adding AS path information to BGP
routes
■ Providing abundant routing policies to implement flexible route filtering and
selection
■ Easy to extend, satisfying new network developments
A router advertising BGP messages is called a BGP speaker, which exchanges new
routing information with other BGP speakers. When a BGP speaker receives a new
route or a route better than the current one from another AS, it will advertise the
route to all the other BGP speakers in the local AS.
BGP speakers call each other peers, and several associated peers form a peer
group.
BGP runs on a router in one of the following two modes:
■ IBGP (Interior BGP)
■ EBGP (External BGP)
BGP is called IBGP when it runs within an AS and is called EBGP when it runs
between ASs.
Formats of BGP
Messages
Header
BGP has five types of messages:
■ Open
■ Update
■ Notification
■ Keep-alive
■ Route-refresh
They have the same header, as shown below:
Figure 115 BGP message header
■ Marker: The 16-byte field is used for BGP authentication. If no authentication
information is available, then the Marker must be all ones.
■ Length: The 2-byte unsigned integer indicates the total length of the message.
■ Type: This 1-byte unsigned integer indicates the type code of the message. The
following type codes are defined: 1-Open, 2-Update, 3-Notification,
Marker
Length Type
2 bytes 1 bytes
16 bytes
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