3Com Switch 8800 Advanced Software V5 Configuration Guide

420 CHAPTER 35: BGP CONFIGURATION
■ Providing abundant routing policies, allowing for implementing 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 message involves five types:
■ Open message
■ Update message
■ Notification message
■ Keep-alive message
■ Route-refresh message
They have the same header, as shown below:
Figure 131 BGP message header
■ Marker: The 16-octet field is used for BGP authentication calculation. If no
authentication information is available, then the Marker must be all ones.
■ Length: The 2-octet unsigned integer indicates the total length of the message.
■ Type: This 1-octet unsigned integer indicates the type code of the message. The
following type codes are defined: 1-Open, 2-Update, 3-Notification,
4-Keepalive, and 5-Route-refresh. The former four are defined in RFC1771, the
last one defined in RFC2918.
Marker
Length Type
2 bytes 1 bytes
16 bytes