3Com Switch 7750 Configuration Guide Guide

336 CHAPTER 36: BGP CONFIGURATION
■ IBGP (Internal BGP)
■ EBGP (External BGP)
When BGP runs inside an AS, it is called interior BGP (IBGP); when BGP runs
among different ASs, it is called exterior BGP (EBGP).
BGP Message Type Format of a BGP packet header
BGP is message-driven. There are five types of BGP packets: Open, Update,
Notification, Keepalive, and Route-refresh. They share the same packet header, the
format of which is shown by
Figure 77.
Figure 77 Packet header format of BGP messages
The fields in a BGP packet header are described as follows.
■ Marker: 16 bytes in length. This filed is used for BGP authentication. When no
authentication is performed, all the bits of this field are 1.
■ Length: 2 bytes in length. This filed indicates the size (in bytes) of a BGP packet,
with the packet header counted in.
■ Type: 1 byte in length. This field indicates the type of a BGP packet. Its value
ranges from 1 to 5, which represent Open, Update, Notification, Keepalive,
and Route-refresh packets. Among these types of BGP packets, the first four
are defined in RFC1771, and the rest one is defined in RFC2918.
Open
Open massage is used to establish connections between BGP speakers. It is sent
when a TCP connection is just established.
Figure 78 shows the format of an Open
message.
Figure 78 BGP Open message format
The fields are described as follows.
■ Version: BGP version. As for BGP-4, the value is 4.
Mar ker
Length Type
0 7 15 31
BGP Identifier
Opt Parm Len
Optional Parameters
0 7 15 31
Version
My Autonomous System
Hold Time