3Com 7757 Configuration Manual page 350

350 C 36: BGP C
HAPTER ONFIGURATION
BGP Message Type
IBGP (Internal BGP)
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EBGP (External BGP)
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When BGP runs inside an AS, it is called interior BGP (IBGP); when BGP runs
among different ASs, it is called exterior BGP (EBGP).
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 80.
Figure 80 Packet header format of BGP messages
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Length
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
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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,
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with the packet header counted in.
Type: 1 byte in length. This field indicates the type of a BGP packet. Its value
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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 81 shows the format of an Open
message.
Figure 81 BGP Open message format
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Version
My Autonomous System
Hold Time
Opt Parm Len
The fields are described as follows.
15
Marker
Type
15
BGP Identifier
Optional Parameters
23 31
31