3Com S7906E - Switch, Configuration Manual

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A router advertising BGP messages is called a BGP speaker. It establishes peer relationships with other
BGP speakers to exchange routing information. 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 peers in the
local AS.
To simplify configuration, multiple peers having an identical policy can be organized as a peer group.
BGP runs on a router in either of the following two modes:
z
iBGP (internal BGP)
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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:
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Open
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Update
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Notification
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Keep-alive
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Route-refresh
They have the same header, as shown below:
Figure 1-1 BGP message header
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Marker: The 16-byte field is used to delimit BGP messages. The Marker must be all ones.
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Length: The 2-byte unsigned integer indicates the total length of the message.
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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, 4–Keepalive, and 5–Route-refresh. The
former four are defined in RFC1771, and the last one is defined in RFC2918.
Open
After a TCP connection is established, the first message sent by each side is an Open message for peer
relationship establishment. An Open message contains the following fields: