3Com Switch 4800G 24-Port Configuration Manual page 367

4-Keepalive, and 5-Route-refresh. The former four are defined in RFC1771, the
last one 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. The Open message contains
the following fields:
Figure 116 BGP open message format
0 7
Version
My Autonomous System
Hold Time
Opt Parm Len
Version: This 1-byte unsigned integer indicates the protocol version number of
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the message. The current BGP version is 4.
My Autonomous System: This 2-byte unsigned integer indicates the
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Autonomous System number of the sender.
Hold Time: When establishing peer relationship, two parties negotiate an
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identical hold time. If no Keepalive or Update is received from a peer after the
hold time, the BGP connection is considered down.
BGP Identifier: In IP address format, identifying the BGP router
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Opt Parm Len (Optional Parameters Length): Length of optional parameters, set
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to 0 if no optional parameter is available
Update
The Update messages are used to exchange routing information between peers. It
can advertise a feasible route or remove multiple unfeasible routes. Its format is
shown below:
Figure 117 BGP Update message format
0
Unfeasible Routes Length
Path Attributes(Variable)
Each Update message can advertise a group of feasible routes with similar
attributes, which are contained in the network layer reachable information (NLRI)
field. The Path Attributes field carries attributes of these routes that are used by
BGP for routing. Each message can also carry multiple withdrawn routes in the
Withdrawn Routes field.
15
BGP Identifier
Optional Parameters
15
Withdrawn Routes(Variable)
Total Path Attribute Length
NLRI(Variable)
BGP Overview 367
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