3Com MSR 50 Series Configuration Manual page 827

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 232 BGP open message format
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Version
My Autonomous System
Hold Time
Opt Parm Len
Version: This 1-octet unsigned integer indicates the protocol version number of
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the message. The current BGP version number is 4.
My Autonomous System: This 2-octet 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 elapses, 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
Update message is used to exchange routing information between peers. It can
advertise a feasible route or remove multiple unfeasible routes. Its format is shown
below:
Figure 233 BGP Update message format
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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 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 Each message can also carry multiple withdrawn routes in the Withdrawn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.
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BGP Identifier
Optional Parameters
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Withdrawn Routes(Variable)
Total Path Attribute Length
NLRI(Variable)
BGP Overview 827
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