3Com 7757 Configuration Manual page 351

Version: BGP version. As for BGP-4, the value is 4.
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My Autonomous System: Local AS number. By comparing this filed of both
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sides, a router can determine whether the connection between itself and the
BGP peer is of EBGP or IBGP.
Hold time: Hold time is to be determined when two BGP speakers negotiate for
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the connection between them. The Hold times of two BGP peers are the same.
A BGP speaker considers the connection between itself and its BGP peer to be
terminated if it receives no Keepalive or Update message from its BGP peer
during the hold time.
BGP Identifier: The IP address of a BGP router.
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Opt Parm Len: The length of the optional parameters. A value of 0 indicates no
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optional parameter is used.
Optional Parameters: Optional parameters used for BGP authentication or
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multi-protocol extensions.
Update
Update message is used to exchange routing information among BGP peers. It can
propagate a reachable route or withdraw multiple pieces of unreachable routes.
Figure 82 shows the format of an Update message.
Figure 82 BGP Update message format
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Unfeasible Routes Length
Path Attributes(Variable)
An Update message can advertise a group of reachable routes with the same path
attribute. These routes are set in the NLRI field. The Path Attributes field carries the
attributes of these routes, according to which BGP chooses routes. An Update
message can also carry multiple unreachable routes. The withdrawn routes are set
in the Withdrawn Routes field.
The fields of an Update message are described as follows.
Unfeasible Routes Length: Length (in bytes) of the unreachable routes field. A
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value of 0 indicates that there is no Withdrawn Routes filed in the message.
Withdrawn Routes: Unreachable route list.
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Total Path Attribute Length: Length (in bytes) of the Path Attributes field. A
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value of 0 indicates that there is no Path Attributes filed in the message.
Path Attributes: Attributes list of all the paths related to NLRI. Each path
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attribute is a TLV (Type-Length-Value) triplet. In BGP, loop avoidance, routing,
and protocol extensions are implemented through these attribute values.
NLRI (Network Layer Reachability Information): Contains the information such
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reachable route suffix and the corresponding suffix length.
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Withdrawn Routes(Variable)
Total Path Attribute Length
NLRI(Variable)
BGP Overview 351
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