H3C S5600 SERIES Operation Manual page 330

Figure 5-2 BGP Open message format
The fields are described as follows.
Version: BGP version. As for BGP-4, the value is 4.
My Autonomous System: Local AS number. By comparing this filed of both 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 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.
Opt Parm Len: The length of the optional parameters. A value of 0 indicates no optional parameter
is used.
Optional Parameters: Optional parameters used for BGP authentication or 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.
Update message.
Figure 5-3 BGP Update message format
An Update message can advertise a group of reachable routes with the same path attribute. These
routes are set in the NLRI (network layer reachability information) 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 The withdrawn routes are set in the Withdrawn Routes field.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 value of 0 indicates
that there is no Withdrawn Routes filed in the message.
Withdrawn Routes: Unreachable route list.
Figure 5-3
5-3
shows the format of an