3Com Switch 4800G 24-Port - Page 367

BGP Overview 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
■ Version: This 1-byte unsigned integer indicates the protocol version number of
the message. The current BGP version is 4.
■ My Autonomous System: This 2-byte unsigned integer indicates the
Autonomous System number of the sender.
■ Hold Time: When establishing peer relationship, two parties negotiate an
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
■ Opt Parm Len (Optional Parameters Length): Length of optional parameters, set
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
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 Each message can also carry multiple withdrawn routes in the
Withdrawn Routes field.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
31
Version
My Autonomous System
Hold Time
Opt Parm Len
Optional Parameters
70
01531
Total Path Attribute Length
Withdrawn Routes(Variable)
Path Attributes(Variable)
NLRI(Variable)
Unfeasible Routes Length