H3C Technologies H3C S7500E Series Switches - Manual (Page 206)

6-3
z Type: This 1-byte unsigned integer indicates the type code of the message. The following type
codes are defined: 1–Open, 2-Update, 3-Notification, 4–Keepalive, and 5–Route-refresh. The
former four are defined in RFC1771, and the last one is 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. An Open message contains the following fields:
Figure 6-2 BGP Open message format
z Version: This 1-byte unsigned integer indicates the protocol version number. The current BGP
version is 4.
z My autonomous system: This 2-byte unsigned integer indicates the Autonomous System number
of the sender.
z Hold time: When establishing a peer relationship, two parties negotiate an identical hold time. If no
Keepalive or Update is received from a peer within the hold time, the BGP connection is
considered down.
z BGP identifier: An IP address that identifies the BGP router
z Opt Parm Len (Optional Parameters Length): Length of optional parameters, which is set to 0 if no
optional parameter is available.
z Optional parameters: Used for multiprotocol extensions, and other functions.
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 6-3 BGP Update message format
Total path attribute length
Withdrawn routes
Path attributes
NLRI
Unfeasible routes length 2 Octets
N Octets
2 Octets
N Octets
N Octets
Each Update message can advertise a group of feasible routes with identical attributes, and the routes
are contained in the network layer reachable information (NLRI) field. The Path Attributes field carries
attributes of these routes. Each Update message can also carry multiple withdrawn routes in the
Withdrawn Routes field.