H3C Technologies H3C S10500 Series Switches - Manual (Page 207)

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Update
The update messages are used to exchange routing information between peers. It can advertise feasible
routes or remove multiple unfeasible routes.
Figure 73 BGP update message format
Each update message can advertise a group of feasible routes with identical attributes, and the routes
are contained in the network layer reachability information (NLRI) field. The path attributes field carries
the attributes of these routes. Each update message can also carry multiple withdrawn routes in the
withdrawn routes field.
Major fields of the BGP update message are as follows:
• Unfeasible routes length: The total length of the withdrawn routes field in bytes. A value of 0
indicates no route is withdrawn from service, nor is the withdrawn routes field present in this update
message.
• Withdrawn routes: This is a variable length field that contains a list of withdrawn IP prefixes.
• Total path attribute length: Total length of the path attributes field in bytes. A value of 0 indicates
that no NLRI field is present in this update message.
• Path attributes: List of path attributes related to NLRI. Each path attribute is a triple <attribute type,
attribute length, attribute value> of variable length. BGP uses these attributes to avoid routing loops,
and perform routing and protocol extensions.
• NLRI: Each feasible route is represented as <length, prefix>.
Notification
A notification message is sent when an error is detected. The BGP connection is closed immediately after
sending it.
Figure 74 BGP notification message format
Major fields of the BGP notification message are as follows:
• Error code—Type of notification
• Error subcode—Specific information about the nature of the reported error
• Data—Used to diagnose the reason for the notification. The contents of the data field depend on
the error code and error subcode. Erroneous data can be recorded in the data field. The data field
length is variable.