H3C Technologies H3C SR8800 - Manual (Page 212)

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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 Each Update message can also carry multiple withdrawn routes in the
Withdrawn Routes field.
• Unfeasible routes length—The total length of the Withdrawn Routes field in bytes.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indicates no route is withdrawn from service, nor is the Withdrawn Routes field present in this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• Withdrawn routes—This is a variable length field that contains a list of withdrawn IP prefixes.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 Network Layer Reachability Information 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
• 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 part of data is recorded. The Data field length is
variable.
Keepalive
Keepalive messages are sent between peers to maintain connectivity. Its format contains only the
message header.