H3C Technologies H3C S7500E Series Switches - Manual (Page 207)

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z Unfeasible routes length: The total length of the Withdrawn Routes field in bytes. A value of 0
indicates no route is withdrawn from service, and the Withdrawn Routes field is not present in this
Update message.
z Withdrawn routes: This is a variable length field that contains a list of withdrawn IP prefixes.
z 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.
z Path attributes: List of path attributes related to NLRI. Each path attribute is a triple
attribute length, attribute value> of variable length. BGP uses these attributes to avoid routing
loops, and perform routing and protocol extensions.
z NLRI (Network Layer Reachability Information): Each feasible route is represented as
prefix>.
Notification
A Notification message is sent when an error is detected. The BGP connection is closed immediately
after sending it. The Notification message format is shown below:
Figure 6-4 BGP Notification message format
z Error code: Type of Notification.
z Error subcode: Specific information about the nature of the reported error.
z Data: Used to diagnose the reason for the Notification. The contents of the Data field depend upon
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.
Route-refresh
A Route-refresh message is sent to a peer to request the resending of the specified address family
routing information. Its format is shown below:
Figure 6-5 BGP Route-refresh message format
z AFI: Address family identifier.
z Res: Reserved. Set to 0.
z SAFI: Subsequent Address Family Identifier.