H3C S7500E Series Operation Manual page 405

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches
Figure 5-3 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message can also carry multiple withdrawn routes in the Withdrawn Routes field.
Unfeasible Routes Length: The total length of the Withdrawn Routes field in bytes.
A value of 0 indicates neither any route is being withdrawn from service, nor
Withdrawn Routes field is present in this Update message.
Withdrawn Routes: This is a variable length field that contains a list of IP prefixes
of routes that are being withdrawn from service.
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, perform routing and protocol extension.
NLRI (Network Layer Reachability Information): Reachability information is
encoded as one or more 2-tuples of the form <length, prefix>.
IV. Notification
A Notification message is sent when an error is detected. The BGP connection is
closed immediately after sending it. Notification message format is shown below:
Figure 5-4 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 upon the Error Code and Error Subcode. Erroneous part of data is
recorded. The Data field length is variable.
5-4
Chapter 5 BGP Configuration