HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 129
Notification, Keepalive, Route-refresh, BGP update message format
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Figure 45 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 Each update message can also carry multiple withdrawn routes in the withdrawn routes field. Major fields of the BGP update message are as follows: • Unfeasible routes length-The total length of the withdrawn routes field in bytes. A value of 0 indicates no route is withdrawn from service, nor is the withdrawn routes field present in this update message. • Withdrawn routes-This is a variable length field that contains a list of withdrawn IP prefixes.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 of variable length. BGP uses these attributes to avoid routing loops, and perform routing and protocol extensions. • NLRI-Each feasible route is represented as . Notification A notification message is sent when an error is detected. The BGP connection is closed immediately after sending it. Figure 46 BGP notification message format 0 7 15 31 Error code Error subCode Data 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. 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 specified address family routing information. 119