Operation Manual – Routing ProtocolH3C S5600 Series Ethernet Switches Chapter 5 BGP Configuration5-4Figure 5-3 BGP Update message formatAn Update message can advertise a group of reachable routes with the same pathattribute. These routes are set in the NLRI (network layer reachability information)field. The Path Attributes field carries the attributes of these routes, according towhich BGP chooses routes. An Update message can also carry multiple unreachableroutes. The withdrawn routes are set in the Withdrawn Routes field.The fields of an Update message are described as follows.z Unfeasible Routes Length: Length (in bytes) of the unreachable routes field. Avalue of 0 indicates that there is no Withdrawn Routes filed in the message.z Withdrawn Routes: Unreachable route list.z Total Path Attribute Length: Length (in bytes) of the Path Attributes field. A valueof 0 indicates that there is no Path Attributes filed in the message.z Path Attributes: Attributes list of all the paths related to NLRI. Each path attributeis a TLV (Type-Length-Value) triplet. In BGP, loop avoidance, routing, andprotocol extensions are implemented through these attribute values.z NLRI (Network Layer Reachability Information): Contains the information suchreachable route suffix and the corresponding suffix length.IV. NotificationWhen BGP detects error state, it sends the Notification message to peers and thentear down the BGP connection. Figure 5-4 shows the format of an Notificationmessage.Figure 5-4 BGP Notification message formatThe fields of a Notification message are described as follows.z Error Code: Error code used to identify the error type.z Error Subcode: Error subcode used to identify the detailed information about theerror type.
