H3C S7500 Series Ethernet Switches Operation Manual

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches Chapter 6 BGP Configuration
6-4
01531
Total Path Attribute Length
Withdrawn Routes(Variable)
Path Attributes(Variable)
NLRI(Variable)
Unfeasible Routes Length
Figure 6-3 BGP Update message format
An Update message can advertise a group of reachable routes with the same path
attribute. These routes are set in the NLRI field. The Path Attributes field carries the
attributes of these routes, according to which BGP chooses routes. An Update
message can also carry multiple unreachable routes. The withdrawn routes are set in
the Withdrawn Routes field.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. A
value of 0 indicates that there is no Withdrawn Routes filed in the message.
z Withdrawn Routes: Unreachable route list.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 value
of 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 attribute
is a TLV (Type-Length-Value) triplet. In BGP, loop avoidance, routing, and
protocol extensions are implemented through these attribute values.
z NLRI (Network Layer Reachability Information): Contains the information such
reachable route suffix and the corresponding suffix length.
IV. Notification
When BGP detects an error state, it sends a Notification message to peers and then
tears down the BGP connection.
Figure 6-4 shows the format of a Notification
message.
07 15 31
Data
Error Code Error SubCode
Figure 6-4 BGP Notification message format
The 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 the
error type.