Huawei Quidway S5600 Series Operation Manual page 349

Operation Manual – Routing Protocol
Quidway S5600 Series Ethernet Switches-Release 1510
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 The withdrawn routes are set in
the Withdrawn Routes field.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.
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value of 0 indicates that there is no Withdrawn Routes filed in the message.
Withdrawn Routes: Unreachable route list.
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.
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.
NLRI (Network Layer Reachability Information): Contains the information such
reachable route suffix and the corresponding suffix length.
IV. Notification
When BGP detects error state, it sends the Notification message to peers and then tear
down the BGP connection. Figure 5-4 shows the format of an Notification message.
0
Error Code
Figure 5-4 BGP Notification message format
The fields of a Notification message are described as follows.
Error Code: Error code used to identify the error type.
Error Subcode: Error subcode used to identify the detailed information about the
error type.
Data: Used to further determine the cause of errors. Its content is the error data
which depends on the specific error code and error subcode. Its length is unfixed.
V. Keepalive
In BGP, Keepalive message keeps BGP connection alive and is exchanged periodically.
A BGP Keepalive message only contains the packet header. No additional fields is
carried.
VI. Route-refresh
Route-refresh message is used to notify the peers that the route refresh function is
available.
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Error Subcode
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Chapter 5 BGP Configuration
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