3Com Switch 7754, Configuration Manual

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352 C
HAPTER
36: BGP C
ONFIGURATION
Notification
When BGP detects error state, it sends the Notification message to peers and then
tear down the BGP connection. Figure 83 shows the format of an Notification
message.
Figure 83 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.
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.
Route-refresh
Route-refresh message is used to notify the peers that the route refresh function is
available.
BGP Routing Mechanism When BGP initially starts on a router, it sends the whole BGP routing table to its
peers to exchange routing information. Afterwards, BGP sends only Update
messages instead of the whole table. During the running, BGP also sends/receives
Keepalive messages to determine whether the connections to its peers are normal.
A router running BGP is also called a BGP speaker because it can send BGP
messages. A BGP speaker can receive routing information as well as generate and
advertise routing information to other BGP speakers. When a BGP speaker receives
a route from another AS and finds this is a new route (a route it does not know) or
a route superior than any of its known routes, the BGP speaker advertises the
route to all other BGP speakers in the AS.
Two BGP speakers capable of exchanging BGP messages with each other are peers
of each other. Multiple BGP peers can form one peer group.
BGP route advertisement policies
In the implementation on the Switch 7750, BGP adopts the following policies to
advertise routes:
■
When there are multiple optional routes, a BGP speaker chooses only the
optimal one;
■
A BGP speaker advertises only the local routes to its peers;
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Data
Error Code Error SubCode