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BPG Common Header
The Version field indicates the BGP version being used by the router sending
the OPEN message. This field allows the BGP speakers to informally nego-
tiate the highest common version numbers that each supports. If a BGP ver-
sion speaker receives a packet indicating a version number of 4, and the BGP
speaker receiving the packet is running a lower version of BGP, then it will
send an error message stating that it does not understand 4 and will termi-
nate the TCP session. The BGPv4 speaker must then reopen the TCP session
using the parameters used in the lower version of BGP.
My Autonomous System
This field indicates the autonomous system number (ASN) membership of
the BGP router sending this OPEN message. Every AS must be identified by
its own unique ASN, which we discussed earlier in the chapter.
Hold Time
This field indicates the amount of time that the sender of the OPEN message
wants to use for its hold-down timer. This hold time indicates the maximum
amount of time that each endpoint will wait for another to send a message
before considering the connection terminated. This means that if an
UPDATE or a KEEPALIVE message is not sent in the indicated amount of
time, the session is considered closed. A value of zero indicates that the
sender does not want to exchange KEEPALIVE messages. This mode is not
recommended since one side will not know if the other has lost communication.
The hold time value is the minimum value set locally on the router or the
advertised hold time value. If the hold time value is not zero, then the hold
time must be at least three seconds. A neighboring endpoint can reject an
OPEN message if the hold time value is unacceptable.
BGP Identifier
This field contains a value that identifies the BGP speaker. This is a random
value chosen by the BGP router when sending an OPEN message. The value
must be unique to all the other BGP speakers communicating with one
another. Although the number can be random, BGP speakers will typically
use the logical IP address assigned to the interface. This number is then used
for every BGP session.
Copyright ©2001 SYBEX , Inc., Alameda, CA