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Chapter 8
Troubleshooting ISDN
PPP
Troubleshooting targets within the PPP protocol are also important when trying
to isolate and resolve ISDN BRI problems. The targets are the steps in PPP and
CHAP negotiation. Let's look at the steps taken by PPP to establish a link.
1.
LCP at the router (TE) sends a configuration request known as a
CONFREQ. Options are specified by the requesting router.
2.
The request is either accepted or denied. If it is accepted, an acknowl-
edgment, known as a CONFACK, is returned to the TE. If the request
is denied, a negative CONFACK is returned. The difference between
a normal and a negative CONFACK is the acceptance or denial of the
request. If the CONFREQ was not recognized by the remote TE, a
configuration reject message, known as a CONFREJ, is sent to the
requesting TE.
3.
If the CONFREQ was recognized and accepted and CHAP is being
used for authentication, the process continues with the three-way
handshake.
A. Challenge is sent to the remote TE.
B. The remote TE responds.
C. If the values match, authentication is given.
The troubleshooting targets in this process are the request/response
sequence between the peers as well as all of the CHAP targets.
Troubleshooting Layer 3
The third layer of ISDN is addressed in the ITU-T I.451 specification, which
is also called q.931. This protocol includes several message commands,
which are viewed with the debug isdn q931 command. These commands
include call setup, connect, release, cancel, status, disconnect, and
user information
.
The output of the show and debug commands will be covered later in the
chapter. It is important to identify the troubleshooting targets that exist in
Layer 3 for ISDN BRI. It is important to recognize that the Layer 3 connec-
tion is between the local router (TE) and the remote ISDN switch (ET).
Just as the q.931 operates on the D channel, so does all debugging.
Troubleshooting targets include the call reference flag, message types, and
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