Common ISDN Problems
437
TEI has different message types that allow the engineer to identify what
type of information is being exchanged, thus identifying any failures in the
TEI process. Refer to Table 8.3 for descriptions of these types.
By using these references, you will be able to understand the exchanges
during the TEI process.
Administrators may also need to review the SAPI, or service access point
identifier. This field may include a SAPI of 0, which represents that Layer 3
signaling is present. Such signaling is provided by q.931. Other values may
include 63, which is a management SAPI for the assignment of the TEI
values, or 64, which is used for call control.
One last target to check while troubleshooting the q.921 with the
debug isdn q.921
command is the SABME messages. The SABME is
exchanged along with the ID verify messages. If the SABME fails and
sends a disconnect response, no further link establishment will occur,
and you should investigate the reason for the SABME failure. If the
SABME succeeds, an acknowledgment is sent and the Layer 2 connec-
tion is complete, and the TE will begin to send INFO frames.
Sample outputs for the show interface and debug isdn q921
T A B L E 8 . 3
TEI Message Types
Type Number
Type Description
1
ID Request
2
ID Assigned
3
ID Denied
4
ID Check Request
5
ID Check Response
6
ID Remove
7
ID Verify
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