You are troubleshooting a PRI connection to the PSTN.
Which of the following state messages most likely indicates a Layer 1 problem?

A.
ACTIVE

B.
DEACTIVATED

C.
TEI_ASSIGNED

D.
MULTIPLE_FRAME_ESTABLISHED

Explanation:
Of the available choices, a state of DEACTIVATED is most likely to indicate a Layer 1 problem with an
Integrated Services Digital Network (ISDN) primary rate interface (PRI) connection to the public switched
telephone network (PSTN). The show isdn status command can be issued to verify or detect signaling
problems on a PRI.
Unless the physical line or interface connecting the PRI to the PSTN is down, the show isdn status command
should always report a Layer 1 status of ACTIVE. If the show isdn status command reports a Layer 1 status of
DEACTIVATED, you should verify that the no shutdown command has been issued on the interface and issue
the show controllers command to verify that the connection is running properly.
The Layer 2 MULTIPLE_FRAME_ESTABLISHED state indicates that the ISDN router is communicating with
the telephone company’s switch. In order to establish Layer 2 connectivity, the ISDN router must first receive an
ISDN Set Asynchronous Balanced Mode Extended (SABME) message and respond with an Unnumbered
Acknowledge (UA) frame. After the frame is sent and the router is synchronized with the switch, Layer 2 frames
are constantly exchanged between the ISDN router and the switch.
If the Layer 2 state is TEI_ASSIGNED when the Layer 1 state is ACTIVE, it is probable that the ISDN router has
not been able to exchange Layer 2 frames with the telephone company’s switch. You can issue the debug isdn
q921 command to further troubleshoot Layer 2 issues on a PRI connection to the PSTN.

Cisco: T1 PRI Troubleshooting: Use the show isdn status Command