You issue the show isdn status command. The device reports that the Layer 1 status is ACTIVE. However, the
Layer 2 status is MULTIPLE_FRAME_ESTABLISHED.
Which of the following statements is true?

A.
Layer 1 and Layer 2 are in their required states.

B.
Neither Layer 1 nor Layer 2 is in its required state.

C.
Only Layer 1 is in its required state.

D.
Only Layer 2 is in its required state.

Explanation:
Both Layer 1 and Layer 2 are in their required states if the output of the show isdn status command reports that
the Layer 1 status is ACTIVE and the Layer 2 status is MULTIPLE_FRAME_ESTABLISHED. The show isdn
status command can be used to verify or detect signaling problems on a primary rate interface (PRI) that is
connected to the public switched telephone network (PSTN).
The Layer 2 MULTIPLE_FRAME_ESTABLISHED state indicates that the Integrated Services Digital Network
(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 isdnq921 command to further troubleshoot Layer 2 issues on a PRI connection to the PSTN.
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.

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