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