350-801 · Question #30
Refer the exhibit. Given this "debug isdn q921" output, what is the problem with the PRI?
The correct answer is D. Layer 2 is down on the controller. ISDN Q.921 is the Layer 2 (Data Link Layer) protocol - specifically LAPD (Link Access Procedure on the D-channel) - used on ISDN PRI interfaces for signaling. The 'debug isdn q921' command displays Layer 2 framing activity including SABME, UA, RR, and DISC frames. When this…
Question
Refer the exhibit. Given this "debug isdn q921" output, what is the problem with the PRI?
Exhibit
Options
- ALayer 1 is down on the controller.
- BPRI does not have an IP address configured on the interface.
- CNoting, the PRI is sending keepalives.
- DLayer 2 is down on the controller.
How the community answered
(26 responses)- A4% (1)
- B4% (1)
- C8% (2)
- D85% (22)
Explanation
ISDN Q.921 is the Layer 2 (Data Link Layer) protocol - specifically LAPD (Link Access Procedure on the D-channel) - used on ISDN PRI interfaces for signaling. The 'debug isdn q921' command displays Layer 2 framing activity including SABME, UA, RR, and DISC frames. When this debug output shows no successful Layer 2 establishment (e.g., missing UA responses, continuous SABME retransmissions, or DISC frames), it indicates a Layer 2 failure - the D-channel cannot establish a data link. Layer 1 status is verified with 'debug isdn q931' or 'show controllers', not q921. A PRI does not use IP addressing, so option B is irrelevant. Keepalives would appear as RR (Receive Ready) frames in a healthy q921 output.
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