350-801 Question #527: Real Exam Question with Answer & Explanation

Question

A collaboration engineer is troubleshooting issues with a T1 PRI. The engineer wants to confirm if the router received an ISDN Set Asynchronous Balanced Mode Extended message and responded with an Unnumbered Acknowledge frame. Which Cisco IOS command is used to debug ISDN Layer 2 issues?

Options

• Adebug voice q921
• Bdebug isdn q921
• Cdebug isdn q931
• Ddebug voip ccapi inout

Explanation

To debug ISDN Layer 2 issues on a T1 PRI, specifically the exchange of SABME and UA frames, the debug isdn q921 Cisco IOS command is used. This command provides visibility into the Data Link Layer D-channel signaling.

Common mistakes.

• A. While q921 is the correct protocol layer, the common and standard Cisco IOS command syntax is debug isdn q921, making this option less precise or potentially invalid in some IOS versions compared to B.
• C. The debug isdn q931 command focuses on ISDN Layer 3 (Network Layer) call setup and teardown messages, not the Layer 2 messages like SABME and UA that establish the D-channel.
• D. The debug voip ccapi inout command debugs the Cisco Call Application Programming Interface (CCAPI) and is used for high-level VoIP call control troubleshooting, not for low-level ISDN D-channel Layer 2 events.

Concept tested. Cisco IOS ISDN Q.921 Debugging

Reference. https://www.cisco.com/c/en/us/support/docs/voice/h323/15479-debug-pri.html

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