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…

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.

The `debug isdn q921` Cisco IOS command is specifically used to display messages related to ISDN Layer 2 (Data Link Layer) signaling, also known as LAPD. This command reveals events such as the exchange of Set Asynchronous Balanced Mode Extended (SABME) messages and Unnumbered Acknowledge (UA) frames, which are crucial for establishing and maintaining the Layer 2 D-channel connection on a PRI.

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.

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.