5.7 Monitoring the ISDN Interface
5.7.5 ISDN debug commands

The following are debug commands that are helpful when troubleshooting ISDN. Two other ISDN debug commands are debug isdn q921 and debug isdn q931.

Note: Click on the topology to view the command outputs.

ISDN Q921
As shown in the Figure , you may use the
debug isdn q921 EXEC command to display data link layer (Layer 2) access procedures that are taking place at the access router on the D channel (LAPD) of its ISDN interface. This command is useful when you want to observe signaling events between the access router and the ISDN switch. The ISDN data link layer interface, provided by the access router, conforms to the user interface specification defined by ITU-T recommendation Q.921.

The debug isdn q921 command output is limited to the commands and responses exchanged during peer-to-peer communication carried over the D channel. This debug information does not include data transmitted over the B channels that are also part of the router ISDN interface. The peers (data link layer entities and layer management entities on the routers) communicate with each other via an ISDN switch over the D channel.

This command can be used with the debug isdn event and the debug isdn q931 commands at the same time. The displays will be intermingled.

ISDN Q931
You may use the
debug isdn q931 EXEC command to display information about call setup and teardown of ISDN network connections (Layer 3) between the local router (user side) and the network. The ISDN network layer interface provided by the access router conforms to the user interface specification defined by ITU-T recommendation Q.931, which is supplemented by other specifications, such as those for switch types VN2 and VN3. The router tracks only activities that occur on the user side (not the network side) of the network connection.

The display information of the debug isdn q931 command output is limited to the commands and responses exchanged during peer-to-peer communication carried over the D channel. This debug information does not include data transmitted over the B channels. The peers (network layers) communicate with each other via an ISDN switch over the D channel.

You may also use this command with the debug isdn event and the debug isdn q921 commands at the same time, but the display will be intermingled.

Other ISDN debugging commands available are debug isdn active (to show active calls) and debug isdn history (to see previous activities).

Figure presents a graphical representation of the different debug commands and their relation to the Open System Interconnection (OSI) model.