The debug isdn q921 Command
The debug isdn q921 command output is limited to 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 with an ISDN switch over the D
channel.
Note The ISDN switch provides the network interface defined by Q.921. This debug
command does not display data link layer access procedures taking place within the ISDN
network (that is, procedures taking place on the network side of the ISDN connection).
A router can be the calling or called party of the ISDN Q.921 data link layer access procedures.
If the router is the calling party, the command displays information about an
outgoing call. If the router is the called party, the command displays information about
an incoming call and the keepalives.
You can use the debug isdn q921 command simultaneously with the debug isdn event,
debug isdn q931, debug isdn q921 frame, and debug isdn q921 detail commands.
However, the output from these commands are intermingled.