The following commands can be used to verify legacy DDR and ISDN:
ping and telnet These are great IP tools for any network. However, your interesting traffic
must dictate that ping and telnet are acceptable as interesting traffic to bring up a link.
After a link is up, you can ping or telnet to your remote router regardless of your interesting
traffic lists.
show dialer This command gives good diagnostic information about your dialer and shows
the number of times the dialer string has been successfully connected, the idle-timeout values
of each B channel, the length of the call, and the name of the router to which the interface is
connected.
show isdn active This command shows the number called and whether a call is in progress.
show isdn status A good command to use before you try to dial, this shows whether your
SPIDs are valid and whether you are connected and communicating with layers 1 through 3 to
the provider’s switch.
show ip route A popular Cisco diagnostics command, this shows all routes that the router
currently knows about.
debug isdn q921 This command is used to see layer 2 information only between the router
and the service provider’s ISDN switch.
debug isdn q931 This command is like debug isdn q921 but is used to see layer 3 information,
including call setup and teardown between the access server and the provider’s
ISDN switch.
debug dialer This command gives you call setup and teardown activity from the dialer’s
standpoint.
isdn disconnect interface bri0 This clears the interface and drops the current connection if
one exists. Performing a shutdown on the interface can give you the same results.