To earn your Cisco CCNA and CCNP certifications, you've got to master ISDN - and despite what some people say, there's still a lot of ISDN out there that needs to be supported. And when it comes to troubleshooting ISDN, there's a lot to look at. Is the correct ISDN switchtype configured? Are the dialer map statements correct? What about the dialer-group and dialer-list commands? And that's just the start.

I always say that all troubleshooting starts at Layer 1, the Physical layer of the OSI model. The usual method of troubleshooting ISDN is sending pings across the link, but the connection can be tested without using pings or even before assigning IP addresses to the BRI interfaces!

It's a good idea to place these test calls before configuring the interfaces - that way, you know you've got a valid connection before beginning the configuration (and there's a lot of config to go along with ISDN!)

To place a test call without using pings, use the isdn call interface command.

R1#isdn call interface bri0 8358662

R1#

03:54:43: BR0 DDR: Attempting to dial 8358662

03:54:43: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up

03:54:44: BR0:1 DDR: dialer protocol up

03:54:45: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up

03:54:49: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2

To tear the test call down correctly, use isdn disconnect interface. IOS Help displays the options with this command.

R1#isdn disconnect interface bri 0 ?

all Disconnect the data call(s) on all b channels

b1 Disconnect the data call on b1 channel

b2 Disconnect the data call on b2 channel

R1#isdn disconnect interface bri 0 all

03:58:36: BR0:1 DDR: disconnecting call

03:58:36: BR0:2 DDR: disconnecting call

03:58:36: %ISDN-6-DISCONNECT: Interface BRI0:1 disconnected from 8358662 R2, call lasted 20 seconds

03:58:36: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down

03:58:36: BR0:1 DDR: disconnecting call

03:58:37: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to down

I say "correctly" because the one thing you don't want to do to end an ISDN call, test or otherwise, is just shut the interface. Telcos don't like it, and ISDN lab devices like it even less. Always let the d-channel do its work and tear the call down in an orderly fashion - don't just cut it off by shutting the interface down.