From the CCNA to the CCIE, ISDN is one of the most important technolgies you’ll work with. It’s also very common in the field ISDN is frequently used as a backup connection in case an organization’s Frame Relay connections go down. Therefore, it’s important to know ISDN basics not only for your particular exam, but for job success.
ISDN is used between two Cisco routers that have BRI or PRI interfaces. Basically, with ISDN one of the routers places a phone call to the other router. It is vital to understand not only what causes one router to dial another, but what makes the link go down.
Why? Since ISDN is basically a phone call from one router to another, you’re getting billed for that phone call — by the minute. If one of your routers dials another, and never hangs up, the connection can theoretically last for days or weeks. The network manager then receives an astronomical phone bill, which leads to bad things for everyone involved!
Cisco routers use the concept of interesting traffic to decide when one router should call another. By default, there is no interesting traffic, so if you don’t define any,...
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