Cisco CCNA Certification: Introduction to ISDN, Part I
by: Chris Bryant, CCIE #12933
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, the routers will never call each other.
Interesting traffic is defined with the dialer-list command. This command
offers many options, so you can tie interesting traffic down not only to what
protocols can bring the link up, but what the source, destination, or even port
number must be for the line to come up.
One common misconception occurs once that link is up. Interesting traffic is
required to bring the link up, but by default, any traffic can then cross the
What makes the link come down? Again, the concept of interesting traffic is
used. Cisco routers have an idle-timeout setting for their dialup interfaces.
If interesting traffic does not cross the link for the amount of time specified
by the idle-timeout, the link comes down.
To summarize: Interesting traffic brings the link up; by default, any
traffic can cross the link once it's up; a lack of interesting traffic is what
brings the link down.
In tomorrow's article, we'll take a look at some common scenarios that make
the ISDN link stay up, and what can be done about it. Keep studying, and I'll
see you tomorrow!
Cisco Certified Internetwork Expert #12933
About The Author
Chris Bryant, CCIE (TM) #12933, has been active in the Cisco
certification community for years. He has written several books that
have helped CCNA candidates around the world achieve the coveted CCNA
certification, including several concentrating on binary math
conversions and subnetting questions that the average CCNA candidate
will need to answer on their CCNA exams.
He is the owner of The Bryant Advantage (http://www.thebryantadvantage.com)
where he teaches affordable world-class CCNA courses via the Internet.
He’s proud to have helped CCNA candidates around the world achieve
their career goals. Mr. Bryant’s books and courses are sold on his
site, on eBay, and on several other major Cisco certification sites.