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Cisco Certification: Introduction To ISDN, Part I |
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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 ISDN link.
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 Part II of this tutorial, we'll take a look at some common
scenarios that make the ISDN link stay up, and what can be
done about it.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, visit the website and download
your free copies. You can also get FREE CCNA and CCNP exam
questions every day! Pass the CCNA exam with The Bryant Advantage!
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