Home
>
Software
>
Introduction To ISDN, Part II

Introduction To ISDN, Part II

In the previous ISDN article, we looked at how and why one router dials another using ISDN. Just as important is knowing what keeps the link up once it is dialed.

Why? Because ISDN acts as a phone call between two routers, and it???s billed that way to your client. The two routers that are connected by this phone call may be located in different area codes, so now we???re talking about a long distance phone call.

If your ISDN link does not have a reason to disconnect, the connection could theoretically last for days or weeks before someone realizes what???s going on. This is particularly true when the ISDN link is used as a backup for another connection type, as is commonly the case with Frame Relay. When the Frame Relay goes down, the backup ISDN link comes up when the Frame Relay link comes back not billed for all that time.

To understand why an ISDN link stays up when it???s not needed, we have to understand why it stays up period. Cisco???s ISDN interfaces use the idle-timeout to determine when an ISDN link should be torn down. By default, this value is two minutes, and it also uses the concept of interesting traffic.

Once interesting traffic brings the link up, by default all traffic can cross the link. However, only interesting traffic resets the idle-timeout. If no interesting traffic crosses the link for two minutes, the idle-timer hits zero and the link comes down.

If the protocol running over the ISDN link is RIP version 2 or EIGRP, the most efficient way to prevent the routing updates from keeping the line up is expressly prohibiting their multicast routing update address in the access-list that is defining interesting traffic. Do not prevent them from crossing the link entirely, or the protocol obviously won???t work correctly.

With OSPF, Cisco offers the ip ospf demand-circuit interface-level command. The OSPF adjacency will form over the ISDN link, but once formed, the Hello packets will be suppressed. However, the adjacency will not be lost. A check of the adjacency table with show ip ospf adjacency will show the adjacency remains at Full, even though Hellos are no longer being sent across the link. The ISDN link can drop without the adjacency being lost. When the link is needed, the adjacency is still in place and data can be sent without waiting for OSPF to go through the usual steps of forming an adjacency.

This OSPF command is vital for Cisco certification candidates at every level, but is particularly important for CCNA candidates. Learn this command now, get used to the fact that the adjacency stays up even though Hellos are suppressed, and add this valuable command to your Cisco toolkit.

One myth about ISDN is that Cisco Discovery Packets keep an ISDN link up. CDP is a Cisco-proprietary protocol that runs between directly connected Cisco devices. There is a school of thought that CDP packets have to be disabled on a BRI interface in order to prevent the link from staying up or dialing when it's not really needed. I've worked with ISDN for years in the field and in the lab, and I've never seen CDP bring up an ISDN link. Try it yourself the next time you're working on a practice rack!

Keep studying!

Chris Bryant, CCIE (TM) #12933, has been active in the Cisco certification community for years. He worked his way up from the CCNA to the CCIE, and knows what CCNA and CCNP candidates need to know to be effective on the job and in the exam room.

He is the owner of http://www.thebryantadvantage.com, where he teaches CCNA and CCNP courses to small groups of exam candidates, ensuring they each receive the individual attention they deserve. Classes are offered over the Internet and in person in select cities. Chris has custom-written the Study Guide and Lab Workbook used in each course - no third-party training materials or simulators are used.

You're invited to visit our site and check out our CCNA and CCNP courses and study aids, and to sign up for our weekly newsletter written personally by Chris. Chris is always glad to hear from Cisco certification candidates at http://www.thebryantadvantage.com.

reliable maid service Highland Park ..

In The News:

Chrome extension spyware disguised as a free VPN service highlights security risks after it captured private browsing data from trusted sites.

New research shows how fatty acids in cooking oil can safely dissolve and recover silver from circuit boards without harmful chemicals or environmental damage.

The Fox News AI newsletter gives you information on the latest AI technology advancements, and about the challenges and opportunities AI presents now and for the future.

Anthropic investigates alarming AI abuse case where hacker automated entire cybercrime campaign using Claude, stealing sensitive data from defense and healthcare firms.

TikTok, Meta and YouTube restrict Charlie Kirk shooting videos with age gates and warnings while X faces criticism for allowing continued circulation.

Cybercriminals use fake troubleshooting websites to trick Mac users into running terminal commands that install Shamos malware through ClickFix tactics.

San Francisco startup Fable launches Showrunner, an AI platform dubbed the 'Netflix of AI' that generates animated episodes from text descriptions with Amazon support.

Apple raised iPhone prices for some models despite receiving tariff relief from President Donald Trump, with the new lineup starting at $799 for the base model.

A two-story 3D concrete printed home in Western Australia demonstrates faster construction methods that could reshape American housing amid rising costs.

Credit scores remain important during retirement for insurance rates and housing applications, while seniors become prime targets for identity theft and financial scams.

Scammers now send unexpected packages with QR codes that redirect victims to fraudulent websites or download malicious software to steal sensitive information.

Meeting AI tools record private conversations alongside work discussions, creating privacy risks that can be managed with proper settings and awareness.

Hotel privacy concerns are valid but rare, with methods to detect hidden tech using smartphone flashlights, mirror tests and scanning apps.

Improve your Wi-Fi speed and reliability with 10 simple router optimization tips that don't require special apps or expensive subscriptions.

A Columbia University breach exposed names, Social Security numbers and academic records of nearly 869,000 people, with notifications beginning in August.

Rental car drivers use AI-powered apps like Proofr to protect themselves from unfair damage fees as major companies deploy automated inspection tools.

Fox News' AI newsletter brings you the latest on technology advancements around artificial intelligence.

OnTrac data breach between April 13-15, 2025, exposed personal information of over 40,000 people including Social Security numbers and medical records.

A woman named Wika announces her engagement to an AI chatbot sparking worldwide debate about virtual relationships and technology.

The notorious people search site National Public Data relaunches despite a previous breach affecting 3 billion individuals, raising fresh privacy concerns.

Revolutionary TRAUMAGEL gel controls life-threatening bleeding from gunshot wounds and traumatic injuries, helping first responders prevent prehospital deaths.

Protect your home network by enabling proper encryption, creating strong passwords, checking connected devices and using VPN and antivirus software.

The Navy's solar-powered Skydweller drone flew nonstop for 73 hours in Mississippi, proving renewable energy can power long-endurance military missions.

Moving and downsizing expose seniors to identity theft and scams as data brokers collect real estate records and personal information to sell to criminals.

ShengShu's Vidar technology revolutionizes humanoid robot training by using AI-generated synthetic video, reducing required training data from hours to just 20 minutes.

Introduction To ISDN, Part II

Software