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Introduction To ISDN, Part II
Author : Chris Bryant
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 #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com), home of free http://www.thebryantadvantage.com/">CCNA and CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. Video courses and training, binary and subnetting help, and corporate training are also available. Pass the http://www.thebryantadvantage.com/UltimateCCNAStudyPackage.html">CCNA exam with Chris Bryant, CCIE #12933! For a copy of his FREE "How To Pass The CCNA" or "How To Pass The CCNP" ebook, write to chris@thebryantadvantage.com! Spam emails More free articles Related articles
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Part II OF Cisco Certification: Introduction To ISDN
Part II OF Cisco Certification: Introduction To ISDN. 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 ...Part II OF Cisco Certification: Introduction To ISDN
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... [[ This is a content summary only. ... Part II OF Cisco Certification: Introduction To ISDN
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