Just as important is knowing what keeps the link up once it is dialed.This OSPF command is vital for Cisco certification candidates at every level, but is particularly important for CCNA candidates. Try it yourself the next time you're working on a practice rack!Keep studying!Chris BryantCisco Certified Internetwork Expert Terminal Blocks12933. 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. However, the adjacency will not be 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. 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. 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.Once interesting traffic brings the link up, by default all traffic can cross the link. Do not prevent them from crossing the link entirely, or the protocol obviously won't work correctly. One myth about ISDN is that Cisco Discovery Packets keep an ISDN link up.Why? Because ISDN acts as a phone call between two routers, and it's billed that way to your client.

The ISDN link can drop without the adjacency being lost. By default, this value is two minutes, and it also uses the concept of interesting traffic. The OSPF adjacency will form over the ISDN link, but once formed, the Hello packets will be suppressed. However, only interesting traffic resets the idle-timeout. Cisco's ISDN interfaces use the idle-timeout to determine when an ISDN link should be torn down. 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. 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.

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. 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.To understand why an ISDN link stays up when it's not needed, we have to understand why it stays up period.In the previous ISDN article, we looked at how and why one router dials another using ISDN.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. CDP is a Cisco-proprietary protocol that runs between directly connected Cisco devices. If no interesting traffic crosses the link for two minutes, the idle-timer hits zero and the link comes down. With OSPF, Cisco offers the ip ospf demand-circuit interface-level command. 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

Megosztás a facebookon