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multiple backup links

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Jul 25th, 2002
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I have 2 routers (A and B) on a LAN, both connected with a third router (C) by an ISDN link. Between A and C a serial link is also present.

I configured OSPF demand circuit feature, giving different costs to the ISDN link A-C (100) and B-C (50).

I saw if the serial link goes down C try to bring up the ISDN link with B (it has a lower cost), but if for some reason it can't establish the connection, C isn't so smart to erase B from OSPF neighbors, stop calling B and proceed calling A.

Have I to give the "dialer wait-for-carrier-time" command?

Thanks to all.


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vcjones Mon, 07/29/2002 - 06:23
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Your challenge is that dialer links are never "down," just "up" or "spoofed." So your basic approach is not going to work as the lower cost dial link will always be attempted in preference to the higher cost link.

If you don't otherwise need OSPF, the easiest solution is to use EIGRP and route advertisement filters to set up a hierarchy of dialer watch or DDR paths. This approach is discussed in chapter 5 of my book High Availability Networking with Cisco, configuration examples from the book are at http://www.networkingunlimited.com/bookpages.html.

You actually face two independent challenges: getting the second backup link up if the first won't come up, and migrating from the second to the first once the first becomes available (which includes discovering that the first has become available while using the second). Depending upon your specific needs and cost structures, you may find that you only really care about solving the former, in which case you could simply define both lines as alternate numbers for the same link.

Good luck and have fun!

Vincent C Jones

atassone Mon, 07/29/2002 - 07:39
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Thank you for your answers...

Unfortunately I do have to use OSPF... I tried di put 2 dialer string in a dialer profile, but the second number is never tried if the first call don't succeed.

At last I used a dialer watch, and legacy DDR on the physical interface (BRI).

For me it's unthinkable an OSPF neighbor will never go down if the dialer interface makes 100 calls and don't come up...




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