backup interface command and routing protocols

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Sep 12th, 2008
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Hi all, I have been looking at a document from cisco about the backup interface command for using isdn, as I dont want the protocol keeping the isdn line up. However, the document only has one of the routers using the backup interface command and not both, is this correct? would you not need to configure it on both routers ?

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guruprasadr Fri, 09/12/2008 - 01:18
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HI Carl, [Pls RATE if HELPS]

It's not advisable to configure the Routing Protocol in the Backup ISDN Interface. The Reason, the HELLO Packets are also an Interesting Traffic for the ISDN Circuit and the same will get trigerred often.

So, direct a default towards BRI Interface with an Higher Metric(220) and direct a normal STATIC towards a BRI Interface for creating the Interesting Traffic once the Primary Fails.

Hope I am Informative.


Best Regards,

Guru Prasad R

andrew.butterworth Fri, 09/12/2008 - 01:24
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I would have thought both routers, however if only one end is configured with a number to dial that would work around it.

I think the logic would be the central site has an ISDN interface that is set to allow all IP traffic to be flagged as 'interesting' using the 'dialer-list' command, however it has no phone number configured so can't actually make a call. The remote site however will only take the ISDN interface out of a shutdown state if the main link is down. This remote site then also allows all IP traffic to be interesting but has an ISDN number to call. When the primary circuit is down the remote site will 'un-shut' the ISDN interface, initiate an ISDN call and the routing keepalives/updates will be enough to keep the line active. When the primary link goes back to an Up state the remote site will put the ISDN interface into the shutdown state and the call will be ended.

Without any more information that is all I can think of?



carl_townshend Fri, 09/12/2008 - 02:48
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Hi There, thanks for the input, the scenario is this, I have a main site and a remote office, on each site there are 2 routers, using hsrp, the primary router has the e1, the secondary has the isdn. the primary tracks the e1 interface and if this goes down the secondary should become active. However I want to use ospf but I dont want the isdn to be kept up by the keepalives etc.

can anyone give me a sample of this?

andrew.butterworth Fri, 09/12/2008 - 02:55
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I would use floating statics on the ISDN routers but not enable OSPF over the ISDN link. If these are remote sites then a default route is probably all that is needed as a floating static on the remote ISDN routers. You will need more specific ones at the core site though.


carl_townshend Fri, 09/12/2008 - 03:15
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Can I use ospf and static routes? would I not need to redistribute the static routes at all? or would I just use a higher admin distance than the ones learnt from ospf ?

Giuseppe Larosa Fri, 09/12/2008 - 09:56
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Hello Carl,

some years ago I had the same scenario: the ISDN backup interface on a second router.

there is a DDR feature that is called watch-list that is able to trigger a call if some subnets disappear from the routing table.

I tried it and it works I think you can use EIGRP or OSPF on the main E1 link and on the lan between the two routers.

a link to an example

Hope to help


Joseph W. Doherty Fri, 09/12/2008 - 04:00
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I have a fuzzy recollection that some routing protocols might support suppressing routine router-to-router communications to keep from bringing up on-demand backup links.


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