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New Member

DSL with ISDN backup

Need some help on configuring ISDN backup for a DSL connection.

Cisco 1700 series with an ADSL WIC and a BRI WIC. The DSL is configured and working ok as a Dialer profile connected to the ATM interface. There is a default route to the dialer interface which is NAT'ed.

Any ideas or working configs on how I implement IDSN backup?

Thanks in advance.

9 REPLIES
Cisco Employee

Re: DSL with ISDN backup

Its tricky to design the backup scheme for DSL connection using isdn...Router need to know "some way" to determine that the dsl line went down...Here are the facts in your case.

1)Dialer interface will not go down at all so can't "backup interface" scheme on that interface

2)No routing protocol so can't use "dialer watch" for the routing update.

So, You can only use the following command "backup interface bri0" under the "interface atm0"..Which makes bri available only when the atm interface went down..But that tracks layer 1 issues only, so there are no way to achive a backup based on layer 3 issues over dsl/atm line.

There is a feature request CSCds88143 to address these scenario..Its not solved yet..Talk to your cisco rep.. to push it..

New Member

Re: DSL with ISDN backup

Thank you for the response.

I currently have dialer1 tied to the ATM interface and there are static port address translations based on dialer1. Am I still going to be able to achieve resilience even if it is just for Layer 1 issues? How do I do this roughly?

Thanks again.

New Member

Re: DSL with ISDN backup

Does anyone know on the progress of the feature request CSCd88143.

Also is there a link to view this request, it's not shown on the Bug search tool.

Thanks in advance.

Cisco Employee

Re: DSL with ISDN backup

Its still under review..talk to your cisco rep for more..The bug is a "feature request" so if many customers pushing it, it will be moved towards resolution early.

New Member

Re: DSL with ISDN backup

Anything I can do for now - just Layer 1 resilience?

I currently have dialer1 tied to the ATM interface and there are static port address translations based on dialer1. Am I still going to be able to achieve resilience even if it is just for Layer 1 issues? How do I do this roughly?

Cheers

New Member

Re: DSL with ISDN backup

try giving the following command on the ADSL wic interface

backup interface bri0

(specify the ISDN BRI interface used)

backup interval 10 60

(10 indicates that after 10s the primary links goes down router will dial the backup link and 60s indicates that 60s after the primary link is restored the router shuts the backup interface)

Silver

Re: DSL with ISDN backup

In the meantime, at the risk of getting burned at the stake for heresy, there are low cost boxes from other vendors which will do the job for you (take a look at the Nexland Pro800 Turbo, for example). You could put one of these in front of your Cisco box and let it detect when the DSL link goes down (it can do so by either pinging your ISP's router, or a variety of other ping or pseudo ping targets). You should check deja news before committing to a specific product, as many of these boxes have a tendency to lock-up if you look at them cross-eyed. Early Nexland boxes, for example, must be upgraded unless you intend to reboot every night.

Another approach is to use spare CPU cycles on a server to ping an appropriate target and reconfigure the router to use BRI if the pings started failing. This is common among Linux/Unix shops where the staff are willing to use 'expect' and shell scripts to do all sorts of funky things.

Good luck and have fun!

VIncent C Jones

www.networkingunlimited.com

New Member

Re: DSL with ISDN backup

Burn Him! Burn Him!

One of my colleagues managed to sort it out thankfully. I hadn't considered the fact that we had a static IP so we could use a NAT pool rather than doing NAT on an interface which was casuing the problem.

Silver

Re: DSL with ISDN backup

Ouch, these flames are HOT!!! Meanwhile, be careful!!! While changing the approach to NAT will allow ISDN to take over when it is activated (an option which was not obvious from your original posting, but that is immaterial), you still have the problem of detecting when the ADSL link is down at the link or network layer so that ISDN can be activated.

The interface on the 1720 only monitors the status of the physical link between you and the DSLAM. There are many other modes of failure which will leave that link up but you disconnected. The only way to detect those failures on current Cisco routers is to run a routing protocol across the link. If you have multiple sites, you could run routing protocols between them (using tunnels if necessary) and use that to trigger the dial backup.

Your best bet is to talk your ISP into running a routing protocol with you (it does not need to be BGP). If they are flexible enough to give you the same public IP for both your DSL and ISDN backup, they should be flexible enough to make it work.

Good luck and have fun!

Vincent C Jones

www.networkingunlimited.com

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