ISDN backup for xDSL line

Posted on 2004-10-20
Medium Priority
Last Modified: 2013-12-14
Currently I have a primary 1M primary DSL link going to internet. I'm using DSL modem and a Cisco 1721 router as my gateway. I have BRI S/T interface and need to use my ISDN line as a backup link.

How do i make my ethernet interface go down when my sdsl link goes down? At the moment at my router i have put 'backup interface dialer 1' on my ethernet interface, however the ethernet wont go down when the sdsl line goes down.

Another method i tried was setting the default route going to internet with the ISDN link having an AD of 2 as shown below;-

ip route BVI1
ip route Dialer1 2

When i disconnected the line, the router wont go through the ISDN even after sometimes. If  I remove the line 'ip route BVI1' then the router goes through the ISDN link. Any way for me to make this automated.

Question by:ason
  • 2
LVL 43

Accepted Solution

JFrederick29 earned 2000 total points
ID: 12368667
This Cisco document explains exactly what you need to do when dealing with an ethernet based connection to your ISP.  The second route is never used because as far as the router is concerned, the ethernet interface connected to the DSL modem is still up/up.  The router will never know a failure exists upstream.  This article explains how to send an ICMP packet to verify the next-hop-address is reachable before using the route.  If the DSL line is down, the router will attempt to ping the next hop via the DSL line, since the line is down, the next hop is unreachable.  The backup line will be verified and if the next hop is reachable, it will be used.


Expert Comment

ID: 12380778

Author Comment

ID: 12407455
Sorry forgot to mention earlier, apparently the SDSL modem used is capable of routing and it is the router next hop before going to the ISP network. So basically the route will never get removed since the link between the router and the modem never goes down. So unless i unplug the ethernet cable between router and modem, the router primary route will still point to the modem.

The modem is provide by the broadband provider, so i have no control over it. Any other method apart from backup interface, floating static routes and dialer watch?

LVL 43

Expert Comment

ID: 12409600
Did you read my post?

Give the link a read, it is exactly what you need to do...

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