Hopefully someone has managed to get this scenario to work:
We have an ADSL link to an ISP with a backup ISDN link. What we need to be able to do is get the router to automatically failover to the ISDN when the ADSL link fails, and to fail back when the ADSL becomes active again. We can not use BGP because the ISP does not support it on ADSL.
The problem seems to be that when the ADSL fails, the dialer remains up (same with ISDN).
This question also comes up routinely on news://comp.dcom.sys.cisco. There is no good solution with Cisco routers (which require routing protocols to detect the outage) but there are other products out there such as the Nexland Pro800 Turbo which do the job. They use tricks like pinging the ISP's router to detect loss of DSL link. With a Cisco router, you could take the same approach but it requires another box (think Linux on a cheap PC) to do the pinging and reconfigure the router to force the dial backup.
Well guys, it took a while but I finally managed to get it to work!!
As I mentioned in the first post it was not possible to use BGP or a dynamic routing protocol. Instead I integrated dialer-watch into the ISDN dialer. The dialer-watch examines the routing table and kicks in as soon as the ADSL next-hop address dissapears. Note that the next-hop address only appears in the table when when the ADSL link is actually connected...
When the ADSL link fails it takes about 10 seconds for the route to be removed from the table, and approximately another 20 seconds for the ISDN interface to dial and connect. When the ADSL link comes up again the ISDN dialer remains active for an additional 5 minutes (in case the ADSL fails again) before disconnecting.
Extensive testing in a production environment has been 100% successful.
Hope this helps the other guys who had similar problems. Let me know if you have any comments or questions.
Is there a way to perform the same function without the requirement of the ADSL link failing? What if the ADSL link to the DSLAM stays active, in which case the static route out the interface stays up... Suppose the ATM link on the egress side of the DSLAM fails, your router's ADSL link would remain up, but you would not be able to reach the ISP's router. Your backup would never dial because the route being watched by the dialer watch statement never goes away.
I am trying to perform essentially the same function, but I want to be able to bring up an ISDN link upon Layer3 reachibility failure across a cable modem. I would like to be able to actively ping an IP address on the ISP's network, and dial the ISDN connection if the ping fails.
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