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[wlug] ADSL failover

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Oliver Jones oliver@d...
Thu Feb 13 13:43:41 NZDT 2003

Actually you should probably not be checking a host so far into the

If you add a route to that forces you to use the
Tempest connection to connect to it you won't be able to contact via the Telecom ADSL link if the tempest one goes
down.  Unless of course you take out the route when you switch to the
Telecom ADSL link.  But then you wouldn't be able to detect if the
tempest came back up again...

A better idea is to find out what your Tempest ADSL pear IP is (the
DSLAM's router IP).  And force a route to that and ping that IP. 
Getting this IP is easy if you use PPPoE.  In our case our Telecom pear
IP is displayed with ifconfig:

ppp0      Link encap:Point-to-Point Protocol  
          inet addr:  P-t-P:  Mask:255....


> One cavaet is that the IP you are checking for the tempest connection 
> *must* have a route out the tempest link. Otherwise when you drop the 
> default gw, you find you can ping the ip again (through the backup) and 
> you bring it back up again... route flap!
> Eg, add a route to via the tempest gw, and ping that. 
> It'll keep pinging out over the tempest link. If the link goes down, 
> it'll still try to ping out over the tempest link, and so when it starts 
> working again your link is fine
> >Would a ping test fired by something like Nagios be enough for this task?
> >The modems support RIP, but I'm not sure what I'd need to do with the Linux
> >machine to use it, and if it's overkill?  Am I missing anything?
> >  

Oliver Jones > Senior Software Engineer > Deeper Design Limited. 
oliver@d...  > > +64 (21) 41-2238

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