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Subject: Failover from VPN
From: "R. Steve McKown" <rsmckown,AT,yahoo,DOT,com>
Date: Mon, 5 Mar 2001 03:49:12 +0100

I'm looking to provide a way to backup a failed CIPE link using dialup 
PPP (or more generally any other network link).  My thoughts on an 
initial solution has limitations I'm not sure are workable with CIPE.  
Would anyone be willing to comment on how to make this solution work, or 
propose an alternative?

My thought is to have the route(s) going through the CIPE tunnel set at 
metric 1.  Another route (or set of routes) to the same destination by 
way of another network link would populate the route tables, but at a 
metric of two or higher.  So, as long as the routes for CIPE were in 
place, the CIPE tunnel would be used.  If those routes were removed, 
packets to the same destination would then begin traversing the backup link.

The problem with this approach is that the presence of the routes 
directing traffic through the CIPE tunnel must coincide precisely with 
the link state of the tunnel (traffic is passing successfully or not).  
I can get part way there with the error settings and ping features.  If 
the tunnel is already up and fails to respond to X pings, then the 
tunnel is closed, the routes would go away, and the backup routes would 
have precedence.

Where I get hung up is starting the tunnel.  The routes appear to get 
created as soon as the CIPE device is initialized and before any traffic 
passes through it.  The failover solution really needs to have CIPE 
bring up the routes through the tunnel _after_ it has verified traffic 
can pass through the tunnel.  That way, packets are routed toward the 
CIPE device only when there is someplace for them to go -- otherwise 
they take the backup path.  This is most obvious when the link is 
failing, most likely due to a network link problem.  One wants CIPE to 
continue to periodically attempt to initialize the tunnel, but not 
override the working backup route(s) until the tunnel has been shown to 
successfully pass traffic.

Any way to make this work?  Are there better and/or workable alternatives?


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