I've got an 1811 series router (and I have a 1700 series with same config with same issue).

We have a crypto tunnel between two locations.

A <=======> WAN <=======> B

Location A has one subnet. 192.168.77.1

Location B has two. 10.10.1.1 and and externally addressable 66.66.66.0/24.

The VPN endpoint for Location B is 66.66.66.254. I have routes on Router A pointing to the internal subnet at B -- 10.10.1.1, everything works good there. As soon as I add a route on router A pointing traffic destined for 66.66.66.1-128, the tunnel will start dropping. It will stay up for exactly 20 seconds, then go down for exactly 10, and repeat. Removing the route will fix the issue. It does not appear to be a recursive routing issue from the log, since my endpoint is not included in the route I'm adding.

I've got this same setup in about a dozen other installs, all working good. Can provide parts of config if that will help...