[c-nsp] GRE tunnel flapping every 15 minutes

[schilling]

I have an ISR 871 behind Comcast residential cable modem with static
IP address, and have GRE tunnel back to our headend. Just plain
point-to-point GRE tunnel. Now it's flapping every 15 minutes, then
comes back up after 30 seconds. debug ip routing just showed tunnel66
down and refreshed the routing table. There is no route
addition/removal or any interface down before the tunnel66 down.  When
the tunnel is down, I am still able to ping the WAN gateway, but not
the WAN IP on the ISR871.

#remote end
interface Tunnel66
  bandwidth 8000
 ip address 192.168.222 255.255.255.252
 no ip redirects
 ip tcp adjust-mss 1436
 load-interval 30
 keepalive 10 10
 tunnel source FastEthernet4
 tunnel destination 192.168.253.19
 tunnel path-mtu-discovery
end
ip route 192.168.253.19 255.255.255.255 WANgateway
ip route 0.0.0.0 0.0.0.0 WANgateway 250
ip route 0.0.0.0 0.0.0.0 tunnel66

#head end similar.

Just upgraded the ISR871 to be running latest
c870-advipservicesk9-mz.124-24.T4.bin, but the issue persists. We
already tried reboot the modem and reboot the switch.

Any insight?

Thanks,

Schilling