I have a router that has a Tunnel0. This has an IP address visible in show ip interface brief. However, when I issue the command show run interface Tunnel0, it has no configuration at all on the interface. However, when I issue the command show interface Tunnel0, I do see an IP address that has been configured on this Tunnel.
Normally I would expect to see an interface configuration on show run, but I don't see it for this Tunnel0. Yet it has an IP, put there somehow by some previous administrator.
Please explain to me what's going on with this mysterious missing configuration.
the 3825 has been end of sale somewhere around 2010, so I assume you are running an old IOS version. There is a bug where the tunnel interface cannot be deleted to some IPv6 configuration, not sure if you have any IPv6 configured ?
Either way, I would guess the tunnel interface disappears (if that is what you want) when you delete it and then reload the router:
3825(config)#no interface tunnel0