I'm not quite sure I follow your question. You have a point to point connection between 2 routers, it is a T1 link, and you want to create a GRE tunnel to pass a single VLAN across this tunnel?

I'm assuming there is some importance to maintaining a single VLAN in this fashion? I'm also assuming there are other VLANs that connect to the trunk port of your router?

I'm not entirely sure that it is possible. For routing to take place you have to route between 2 different networks, and each interface on a router has to have a different network address. The way I am thinking of this happening would not work.

Here is why, the router would have 2 paths to get to the same network, a directly connected interface (which would be your trunk port) and the GRE tunnel (your external link). The directly connected link would be preferred for all traffic destined to VLAN 10 and would never traverse the GRE tunnel, andything that would travers the GRE tunnel would create a routing loop.

Now if you are attempting to have 1 VLAN with multiple subnets, that isn't a very good practice. General rule of thumb and best practice described by Cisco is 1 subnet per vlan and 1 vlan per subnet.

Is GRE Tunneling a single VLAN the only plausible scenario?

Examples where GRE tunnels are used are for multicasting acrossed other devices that might not support multicast. Such as a router connected to an encryption device, if the encryption device does not support multicast (for routing protocols like OSPF) the only way to pass your OSPF information ot the router on the distant end is by using a GRE tunnel.

Make sure you set the MTU and MSS to compensate for the additional GRE overhead on your packet and set the dont fragment bit, otherwise you will have packet loss due to runts and giants.

Hope this helps,

Craig

I've had a similar requirement in the past, where layer 2 connectivity was required between devices in differing locations. Our solution was to build a L2TPv3 pseudowire between router LAN interfaces; to the switches at each site it appeared that they were directly connected.

A configuration guide can be found here: http://cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00804420e7.html

Hope this helps.

One last thought, you have a T1 link, I don't know what else shares the bandwidth of this link, and I also don't know the amount of traffic VLAN 10, but consider the added bandwidth caused by this setup.

CDP traffic every 60 seconds

BPDU packets every 2 seconds

STP packets

All of that can be pruned manually though

But any packets where the destination is unknown to the switch will be broadcasted across that link.

Just a word of caution.