Vlans through GRE tunnel

Unanswered Question
Jul 1st, 2007


This is krishna. We have a peculiar requirement, We have two different locations connected through a T1 link and site A we have a vlan 10 which should be reachable through the T1 at site B. I mean extending a vlan to site B I heard that it is possible through GRE tunnel. Can anyone help me how to configure the cisco routers.



I have this problem too.
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dsweeny Fri, 07/06/2007 - 08:30

I think Vlans through GRE tunnel can be possible. When you configure VLANs on access points, the Native VLAN must be VLAN1. In a single architecture, client traffic received by the access point is tunneled through an IP-GRE tunnel, which is established on the access point's Ethernet interface native VLAN. Because of the IP-GRE tunnel, some users may confgure another switch port as VLAN1. This misconfiguration causes errors on the switch port.


xcz504d1114 Fri, 07/06/2007 - 09:15

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,


xcz504d1114 Fri, 07/06/2007 - 09:34

I completely agree with the L2TPv3 as a viable solution, my only question is, does it apply to the entire trunk or can you single out a specific VLAN to run across the psuedowire?

Our implementation applied to the entire trunk so the following is merely conjecture.

The xconnect statement which creates the psuedowire is applied to the interfaces you'd like to connect, it would seem that you would be able to apply this to a sub-interface on each end of the tunnel connecting that vlan and allowing standard IP routing on all other vlans.

Again I have not tested this but it appears to theoretically solve the problem. If anyone else has more information on this please let me know.

xcz504d1114 Fri, 07/06/2007 - 10:00

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.


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