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Trunking over WAN

Hi all,

i'm studying a different solutions for my network topology.i'd like share the same lan in two different sites.

I use a catalyst 3550 and a router cisco 26XM for the WAN,and i create the vlans in the cat and after i make encapsulation 802.1q or ISL on the router.

How can i create a trunking between two router over WAN?

I'm thinking to use the bridge open on serial interface , so i dedicate the link only the transport for multi-vlan.

What do you think????


Roberto Paoletti


Hi roberto,

To the best of my knowledge it is not possible to trunk across a WAN link.

I don't think bridging across the WAN is a good idea unless it is absolutely necessary.

Question: Why would you like to share the same lan in two different sites ?

Yes, as told above its better to have routing across WAN instead of bridging bcoz lot of unwanted traffic can flow over the WAN.Its better you have separate VTP domains for Cat switches at every site. You can use static/ dynamic routing to reach your vlans.


i must share the same lan in two different sites, it's a migration of my central office.

I'm trying use encapsulation and bridging.

I known that the link WAN must be dedicate for transport , but it's not clear if works correctly with tagging.

I try and after i tell you....

Thanks for collaboration


We are going through the same process. One office is moving and we need to keep the same addresses across both sites during the migration.

We moved one of our WAN routers and a core/distribution switch to the new site. An ATM PVC was added between the WAN router at the old and new site. Bridging has been configured over the PVC between spare FE interfaces on the routers. The router FE interfaces connect to switch interfaces configured as 802.1Q trunks. This was done yesterday with testing continueing today. So far so good.

This is not desirable long-term set up but will allow the site move with minimal changes.


I believe the only way you could trunk over a WAN is via ATM Lane. If you try to bridge, I'd imagine you'ld lose all tagging which defeats what you want to do.

Anyway you slice it, you'll want to route.


I assume that you have thought about the Broadcast traffic load that the WAN link will be subjected to if you Bridge the connection? Unless you have purchased a fair amount of bandwidth, this could quickly render the link unusable, and impact significantly on the performance for the users on the LAN's.

Darren Ramsey

We use a box from Tasman Networks to accomplish dot1q across T1 links. It's purpose built to support MLPPP and trunking across the Wan and they are cheap too. However, I have wondered if you could accomplish the same setup on 2 Cisco routers. Setup the T1 and connect your 3550 to E0 on each router. Configure bridge group 1 end to end, and then set the MTU to 1524 and dot1q packets would traverse? Should be easy to test in the lab.

You might also try BCP

BCP Support


The Bridge Control Protocol (BCP) Support feature provides support for BCP to Cisco devices, as described in RFC 3518. The Cisco implementation of BCP is a VLAN infrastructure that does not require the use of subinterfaces to group Ethernet 802.1Q trunks and the corresponding PPP links. This approach enables users to process VLAN encapsulated packets without having to configure subinterfaces for every possible VLAN configuration.