Bridging the same subnet over a WAN link

Unanswered Question
Mar 19th, 2008

We have the following:

Two sites connected over MPLS backbone.

We need to configure one subnet in both locations and have this subnet spanned across the WAN.

The reason for this is we have a security system that needs to monitor both sites using a private IP subnet. the security servers can be connected to 3750 switches in both locations.

Any simple solutions?



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Jon Marshall Wed, 03/19/2008 - 05:57


You could talk to your Service provider and see if they support virtual pseudowires which is a L2 P2P link over a L3 MPLS network. Attached is a link to some info on MPLS L2 VPN's.

Alternatively you can use L2TPv3 and configure this yourself.



ziutek Wed, 03/19/2008 - 06:04

We do not want to modfy our MPLS connections.

Can L2TP be runn between 2 L3 ports on the 3750s?

Thanks for the advice.


Paolo Bevilacqua Wed, 03/19/2008 - 07:26

I suppose the "security server" insists on having the systems on a directly connected lan and would not work otherwise ?

I so there is a simple way to fool it while keeping a regular routed network.

Suppose the two locations are numbered and 10.0.34/24.

On the picky server configure and address that falls in local network, but netmask /16. It will belive all the systems are local. The router will answer to requests by proxy-arp and it thing should work normally.

Jon Marshall Wed, 03/19/2008 - 08:05


I don't believe L2TPv3 is supported on 3750 switches. So unless you talk to your SP you are limited in what you can do.


ziutek Wed, 03/19/2008 - 08:15

What about ethernet subinterfaces? The L3s are each behind a 2811. Could I run L2TP between the 2 2811s, with the end points on ethernet subinterfaces?

Jon Marshall Wed, 03/19/2008 - 08:16

Yes you could as long as the network you are trying to bridge terminates on the subinterface on each respective 2811.


wilson_1234_2 Wed, 03/19/2008 - 16:00

Just a curiosity question:

Do bridged connections no work across mpls?

Only point-to-point?

We have a site bridged across a point-to-point that is in the same subnet on the remote side as the local side.

Jon Marshall Wed, 03/19/2008 - 16:07


MPLS supports 2 types of L2 bridging

1) Virtual pseudowire - a P2P layer link running over a L3 MPLS network

2) VPLS - a many to many L2 network ie just like ethernet extending across a L3 MPLS network.

So you can bridge vlans across MPLS. These 2 are provided by the service provider of the MPLS cloud.

You can also use L2TPv3 where you can bridge a vlan between 2 of your routers on at either end. The service provider is not needed here.

Hope this makes sense



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