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New Member

Same VLANs in two different physical locations -possible ?

I am conecting my two offices with dark fiber. At the same time both sites are connected with MPLS.I want to pass Vlans between these two sites. More prcisely I would like to have for example Vlan 100 in site A and in site B. I configured dark fiber connection as Layer 2. VTP distributed all VLANS accross. When tracing from site A to site B traffic passes through MPLS and not through dark fiber. Right now I am able to route between these two networks if dark fiber is configured as Layer3. Do I need to configure Inter Vlan Routing? If yes, can someone give me an example please. I am attaching two diagrams. First is showing present connection, second is showing desirable connection.

Any help would appreciated.

Thank you


Re: Same VLANs in two different physical locations -possible ?


I don't see why you should not be able to span a vlan across the two sites. That's one of the main reasons that enterprises buy a Metro-E connection. SAN replication, for example.

You can maintain the L3 isolation between your core switches at each site and the 3750 switch stacks. It depends on where you want the L3/L2 boundry to be. If the vlan must be extended to the core, then of course migrate to a L2 connection between core and edge.

As for the "dark fiber" TELCO trunks between the 3750 stacks at each site, typically they are configured as dot1q trunks with trunk negotiation disabled (switchport mode trunk). The carrier will carry all customer vlans in one carrier-defined vlan (QinQ tunneling).

Does this help you?


New Member

Re: Same VLANs in two different physical locations -possible ?

Thank you for reply.

Maybe I didn't explain my problem properly. When I enable Layer 2 between core switches switches (3750G and 4506) than I can distribute all Vlans in both locations. For example: I can see Vlan 100 and 150 in both locations. However, when I trace from Vlan 100 in location B to Vlan 150 in location A than traffic goes through MPLS and not through "dark fiber" This is because in location B 4506 which is a default gateway sends traffic to MPLS. Same in locatin A, 3750 is a default gateway and sends traffic from Vlan 150 to Vlan 100 trough MPLS which is Layer 3. My question is how to enable routing through Layer 2 connection (dar fiber). Do I need to create Vlan interfaces in both locations with IP addresses in the same subnet? For exaplme Vlan Interface 200 with ip in location A and ip in location B.


Re: Same VLANs in two different physical locations -possible ?

Now I understand better.

Can you post the output of a 'sh ip eigrp topology' for vlan 100 and 150 at each site?

And also a 'sh ip route' for those two vlans, too?

And lastly, a 'sh ip eigrp nei' at the core switch at both sites?



Re: Same VLANs in two different physical locations -possible ?

You can't have layer3 definitions for the same subnet in both locations . Say vlan 100 is in site A , create the layer 3svi on "A" . Allow vlan 100 across trunk to "B" . "B" cannot have a layer 3 definition on that side only layer 2 . So anything that needs to be routed for say vlan 100 goes all the way back to site "A" to be routed to any other vlan . If you don't have a layer 3 instance on "B" for vlan 100 it is impossible for it to be routed across the mpls. You have to decide where you want your routing boundaries for any shared vlans.


Re: Same VLANs in two different physical locations -possible ?

"You can't have layer3 definitions for the same subnet in both locations ."

Sure you can. Why not?

New Member

Re: Same VLANs in two different physical locations -possible ?

Going at first glance on this.

You really don't want to do this. Why? Redundancy. I have lived this nightmare before. By spanning the Lan will break any routing you have.

Why? If v100 lives in office A&B and the link between is broken, how will the packets get routed back via your MPLS?

Even if you setup HSRP between the 3750's, you still have a problem where the default gateway is the same for two sites that are not connected.

If you are happy with having no ability to route over your MPLS if the 10G fiber breaks, then proceed.

Another suggestion would be to prioritize each sites HSRP so the traffic isn't being pushed over the 10g. If the the majority of services in OfficeA use V100, make that hsrp priority.

Also, if your MPLS isn't designed to carry the 10g traffic, then the problem is solved, you can Span.

New Member

Re: Same VLANs in two different physical locations -possible ?

Thank you all of you for help. I have some concerns over the issues mentionned in the last post, but I think I will still proceed with Lan span over two sites. Only because I have two completely independent fibers running over different path and then I have stack of two 3750e in both location. Chances that all of these fails is a remote possibility. I will limit number of devices being in the same Vlan in both locations.

Thank you guys. Great community. Appreciate