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Can I use layer 2 and layer 3 over a fibre link?

Posted on 2009-07-03
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Last Modified: 2012-05-07
We have multiple sites that are connected by our own in-ground fiber.  We are setting up VMware Site Recovery Manager which allows us to redeploy our main location VMs in our Disaster Recovery site in the event of a disaster.  Optimally, we'd like to have these redeployed servers have the same IP addresses as they normally do, which requires the VLANs and subnets be available at the DR site.  In order to do this, I realize I need layer 2 trunking on our fiber.

Right now, we just have layer 3 IP routing which prevents this.  Is there a way to implement layer 2 (dot1q) trunking in addition to IP routing so that I don't have to change how things work now but have the extra ability to have this one subnet available in our DR location?

We have all Cisco switches, 3750 and 3550 models.  Our main site (site 1) connects through fiber to an intermediate site (site 2) and then another fiber comes out from there to our DR site (site 3).
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Question by:Phaelium
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Author Comment

by:Phaelium
ID: 24774316
Upped points
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Expert Comment

by:Don Johnston
ID: 24774492
You can't have an interface with an IP address and trunking. They're mutually exclusive.
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Author Comment

by:Phaelium
ID: 24774638
That's what I thought, but what about these links:

Question:
http://newsgroups.derkeiler.com/Archive/Comp/comp.dcom.sys.cisco/2007-09/msg00308.html

Answer:
http://newsgroups.derkeiler.com/Archive/Comp/comp.dcom.sys.cisco/2007-09/msg00310.html


Maybe I'm reading it wrong, but it looks like he's trying to do a similar thing?  
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Expert Comment

by:lrmoore
ID: 24774674
The simple answer is that direct fiber connection allows you complete L2 connectivity to do exactly what you want. The fact that is it L3 today simply means that you need to plan very carefully how to change it to L2.
Generally speaking, L3 in any flavor prevents the same IP subnet on both sides of the L3 link no matter what you do.
You might be able to get creative with L2 VRF tunnels, but I would try to keep it as simple as possible at L2.
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Don Johnston earned 100 total points
ID: 24774681
Yep, you're reading it wrong. :-)

The link would be a layer 2 link. But the switch would have a layer 3 interface to that VLAN.

Do the two sites need to be on the same IP network? Do they need to be on different IP networks? Are there any other networks that you need connectivity with?

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Assisted Solution

by:lrmoore
lrmoore earned 100 total points
ID: 24774820
Maybe. You can setup L3 interface on 3750/3550 without having it in a vlan, just like a router.
interface Fast 1/0/18
 no switch
 ip add 1.23.4.5 255.255.255.252

No l3 vlan interface.
>have the extra ability to have this one subnet available in our DR location
I read this to mean yes, they need the same IP address space on both sides of the fiber.

Trunking between sites, 1 vlan at L2 with same subnet on both sides, another vlan with l3 interface between them.
done.
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Author Closing Comment

by:Phaelium
ID: 31599613
Thanks guys, I think we're going to use a separate subnet just so I don't mess with an already working configuration.  It may cause a few more headaches but I think we can work through it.
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