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I'm doing a graduate degree in Data- and Telecommunication and as part of my last year I'm doing a project within a company.

My company wants me to redesign their inter-datacenter connections, moving them from layer 2 to layer 3 and putting them all in 1 vlan.

Their reasoning for doing this is to make it more manageable, but I'm looking for a more theoretical approach to this.

The question / dl;dr Why is L3 better than L2 for traffic between data centers?

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It does seem a little in conflict to say you want to eliminate a layer 2 connection, but then put everything in one VLAN, by nature if you're separate by a layer 3 link you will have different subnets and in turn different VLANs on each side. – SpacemanSpiff Jan 9 '12 at 16:53

Layer 3 datacenter interconnects FORCE you to make certain compromises. A given subnet can only exist either on one side or the other. When the datacenter interconnect fails, you can fail an entire network over, or not. Now, L2 is simple, but can cause some serious issues when it comes to performance and troubleshooting as you've basically one or more shared broadcast domains between sites.

So in essence, to give you a short answer: Layer 3 DCI's force you to make design compromises that in the long term will probably result in performance and reliability gains, but these architecture decisions can and probably will lead you to having to do more engineering and architecture.

Each of these decisions need to come for a reason and unless you're having a PROBLEM, I would say why change something that's not broken? You need to define your objectives and reasons for them before engineering a best fit for your organization. A small company with relatively few business dependent DR needs can probably use a L2 connection and some trunked VLANs for some nice simplicity. Someone building a multi-server farm of databases or application servers made available to the Internet and in turn used to RUN the business will have higher requirements.

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I recommend Ivan Pepelnjak's blog for further reading on these subjects: – SpacemanSpiff Jan 9 '12 at 16:36

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