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I was wondering if two different OSPF areas connected via an MPLS/GRE tunneled link run OSPF across them? May be keeping the interconnecting MPLS/GRE routers in OSPF backbone area 0?

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  • I know I can use OSPF virtual links which are tunneled to connect an disconnected OSPF area to backbone network. But was wondering about the specific use case where I connect two disconnected networks via a tunnel into a single OSPF area – Subraminion Sep 24 '14 at 0:05
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You can certainly run OSPF on GRE interfaces and routers will exchange routes quite happily via the GRE tunnel. A GRE interface should be just like any other interface to OSPF, so normal rules would apply for area assignments just the same as they would apply to other interface types.

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I think you're looking for the OSPF "Sham-Link" feature. This feature builds virtual OSPF links over an MPLS infrastructure to connect OSPF networks. The feature is available on Cisco and Juniper gear at least.

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Yes - this is commonly done over metro Ethernet circuits via an L2VPN solution (ie pseudo-wire or VLL) or can also be done with L3VPN services from a carrier - but this means that you need to participate in routing with your carrier - more often than not however this will require more complexity in your L3 domain; so rather than doing this, it is possible to run OSPF over a GRE tunnel, as @JeffMcAdams has mentioned. It's usually more desirable to utilize an L2VPN-based solution.

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