From CCIE Study Wiki

OSPF Virtual Links are used to provide virtual OSPF connectivity between routers when the direct physical connectivity does not exist.

Theory

OSPF virtual links allow a pair of routers to tunnel OSPF packets inside IP packets to some other router that is not on the same data link.

OSPF virtual links can help overcome problems in a couple of situations:

1. OSPF requires that each non-backbone area be connected to the backbone area, and an area does not have a direct connection
2. OSPF requires that each router in an area have a path to every other router in the area (without that LSA flooding would fail) - virtual links can help overcome that.

Optionally, you can set up the virtual link with it's own unique authentication and timing settings that would be typical to a normal link.

The IOS treats virtual-links as unnumbered point-to-point links.

Commands

• area area-id virtual-link router-id OPTIONS (router) - Area-id is the transit area and route-id is the router-id of the destination router.

Default Settings

• No virtual links ever set up by default.

• When using virtual-links the "transmit capability" is on by default.

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• A virtual link cannot pass through a transit area that is stubby
• Carl over at the CCIE Candidate Site has a posting describing the fix for a "recursive routing" error he was getting when using a GRE tunnel with a OSPF virtual link.
• An example of an OSPF filtering issue when using virtual links from CCIE to Be.

Online Resources

• An overview of OSPF virtual-links from Cisco Network Engineer.

• CCIE To Be investigates the OSPF virtual-link transmit capability.

• Another overview of virtual-links, this time from Fabio of Ciscozine.

• Networkers Online compares OSPF virtual links and GRE tunnels.

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