How to configure OSPF Virtual Link

Lessons Discussion
104

Hello Sathish

Yes, you’re on the right track. The reason the LSAs learned through the virtual link are still in the LSDB even after removing the virtual link is due to the DNA flag. When an LSA has the DNA flag set, it prevents the LSA from aging out of the LSDB, meaning the LSA stays indefinitely until explicitly removed or until the OSPF process is removed, or the device is reset.

This behavior occurs because OSPF’s DNA flag is typically set for LSAs that originate from interfaces marked as point-to-point or virtual links to ensure they persist as long as possible across the network.

Now depending on the vendor and the implementation, simply clearing the OSPF process doesn’t always clear out LSAs marked with DNA. Clearing the OSPF process typically initiates a soft reset rather than a full teardown of the OSPF database. When you clear the process, OSPF re-establishes neighbor relationships, refreshes LSAs, and recalculates the routing tables, but it doesn’t entirely flush LSAs marked with the DNA flag out of the LSDB.

To remove them completely, a reload or a removal of the OSPF config is necessary.

I hope this has been helpful!

Laz

105

Hi,
I was just trying configuring the VL in my lab and noticed the virtual link didnt come up until I changed the network type between routers to point-to-pint. is there any restriction with other type of links?
Thanks

106

Hello Yasamin

Hmm, that’s interesting. There is no restriction to creating a point-to-point network type in order for a virtual link to come up. However, the network type of the intermediate area, where the virtual link is configured, influences how OSPF adjacencies form and how LSAs are propagated.

Make sure that the network type is the same on both ends of the virtual link in order to make it work. I suggest you check the config on both of those routers, and let us know what you find. You can also share your configs on the two routers becoming OSPF neighbors over the virtual link so that we can help you further…

I hope this has been helpful!

Laz