OSPF Virtual-Link

[network_geek]

Hi All,

I have two questions regarding OSPF virtual-links. Please refer to the attached topology to get a better understanding of what I am trying to emphasize.

Question 1): Is OSPF virtual-link two way communication or one way?

Question 2): Suppose a transit link in the path of virtual-link goes down(link between R9 and R10), how does OSPF virtual-link behave? Will the summary LSA generated ever will get removed from LSDB? How will the routing table be effected?

Thanks in advance.

[Georg Pauwen]

Hello,

I am not really sure what you are after...

Question 1): Is OSPF virtual-link two way communication or one way?

--> What do you mean by one or two way communication ? The virtual link will enable area 100 to be connected to the backbone, any communication is always two way...

Question 2): Suppose a transit link in the path of virtual-link goes down(link between R9 and R10), how does OSPF virtual-link behave? Will the summary LSA generated ever will get removed from LSDB? How will the routing table be effected?

--> If the virtual link goes down, the far end of the VL is not an ABR anymore, and the summary LSA will be removed/not advertised.

[Giuseppe Larosa]

Hello @network_geek ,

>> Question 1): Is OSPF virtual-link two way communication or one way?

An OSPF Virtual Link is NOT a form of tunneling that carries user data,  it exists only in the control plane / routing plane and it can be seen as point to point unnumbered link that allows to reach area 0 via a STANDARD area making possible the exchange of LSA data structures.

>> Question 2): Suppose a transit link in the path of virtual-link goes down(link between R9 and R10), how does OSPF virtual-link behave? Will the summary LSA generated ever will get removed from LSDB? How will the routing table be effected?

the OSPF virtual link exists until there is an alternate path between OSPF RIDs of the two devices where you have configured it.

In the case of your topology if the link between R9 and R10 fails there is no alternate path within area 10 that interconnects the two routers the ABR( 0, 10 )  and the "ABR" between area 10  "ABR" (10,100) and area 100.

Final note: virtual links are not used in production networks, there are other methods that can be used they are a lab tricky subject.

If you make tests without advertising in OSPF the OSPF RIDs the Virtual link is able to go up.

I had found strange this fact and I asked this in a Ask the Expert session to Vinit Jain a Cisco SME expert and he answered back that the RID even if not advertised can be used as key to seach in OSPF DB the two router LSAs and the nearest interfaces active in OSPF are used to build the control plane only virtual link communication.

Hope to help

Giuseppe

[network_geek]

Hi @Giuseppe Larosa,

What amazes me about OSPF virtual-link is that the LSA does not get deleted. I see an ICMP unreachable message between tunnel source and destination and nothing else. After that the route gets deleted from RIB but the LSA is still there. Do you know what mechanism might be causing this?

Lastly, can we agree that OSPF virtual-link is a one-way communication where both sides after sharing their LSAs with DNA(Do Not Age) Flag set never care about each other?