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2477 Views 14 Replies Latest reply: Oct 20, 2011 7:21 PM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

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What do OSPF virtual links do?

Oct 12, 2011 8:07 AM

Mike Gannon 483 posts since
Jul 1, 2008


What exactly do OSPF virtual links do?

I've set up a lab on GNS where R4 and R6 have subnets in area 0 (172.16.x.0/24), and area 1 is the backbone across R5 (10.10.10.x/30).


I was suprised to find that R4 still learned about routes on R6 without a virtual link. I thought only area 0 could be used as a transit network?


I then configured a virtual link between R4 and R6 to see what the effect was on the routing table.

I noticed that the only change appeared to be that R4 treated R6's networks as intra area routes (O) instead of inter area routes (O-IA), but there was no effect on the cost of the route (still 21).


So all that virtual links do is allow routers to exchange type 1 LSAs when two routers use the same area ID, but the areas have no direct connection.

The virtual link is not used for routing network traffic. Network traffic is still routed through two hops?

Is that right.

  • cadetalain 2,642 posts since
    Sep 18, 2008
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    1. Oct 12, 2011 8:12 AM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?



    Can you post your GNS3 net file and configs.



  • Currently Being Moderated
    2. Oct 12, 2011 8:43 AM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Virtual links are used to connect non-backbone connected areas to the backbone via a transit area, or as in this case, connect two area 0s that are disconnected.


    I'm not sure what you mean by Area 1 is the backbone though. Please explain further...

  • Rob 423 posts since
    Feb 21, 2011
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    4. Oct 12, 2011 2:45 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Mike, isn't the described behaviour perfectly normal? Your setup may be a bit abnormal, but in effect the two routers in area 0 are intra-area routers - sharing the same area. The fact that they are separated by router 5 is in my opinion irrelevant. I haven't tried this kind of setup myself but believe that having the virtual link in the middle shouldn't change a thing. I presume that you have auto-summary switched off? Otherwise you could have fun seeing the routes between R4-R6.



  • Brian McGahan - 4 x CCIE, CCDE 645 posts since
    May 29, 2008
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    7. Oct 12, 2011 3:00 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Put another router behind R4 or R6 in area 0 and you'll see why it's broken.

  • Currently Being Moderated
    8. Oct 12, 2011 3:22 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Hello Mike,


    The problem lies with backbone area being area 0. Even if you use area 1 to connect the 2 areas, area 1 will not be the backbone. It will be area 1 connected to the backbone through two disconnected ABRs...This is not ideal, and in fact is a serious design problem.


    So what happens if you have no virtual link and add additional routers in area 0 off of R4 and R6? By design, ABRs will not inject a summary LSA into the backbone area for interarea routes. This would be a problem. The other area 0 routers would not know about the routes of the other area 0 routers.


    What if your network was larger than two areas, and connected to additional routers in area 0?

  • wael 1 posts since
    Sep 29, 2011
    Currently Being Moderated
    9. Oct 12, 2011 4:11 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Hi mike


    I agree with Del and Brian  virtual link is used to connect routers from deffernt areas other than the backbone 0 for example if you wanna connect ABR router in area 34 and othe ABR router in area 60 u have to configure virtual link between the Abr and the backbone router that is in are 0 I hope that will answer the Question.

  • James W 23 posts since
    May 15, 2010
    Currently Being Moderated
    10. Oct 12, 2011 7:15 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Area 1 can not be a backbone area. That is reserved for area 0. Every area has to be directly connected to area 0, the backbone area, and every area has to be continious, not segmented. In this example, area 0, the backbone area, is segmented, so you would use virtual links to connect the segmented area 0's together.


    In other examples if you have:


    Area 2 --> Area 1 --> Area 0


    You would use virtual links to connect area 2 directly to area 0.

  • Currently Being Moderated
    12. Oct 19, 2011 7:07 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    In OSPF, everything flows through the backbone area, hence the name backbone. So if an area is separated from this area, the domain will not have all of the topology information. 


    Virtual-links create a tunnel so that neighborships can form between routers, and the topology data can be exchanged appropriately. In OSPF there are two primary scenarios where we use virtual-links:


    1. To connect two disconnected backbone areas.

    2. To connect an area that has no connection to area 0.





  • Efx 629 posts since
    Jun 26, 2010
    Currently Being Moderated
    13. Oct 20, 2011 1:48 AM (in response to DelVonte)
    Re: What do OSPF virtual links do?

    <...1. To connect two disconnected backbone areas...>


    Isn't this situation there showed , in the first post of this thread?

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,398 posts since
    Oct 7, 2008
    Currently Being Moderated
    14. Oct 20, 2011 7:21 PM (in response to Mike Gannon)
    Re: What do OSPF virtual links do?

    Ahhhh...   THIS would explain a strange conversation and whiteboard moment I had with someone last week. 


    I didn't realize there was a question out here...  I guess I've been away too long!


    Anyway, in the diagram you've provided, other than connecting the discontiguous area 0's, your VL wouldn't actually accomplish anything.   Once a route becomes a summary (type 3), there's no area information.  The only thing is that type 3 LSAs don't EXIT regular areas, just the backbone.


    So if you expanded your diagram with other routers and other areas outside of each area 0, THEN you'd see a more noticeable difference in life  with and without the VL there.






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