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Home Forums How to configure OSPF Virtual Link

This topic contains 41 replies, has 13 voices, and was last updated by   Andrew P 4 days, 17 hours ago.

Viewing 15 posts - 1 through 15 (of 41 total)
  • Author
    Posts
  • #14325

    Vinod Agrahari

    Rene ! this is really grt !

    #14326

    Celso

    Hi, thank you very much for this very easy to understand explanation. Great job rene.

    #14327

    Rene Molenaar
    Keymaster

    Glad to hear you like it!

    #14328

    Jurati Fundikira

    Is it possible to build a virtual link over frame-relay link ? for example in the diagram above if the router in between was a frame-relay switch

    #14329

    Rene Molenaar
    Keymaster

    Hi Jurati,

    Yes this is possible, when your OSPF neighbor adjacency is working over the frame relay link you should be able to configure a virtual link.

    Rene

    #14330

    Nitin

    Hi Rene , according to OSPF ” RID is highest IP of loopback interface or software interfaces which are active when OSPF routers start up ,in absence of software/loopback interfaces RID is the highest IP of the active interfaces when OSPF starts ”

    in this example RID for router Nancy is 1.1.1.1 since it’s the loopback interface for that area

    How come RID for router Susan is 192.168.23.3 , for router Susan It’s RID should be of it’s loopback interface 3.3.3.1

    here why does router selects RID as 192168.23.3 since loopback interface is also present in that area .

    #14331

    Rene Molenaar
    Keymaster

    Hi Nitin,

    What you say is correct. The thing with the OSPF router ID however is that it’s non preemptive which means that once it has selected an ID, it will not change it unless you reboot the router or reset the OSPF process. In this example, I probably configure OSPF before adding some of the loopback interfaces. Hope that helps.

    Rene

    #14332

    Nicholas Russo
    Member

    Another bit of information: OSPF VLs cannot be constructed if the interface from which the VL would recurse, on either end, is unnumbered. In the first example the Nancy Fa0/0 and Donna Fa0/0 would have to have real IP addresses on them. However, OSPF VLs can transit unnumbered interfaces if those links are in the middle of the area. If area 1 were much larger, and some of the intermediary links were unnumbered, that would work fine. Reason is because although OSPF VLs behave like demand circuits, initially they send unicast hellos between exit interfaces and this must have a real IP address.

    #14333

    Chandan K
    Participant

    Hi Rene,

    I configured Discontiguous area 0 for virtual links,by default if we enable loopback interface on the router,it takes loop back interface., i.e 3.3.3.1 as the router ID and one more thing with lab,i get error message like mismatch in the area ID,before I configure a virtual link between router Nancy and Susan.

    This lab dint work the way its show in the figure.Need your help in this rene.

    #14334

    Apostolis

    Hi Rene
    Is it possible for VL’s to be used over different AS? In your example, instead of OSPF area 1, could there be a network running i.e. EIGRP (Nancy and Donna would be ASBR’s), so OSPF area 2 would be directly connected to OSPF area 0 passing through a different AS? If for any reason areas 0 and 2 should belong to the same AS (let’s say after consolidation of two companies, one running only OSPF and the other running both EIGRP and OSPF), could we use VL’s instead of redistribution?

    #14335

    Rene Molenaar
    Keymaster

    Hi Apostolis,

    The virtual link is a OSPF-only feature. Also keep in mind that it doesn’t work with AS numbers, only EIGRP (and BGP) work with AS numbers.

    You could make it work with a GRE tunnel though. If you run EIGRP on the 192.168.12.0 /24 instead of OSPF then you can create a GRE tunnel between Nancy / Donna and connect the two distance OSPF areas with that.

    Rene

    #14336

    J.A.III

    Is possible to use virtual links within the same area?

    #14337

    Rene Molenaar
    Keymaster

    You probably could configure a virtual link between two routers in the same area but there’s no point in doing so 🙂

    #14338

    Lee

    Rene

    Is it possible for a virtual link to transit two areas?

    For instance, there are areas 0, 1, 2, and 3.

    How to create a virtual link that connects area 1 and area 3 together?

    Thank you for your detailed explanation!

    Lee

    #14339

    Rene Molenaar
    Keymaster

    Hi Lee,

    It’s possible but you have to do it in steps.

    First create a virtual link between Area 0 and Area 1 so you can reach Area 2. Then you can configure another one from Area 2 to Area 3.

    Rene

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