We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is Why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 588 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)


314 New Members signed up the last 30 days!


100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Forum Replies

  1. 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.

  2. 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 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.

  3. Will,
    I am trying to visualize your area layout, but I need you to confirm. Would it be something like this?

    0 - 1 - 2

    If this is the case, you will not be able to do summarization in areas 1 and 3, because any area through which a Virtual-Link is created is considered a transit area. Transit areas are not allowed to do summarization. I previously wrote a forum response on this (see below). You might find it useful.


  4. Hi Rene,
    I am tying to understand some of the mechanics of the virtual links.
    I am assuming that the virtual link LSAs are unicast directly but that data packets traverse the transit area normally.

    when we have area 0 - 1 - 2 arrangement. The ABR between areas 1 and 2 has a virtual link to area 0.
    Does the area 2 ABR send type 3 LSAs into area 1? and are the area 1 routers happy to send packets direct to area 2 ?

    where we have a 0 - 1 - 0 setup and we bridge the two area 0's
    From area 1's perspective does each ABR to area 0 send different set of type 3 LSA ?

    Area 1 routers must not assume area 0 is continuous.
    So the ABRs must not send LSAs learnt over the virtual link into the transit area ?


  5. Hello Stuart.

    Here is the diagram that Rene has in his lesson:

    A virtual link has been created between the L0 interface of R1 and the Fa1/0 interface of R2.

    The OSPF packets between the two ends of the virtual link are not multicast packets. (Note the two ends of the virtual link are the L0 interface of R1 and the Fa1/0 interface of R2). LSAs that are sent over the virtual link are actually tunnelled packets between and, based on the network diagram in the lesson. However, the LSA packets as they continue their journey from R2 to R3, the ARE multicast packets just like any ordinary OSPF LSAs.

    Secondly, yes, data packets traverse the whole network normally independent of any virtual links.

    Concerning your second question:

    According to Cisco, an ABR is a router with at least ONE interface in area 0 and at least ONE interface in another area. In other words, there is no such thing as an ABR between area 1 and 2. One of the areas has to be area 0.

    Practially, an OSPF router configured as an ABR, without a connection to area 0, will route traffic within and even between its connected areas. However, it won't share one area's routing information with another area's OSPF neighbors without an area 0.

    Your third question,

    From the point of view of the routers, this is the same as having two ABRs link between area 0 and area 1 without a virtual link. In this case type 3 LSAs will be injected into area 1 from both R1 and R2, however, the LSAs will be different based on the destinations that exist in each of the two "parts" of area 0.

    Area 1 routers must not assume area 0 is continuous.
    So the ABRs must not send LSAs learnt over the virtual link into the transit area ?

    Area 1 routers don't care if area 0 is contiguous or not. They are not aware of how the areas interconnect, weather they connect via virtual link or not. ABRs in area 1 will advertise the type 3 LSAs that the receive from each discontiguous part of the area 0 thus informing the routers in area 1 of the correct route to the destinations within each "part" of area 0.

    I hope this has been helpful!


47 more replies! Ask a question or join the discussion by visiting our Community Forum