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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. 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: [quote=“stuart.w.gall, post:46, topic:942”]
    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 ?

    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!


  4. Hi Zeko,

    If you enable authentication for virtual links, you have to enable it globally for area 0 and set the password on the virtual link command. Here’s an example:

    R1#show run | begin ospf
    router ospf 1
     area 0 authentication
     area 1 virtual-link authentication-key NWL

    Authentication is enabled for area 0, the virtual link goes through area 1 and has the password. You can see it works with this command:

    R1#show ip ospf virtual-links 
    Virtual Link OSPF_VL0 to router is up
      Run as demand circuit
      DoNotAge LSA allowed.
      Transit area 1, via interface GigabitEthernet0/1
     Topology-MTID    Cost    Disabled     Shutdown      Topology Name
            0           1         no          no            Base
      Transmit Delay is 1 sec, State POINT_TO_POINT,
      Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
        Hello due in 00:00:04
        Adjacency State FULL (Hello suppressed)
        Index 1/1/2, retransmission queue length 0, number of retransmission 0
        First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)
        Last retransmission scan length is 0, maximum is 0
        Last retransmission scan time is 0 msec, maximum is 0 msec
      Simple password authentication enabled

    Note the last line that says simple password authentication enabled.

    Hope this helps!


  5. Hello Sovandara,

    You can create more than one virtual link if needed. For example, let’s say you have a topology like this:

    (area 0) R1 (area 1) R2 (area 2) R3 (area 3)

    You can configure a virtual link between R1-R2 to get area 2 connected to area 0. The virtual link is like a tunnel that gives R2 access to area 0.

    You can then configure a virtual link between R2 and R3 to connect area 3 to area 0.

    Hope this helps!


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