• Inter-Area routing relies on LSA Type 3.
  • Generated by ABRs into area 0
  • A discontiguous area is when there’s an area that’s not directly connected to area 0.
    • Does not work.

Virtual Link Requirements

  • Must be used over a normal area.
    • no stub.
  • No filtering applied over transit area.
  • Inherits cost through SPF.
    • Must be lower than 65535
  • Runs as demand circuit.

In the image below there are four routers. Core and COLO are both running area 0, but COLO to Branch is running Area 1. From Branch to WAN router is running area 0 again, which obviously doesn’t work.

When advertising Loopback interfaces from Core into the OSPF domain, Branch and WAN are failing to receive any updates, but COLO receives them just fine. A virtual link can be created to fix this poor topology. The virtual link will start under the COLO router.

The virtual link on the COLO router will receive the proper command with the neighboring router-id of Branch, where we’re terminating the virtual link.

Same config on the Branch side, but with the OSPF router-id of COLO.

Immediately after its entered there’s a console message showing there’s an adjacency, but on VL0.

And now reachability is allowed from both WAN and Branch routers.

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