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OSPF VIRTUAL LINKS  An Open Shortest Path First (OSPF) autonomous system has all areas physically connected to the backbone area (Area 0). In some cases, if this is not possible, we can use a virtual link to connect to the backbone (Area 0) through a non-backbone area. We can also use the virtual links to connect two parts of a partitioned backbone (Area 0) through a non-backbone area. The area through which we configure the virtual link, known as a transit area, must have full routing information. Remember, the transit area cannot be a stub area. Virtual link must be configuring both sides with the same area ID and the corresponding virtual link OSPF neighbor router ID. Show IP OSPF neighbors command give you the router ID information. OSPF Transit Area from Tunnel When configuring OSPF with tunnel interfaces as transit areas, there are several important considerations to ensure proper routing functionality. Here's what you need to know: Key Concepts Transit Area : In OS...

  Open Shortest Path First (OSPF)  Open shortest path first is an open standard routing protocol implemented by a wide variety of network vendors, including Cisco. And it’s that open standard characteristic that’s the key to OSPF flexibility and popularity. OSPF uses the Dijkstra algorithm to initially construct a shortest path tree and follows that by populating the routing table with the resulting best route. Its quick convergence is another reason it’s a favorite. Another two great advantages OSPF offers are that it supports multiple, equal-cost routes to the same destination and it also supports both IPv4 and IPv6 routed protocols read more... OSPF STUBS OSPF stubs allow the OSPF routers in an area to use default routes for forwarding packets to Area Border Routers (ABRs), rather than more specific routes. OSPF stubs reduce memory consumption and CPU processing time on the routers inside the area because the routers in that area can have fewer LSAs in their LSDBs Read more...