Every area should have at least one router with an interface in area 0. So taking your second example first
area1 does have a router with one interface in area0 and that is the interface on R2 that connects to area0. So R2 connects area0 and area1 together. R3 doesn't need a direct connection to area0 because it can get to area0 via R2.
Now look at your other diagram. The router R4 that is in Area 4 has no interface in Area 0 and it is not connected to any other routers in Area 4 that have interfaces in Area 0 so drawing it the same way
-- area 0 -- -- area 34 -- -- area 4
it becomes obvious that for Area4 to connect to area0 there would have to be a physical interface on R4 in area0 but there isn't. That is why you need a virtual link.
Sorry about that, lets look at both diagrams again -
1) -- area 0 -- -- area 34 -- -- area 4
If you look at the above R2/R3 have interfaces in both area0 and area34. R4 has interfaces in area34 and area4. R4 therefore is the router that needs a connection to area0
Looking at this one, R2 has interfaces in both area0 and area1. R3 has interfaces in area1 and area2 BUT no interfaces in area0. So R3 needs a connection to area0. If R3 gets a connection to area0 then R4 has path to area0 via R3.
There is a significant difference between your first example and second example. The first example specifies that no router in area 4 has a physical connection to area 0. In example 2 a router in area 1 does have a physical connection to area 0.
In OSPF every non-zero area must have a connection to area 0. If there is a physical connection, then that is good and the requirement is satisfied. If there is not a physical connection to area 0 then there must be a virtual link which establishes a virtual connection from the area to area 0.
So in example 1 you must configure a virtual link because there is not a physical connection. In example 2 you do not need a virtual link because there is a physical connection to area 0. It does not have anything to do with Frame Relay.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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