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Community Member

FIRST EXAMPLE:

Text of book says:

"

7 REPLIES
Hall of Fame Super Blue

Every area should have at least one router with an interface in area 0. So taking your second example first

R1>--area0----area1--

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.

Jon

Community Member

Am I right, that in this one new topology

--area0----area1----area2--

Router R4 should have virtual-link configured on?

Hall of Fame Super Blue

R3 would need the virtual link not R4 because it is area2 that is not connected to area0.

Jon

Community Member

You have confused me

Hall of Fame Super Blue

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

2) --area0----area1----area2--

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.

Jon

Hall of Fame Super Gold

Alexandr

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.

HTH

Rick

Purple

hi guys

also if we have it like

r1--area0--r2--area1---r3--area3--r4

we can make a connectivity without using vertual-link

which is by makeing a gre tunnel interface from r3 to r2 and put that tunnel in area 0

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