Monday, August 3, 2009

Virtual Links

Yesterday I was thinking of writing something related to OSPF so something crossed my mind and on that very moment I start to write about it, so here we go that is “VIRTUAL LINKS”.
Suppose we have a complicated network and we use OSPF as our IGP and we configure different areas in it, I assume that the reader of virtual link must know how to configure multiple area OSPF and what is the concept behind it, I will talk directly about the virtual link, what is it, how it works, and what is the purpose of virtual links.

When an area is not directly connected to area 0 (backbone area) so it can’t communicate, a concept is used for it and that is called Virtual Links make able the area to communicate. See figure a, we have two routers namely R1 and R2, The fastethernet of R1 is configured under area 0, link between R2 and R1 is configured under area 1 and R2 fastethernet is configured under area2, now the LAN of R2 will not be able to communicated to R1 as it is not directly connected to R1 so we have to configure Virtual Link between R1 and R2 so what it will do is that the area 0 will be expand logically to R2 covering area 1 with it, now from R2 point of view there will two areas area 0 and area 2.

1: In the figure, Area 2 is not directly connected to Area 0 so we have to create a transit link on Area 1 to make able the communication between Area 2 and Area 0.
2: Here the Area 0 expand logically
3: After Virtual the R2 becomes Area Boarder Router (ABR).

NOTE: If a router connects two areas but none of the area is Area 0 so the router will never be ABR.

Conditions for Virtual Links

1: Link between two routers when made so one of the router must be part of back-bone area (Area 0).
R1-------R2-------> Link can be configured
If we another router R3 connected to router R2
R2------R3------> Can’t configure link as non of the router is directly connected to area 0.
2: When link is made between two routers, so one area must be common between them like
R1-----R2 (Area 1 is common)
R1-----R3 (Nothing is common)

NOTE: For Virtual Link both he condition must be satisfied.

OSPF Configuration

A(config) # router ospf 1
A(config) # network area 2
A(config) # network area 2

B(config) # router ospf 1
B(config) # network area 2
B(config) # network area 1
B(config) # network area 1

C(config) # router ospf 1
C(config) # network area 1
C(config) # network area 0

Virtual Link Configuration

B(config) # router ospf 1
B(config) # area 1 virtual-link

C(config) # router ospf 1
C(config) # area 1 virtual-link

Now check your Router-A routing table , the Router-C route will be there by using following command.

A # show ip route


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