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OSPF Virtual Link

OSPF Virtual Link

OSPF virtual link is use for that area far from backbone area 0.

At this figure 1.1 area 2 is far from area 0 so area 2 can't reach other area.
Figure 1.1
After configure OSPF, neighbor is up but router 2 don't see 3.3.3.0/24 network.

R3#sh ip ospf nei
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/DR         00:00:39    10.1.2.1        FastEthernet0/0

R2#sh ip route
     10.0.0.0/30 is subnetted, 2 subnets
C       10.1.2.0 is directly connected, FastEthernet1/0
C       10.1.1.0 is directly connected, FastEthernet0/0

One of OSPF rule all area must connect to area 0.In this cause we can use virtual link at area 1.

R2
router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 area 1 virtual-link 3.3.3.3

R3
 router-id 3.3.3.3
 log-adjacency-changes
 area 1 virtual-link 2.2.2.2

*Mar  1 00:23:02.203: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on OSPF_VL1 from LOADING to FULL, Loading Done

Now Virtual link is up and we will see all all area are connected.

R2
     3.0.0.0/24 is subnetted, 1 subnets
O IA    3.3.3.0 [110/2] via 10.1.2.2, 00:03:59, FastEthernet1/0
     10.0.0.0/30 is subnetted, 2 subnets
C       10.1.2.0 is directly connected, FastEthernet1/0
C       10.1.1.0 is directly connected, FastEthernet0/0

R1
R1#sh ip route ospf

     3.0.0.0/24 is subnetted, 1 subnets

O IA    3.3.3.0 [110/3] via 10.1.1.2, 00:05:07, FastEthernet0/0

R1#sh ip ospf data

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         388         0x80000002 0x00947B 1
2.2.2.2         2.2.2.2         380         0x80000004 0x004097 2
3.3.3.3         3.3.3.3         2     (DNA) 0x80000003 0x0019E5 1

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.1.1        1.1.1.1         387         0x80000001 0x0052CD

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
3.3.3.0         3.3.3.3         11    (DNA) 0x80000001 0x00CC5A
10.1.2.0        2.2.2.2         395         0x80000001 0x00A089
10.1.2.0        3.3.3.3         11    (DNA) 0x80000001 0x0082A3

But Virtual link can't support Stub area and NSSA area.

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