In this lesson, I’ll explain the concept of Virtual Link in OSPF, the scenarios in which it is required, and how to configure it on Juniper routers. A virtual link is a logical connection created to ensure connectivity between a remote area and Area 0 (the backbone area) when there is no direct physical connection. This is achieved by designating an intermediary area (transit area) to act as a bridge. This is required because, as I said earlier in the previous lessons, all areas on OSPF should connect to area 0 which is the backbone area.
It happens sometimes that there is no physical connection between a remote area and area 0 and there is an area between them. Then we will use this intermediate area to be a transit area using virtual link allowing the remote area to reach to area 0.
Let me show you this in a LAB directly.
As you can see, area 2 is not connected to area 0 and there is a transit area (area 1) between them. I will use area 1 to form a virtual link allowing area 2 to reach area 0. Let’s do this work in a LAB.
Step 1: Verify Existing OSPF Configuration
Ensure OSPF and IP addresses are configured on all routers.
R1# show ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.10.10.2 1 Full/DR 00:00:34 10.10.10.2 ge-0/0/1.0
R2# show ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.10.10.1 1 Full/BDR 00:00:33 10.10.10.1 fe-0/0/1.0
3.3.3.3 1 Full/DR 00:00:34 192.168.23.3 fe-0/0/2.0
R3# show ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.34.4 1 Full/DR 00:00:35 192.168.34.4 ge-0/0/3.0
Step 2: Check Router 1’s Routing Table
Verify that R1 does not see networks from Area 2.
R1# show route
inet.0: 11 destinations, 12 routes (11 active, 0 holddown, 0 hidden) + = Active Route, – = Last Active, * = Both
10.10.10.0/24 *[OSPF/10] 00:05:26, metric 10
to 10.10.10.2 via ge-0/0/1.0
192.168.23.0/24 *[OSPF/10] 00:05:26, metric 20
to 10.10.10.2 via ge-0/0/1.0
Area 2 routes (e.g., 4.4.4.4/32) are not visible.
Step 3: Note the Router IDs of R2 and R3
Find the Router IDs of R2 and R3 for the virtual link configuration.
R2# show ospf overview
Instance: master
Router ID: 10.10.10.2
SPF Calculation Count: 5
R3# show ospf overview
Instance: master
Router ID: 3.3.3.3
SPF Calculation Count: 5
Step 4: Configure Virtual Link on R2
Create the virtual link on R2 using the Router ID of R3 and transit area 1.
[edit]
R2# set protocols ospf area 0 virtual-link neighbor-id 3.3.3.3 transit-area 1
R2# commit
Step 5: Configure Virtual Link on R3
Complete the virtual link configuration on R3 using the Router ID of R2 and transit area 1.
[edit]
R3# set protocols ospf area 0 virtual-link neighbor-id 10.10.10.2 transit-area 1
R3# commit
Step 6: Verify the Virtual Link
Ensure that the virtual link is established and functioning.
R2# show ospf virtual-links
Neighbor ID Transit Area State Dead Time
3.3.3.3 1 Full/DR 00:00:35
R3# show ospf virtual-links
Neighbor ID Transit Area State Dead Time
10.10.10.2 1 Full/BDR 00:00:33
Step 7: Check Router 1’s Routing Table Again
Verify that R1 now sees the networks in Area 2.
R1# show route
inet.0: 13 destinations, 14 routes (13 active, 0 holddown, 0 hidden)
+ = Active Route, – = Last Active, * = Both
4.4.4.4/32 *[OSPF/10] 00:01:12, metric 30
to 10.10.10.2 via ge-0/0/1.0
192.168.34.0/24 *[OSPF/10] 00:01:12, metric 30
to 10.10.10.2 via ge-0/0/1.0
Conclusion
The virtual link successfully connected Area 2 to Area 0 via Area 1. R1 can now see networks in Area 2, and vice versa. Virtual links are an effective solution for extending connectivity between areas not directly connected to Area 0.
This concludes the lesson. I hope it was informative, and I’ll see you in the next one!
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