OSPF Virtual Link Configuration
What do you do when the OSPF area has no direct connection to the backbone? Read our post on this and other solutions for IT Pros. Read more.
Notice: This blog post was originally published on Indeni before its acquisition by BlueCat.
The content reflects the expertise and perspectives of the Indeni team at the time of writing. While some references may be outdated, the insights remain valuable. For the latest updates and solutions, explore the rest of our blog
Key takeawaysKey takeaways are generated with AI assistance. Because automated summaries can occasionally contain errors or miss important context, always refer to the full blog post for complete information.This article explains how to use an OSPF virtual link to connect an OSPF area that lacks a direct physical connection to the backbone area 0, solving routing update and convergence problems in enterprise networks. It outlines a practical configuration and verification example where R3 has no direct link to area 0 and is connected to the backbone via a virtual link created through transit area 100 across R1 and R2. The result is that R3 can receive routing updates from area 0 and the network achieves full OSPF convergence when the virtual link is up.
What problem does an OSPF virtual link solve in the provided example?
In the example, R3 has no physical connection to OSPF backbone area 0, which prevents it from receiving routing updates and participating fully in the OSPF domain. The virtual link provides a logical adjacency across a transit area (area 100) between routers that are connected to the backbone, effectively extending area 0 to R3. This allows R3 to receive area 0 LSAs and become fully converged with the rest of the OSPF network without changing the physical topology.
What configuration steps are needed on R1 and R2 to create the virtual link?
First, enable OSPF with appropriate network statements so R1 places the 192.168.1.0/24 in area 0 and 10.0.0.0/24 in area 100, and R2 places 10.0.0.0/24 in area 100 and 172.16.0.0/24 in area 200. Then configure the virtual link commands referencing the neighboring router IDs in the transit area: on R1 use ‘area 1 virtual-link 10.0.0.1’ and on R2 use ‘area 1 virtual-link 10.0.0.2’ (as shown in the example). These commands create the logical link through transit area 100 to connect the isolated area to area 0.
How can you verify the virtual link is operational and what output indicates success?
Use the ‘show ip ospf virtual-links’ command on the router to verify the virtual link status. A successful output will show the virtual link to the remote router is up, the transit area (e.g., area 100), the interface used (for example FastEthernet0/0), state ‘POINT_TO_POINT’, timer intervals (Hello, Dead), and ‘Adjacency State FULL’. The example output includes ‘Virtual Link OSPF_VL0 to router 10.0.0.2 is up’ and ‘Adjacency State FULL (Hello suppressed)’, indicating the virtual link is operational and will allow routing updates to pass to the formerly isolated area.
We all know about OSPF (Open Shortest Path First) protocol–a fast link state protocol that is a go-to IGP (Interior Gateway Protocol) routing in large enterprises. It is also an open source protocol.
OSPF calculates shortest path for each route using Dijkstra’s algorithm. It uses area 0 as a backbone and every other area must be directly connected to it. There are cases in which we need to circumvent this rule and connect the OSPF area through the transit area to area 0. This occurs, for example, when the OSPF area has no direct connection to the backbone.
The configuration solution is an OSPF virtual link.
Let’s look at the following example:
R3 has no physical connection to the backbone, which means it cannot get routing updates. We will create an OSPF virtual link through R1 and R2 to connect R3 to area 0 by going through transit area AS100.
OSPF Configuration Virtual Link example:
Follow these steps to configure an OSPF virtual link:
- Configure OSPF
R1:
router ospf 1 log-adjacency-changes network 192.168.1.0 0.0.0.255 area 0 network 10.0.0.0 0.0.0.255 area 100
R2:
router ospf 1 log-adjacency-changes network 10.0.0.0 0.0.0.255 area 100 network 172.16.0.0 0.0.0.255 area 200
- Apply a virtual link with the following commands:
R1:
area 1 virtual-link 10.0.0.1
R2:
area 1 virtual-link 10.0.0.2
- To verify the virtual link, use the following command:
show ip ospf virtual-links
This is the output you should get:
R1#
show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 10.0.0.2 is up
Run as demand circuit DoNotAge LSA allowed.
Transit area 100, via interface FastEthernet0/0, Cost of using 1 Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:06 Adjacency State FULL (Hello suppressed) Index 1/2, retransmission queue length 0, number of retransmision 0 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 0, maximum is 0 Last retransmission scan time is 0 msec, maximum is 0 msec
That’s it! Now R3 will be able to receive routing updates from area 0 and you will have a converged network.
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