OSPF Virtual Link - CISCO -

The Backbone and Area 0

OSPF has special restrictions when multiple areas are involved. If more than one area is configured, one of these areas has to be area 0. This is called the backbone.

The backbone has to be at the center of all other areas, every other areas have to be physically connected to the area 0 (backbone area). The reasoning for this is that OSPF expects all areas to inject routing information into the backbone and in turn the backbone will spread out that information into other areas. When designing networks it is good practice to start with area 0 and then expand into other areas later on. 

If it is not possible to connect area to the backbone area 0 there is solution for that, we have tool that can be used, and that would be virtual link.

Virtual links are used for two purposes: 

• Linking an area that does not have a physical connection to the backbone.              

         See Example 1.

• Patching the backbone in case discontinuity of area 0 occurs. 

         See Example 2.

 

Example 1: Areas Not Physically Connected to Area 0

 

As mentioned earlier, area 0 has to be at the center of all other areas. In some rare case where it is impossible to have an area physically connected to the backbone, a virtual link is used. The virtual link will provide the disconnected area a logical path to the backbone. The virtual link has to be established between two ABRs that have a common area, with one ABR connected to the backbone. This is illustrated in the picture 1:

As you can see area 2 does not have a direct physical connection into area 0 (Backbone area). A virtual link has to be configured between R1 and R2. Area 1 is to be used as a transit area and R1 is the entry point into area 0. This way R2 and area 2 will have a logical connection to the backbone Area 0.

In order to configure a virtual link, use the area <area-id> virtual-link <RID> router OSPF sub-command on both R1 and R2, where area-id is the transit area. In this case, that would be area 1. The RID is the router-id. The OSPF router-id is usually the highest IP address on the box, or the highest loopback address if one exists. To find the router-id, use the show ip ospf interface command. 

To set router ID manually, use the router-id command in router configuration mode.

Note:

Router ID for R1 is 1.1.1.1

Router ID for R2 is 2.2.2.2

R1#

router ospf 10

area 1 virtual-link 2.2.2.2

R2#

router ospf 10

area 1 virtual-link 1.1.1.1

Example 2: Partitioning the Backbone

 

OSPF allows for linking discontinuous parts of the Area 0 using a virtual link. In some cases, different area 0s need to be linked together. This can occur if a company has to merge two separate OSPF networks into one network with a common area 0. In other instances, virtual-links are added for redundancy in case some router failure causes the backbone to be split into two.

A virtual link can be configured between separate ABRs that touch area 0 from each side and having a common area. This is illustrated in the Picture 2:

 

 

Two Area 0s are linked together via a virtual link. In case a common area does not exist, an additional area, such as area 3, could be created to become the transit area.

Source: cisco.com