Transitioning from one IGP to another can be quite a challenge. The task of merging two networks that use the same IGP also has its own challenges to overcome. The classic example is the merging of two companies: Company A has bought Company B and the two organizations must now join together. Part of this effort involves merging the companies’ IT infrastructure. From a networking perspective, two separate administrative domains must be merged into one, a problem that can be quite a challenge to solve.
There are a variety of factors to consider when merging two (or more) networks—primarily area design, but also matching configuration parameters and tunneling.
The biggest factor in merging the two networks is area design, and the most important design consideration is how to deal with the networks’ backbones. If the networks are running centralized backbones, as required by OSPF and as shown in Figure 19-9, significant work needs to be done to determine how and where to join the two networks’ backbone areas.
Figure 19-9. Two companies using OSPF and centralized backbones
The simplest solution may involve creating physical links between the backbone areas, effectively creating a single larger backbone, as shown in Figure 19-10.