Using Border Gateway Protocol (BGP) to scale beyond core

As your network grows toward becoming an international Juggernaut, you will find that taking the load off the core routers is not enough - it is time to extend the use of BGP to the rest of the network. Three different approaches may be followed in general:

iBGP requires a full internal mesh to ensure routing consistency. This internal mesh grows larger as BGP extends throughout the network and, of course, as the network grows. The last two approaches in the preceding list present a scalable way to reduce the number of neighbors while maintaining consistency. Dividing your network up into separate AS's and reducing the number of internal neighbors will be covered first.

The first two approaches are very similar. In fact, both require that you follow these three "easy" steps:

1. Divide the network into multiple regions/areas.
2. Select and configure an IGP for each region/area.
3. Connect each region using BGP.

The "divide and conquer" option that you chose will depend on a combination of the topology (resulting from the division) and the external connectivity. The following thumb rule is offered to aid in the decision:

1. Is your network connected to the Internet or are you planning to connect it?

2. Did the division result in a two level hierarchy with a core AS and all the others connecting to it (and not among themselves)?

3. Where are the connections to the Internet located?

This is just a small effor to share my views on chosing the right technology at right place and even if we all agree to this magic formula, we would still need to analyze our network needs everytime we plan to use BGP. Only your gut feeling and your experience can suggest you the best solution.