BGP has long been used as the external routing protocol, since the beginning of internet. It is meant for exchanging routing information between different autonomous systems without having a centralized management system. As the network starts to grow exponentially, traditional IGPs, such as IS-IS and OSPF, are not able to keep up with the resources required to keep up with the growth. The nature of BGP is also useful when different internal entities needs to operate over a common network infrastructure. An example would be to have a common core for inter-site networks in one autonomous system, a different distribution autonomous system in each data center, and a different autonomous system for each division behind the distribution fabric. In many large networks, BGP has become the routing protocol of choice in both internal and external networks.
The...
