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 Ryu framework includes a BGP speaker library (http://ryu.readthedocs.io/en/latest/library_bgp_speaker.html#) that greatly simplifies the task if your SDN controller needs to communicate externally using BGP. We will look at using the Ryu SDN controller with BGP in detail in Chapter 13, Hybrid SDN, but this section will serve as a good introduction.

There are two ways to use the Ryu controller with BGP:

• Directly using Python and the BGP speaker library
• As a Ryu application, using ryu/services/protocols/bgp/application.py

We can use the VIRL router to verify our Ryu BGP configuration. Let's take a look at setting up the iosv routers.