Interdomain Routing: Border Gateway Protocol

Introduction

Just the way packets need to move around within a domain or autonomous system, packets need to move across them too. To enable all domains to communicate with each other, they need to be talking in the same language, in other words, they need to be using the same protocol. Hence, there exists just one interdomain routing protocol and it’s called the Border Gateway Protocol (BGP). We’ll spend a bit of time on this one as it is essentially the glue that holds the Internet together!

The Role of BGP

BGP routers have two key responsibilities:

1. To make each domain announce its presence to other domains. If it weren’t for this, no domain would know about the existence of other domains, and each domain would be an isolated island. This is done by each domain obtaining prefix reachability information from neighboring domains.
2. To determine the best route to each prefix.

Let’s study how each objective is achieved.

Advertising BGP Route Information

Internal Routers & Gateway Routers

• An internal router is only connected to routers within the domain it resides in.
• A gateway router is connected directly to one or many routers in one or many domains.

• For a domain to advertise its existence with BGP, gateway routers establish a TCP connection on port 179.

• In BGP, each domain is identified by a unique Autonomous System (AS) number.

• The BGP routers exchange routing information with each other over this TCP connection. The connection and the information exchange is known as a BGP session.

• A BGP session across two domains is called an eBGP session.

• A BGP session within a domain is called an iBGP session.

Propagating Information

In order to propagate BGP information, both iBGP and eBGP links are used. In the example above, consider router $R12$ ...