Chances are, if you don’t work for an Internet Service Provider (ISP) or the phone company, you’ve probably never heard of Border Gateway Protocol Version 4 (BGP4). However, BGP4 is responsible for building the complex routing table used to traverse the global Internet.

What is BGP4?
BGP4 is an extremely complex routing protocol used to provide routing information for very large networks, such as those found at an ISP or a multinational corporation such as AT&T. BGP4 does not provide detailed routing information based on individual subnets. Instead, BGP4 stores routing information in the form of network summarization.

Summarizing routes is very important for an environment that can support literally thousands of networks. Additionally, BGP4 uses complex metrics that determine the best route to a destination network. Because of its complexity and scalability, BGP4 is best suited only for the largest of networks.

How does BGP4 work?
Unlike the majority of routing protocols, BGP4 is a connection-oriented protocol. When BGP4 is initiated on a router, it establishes a Transmission Control Protocol (TCP) connection with its neighboring router(s). Using this TCP connection, the router exchanges a complete copy of its routing table. Once the exchange of routing information is complete, BGP4 only sends out incremental updates, as changes occur in the network.

Two types of routes are defined by BGP4. These routes are internal BGP4 routes and external BGP4 routes. Internal BGP4 routes are routes within a single autonomous system. For example, all of the routes available through an entire college campus may be considered an internal BGP4 route.

An external BGP4 route is a route between two different autonomous systems. An example of an external BGP4 route could be a route between two college campuses.

Want more information?
If your large enterprise network has grown considerably, and Open Shortest Path First (OSPF) id no longer efficiently supports the routes through your internetwork (for more information on OSPF, check out “The path of least resistance: OSPF boasts enterprise benefits”), it may be time to upgrade to BGP4.

Keep in mind that BGP4 is extremely complex and requires much administration. If you want more information on implementing BGP4, check out Internet Routing Architectures by Cisco Press.

Warren Heaton CCDA, CCNA, MCSE+I is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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