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When it comes to routing protocols, it doesn’t get much bigger than the Border Gateway Protocol (BGP). It’s the biggest one in the world because it’s the protocol that’s responsible for carrying all that wonderful traffic across the Internet.
BGP is most commonly used as an external routing protocol for large networks with multiple connections to the Internet. Companies or institutions that use BGP will have a unique autonomous system number that is exchanged with other BGP networks to create peering relationships with other autonomous systems. Through these partnerships, large networks and service providers provide BGP-controlled routes to, from, and through each other’s autonomous systems to give the Internet increased speed and efficiency, as well as a high level of redundancy.
One of the main benefits of BGP is that it’s a highly configurable routing protocol. Internally, BGP has a large set of criteria to determine the best route. That list of criteria can be altered by adjusting BGP attributes, creating prefix lists that affect what routes are advertised as well as access lists and routes maps that uniquely specify what routes are accepted and possibly set the cost of different available routes.
CBT Nuggets trainer and Cisco expert Jeremy Cioara explains BGP fundamentals in the video below:
Keep in mind, though, BGP is a “slow to converge” protocol. Routing changes on the Internet occur all the time. If BGP had to react to every change, it would flood the Internet with routing updates that could slow traffic all over the globe. So, BGP plays a waiting game to give routes time to settle down. Of course, this is not what you would want to occur inside your company's internal network. Your internal network needs to react to issues quickly to be effective. This is why BGP is seldom used as an internal routing protocol.
Understanding how BGP works, knowing its attributes, and understanding the route selection process enables you, as a network engineer, to have granular control over your Internet traffic -- and gives you the route redundancy needed to maximize your network availability.
To learn more about BGP, check out Jeremy Cioara’s “Cisco CCIP BGP 642-661” training course at CBT Nuggets (free trial required).