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Introduction to BGP

Cisco certification candidates are introduced to BGP at the CCNP level, but the reaction to its introduction always reminds me of something I see often at the CCNA level.

Whenever I teach distance-vector protocols in my class, I make sure my students understand the many rules of distance-vector routing thoroughly. After that, we move on to OSPF and link-state routing.

And what do I tell my students before we move on to OSPF and link-state? "Take all that great stuff you just learned about distance-vector routing and put it aside, because none of it applies here!"

That's about the time I get astonished looks and a few things thrown at me. :)

What's this got to do with BGP? Well, when you start studying BGP, you have to put a lot of preconceptions aside. It sounds like just another routing protocol, like RIP or OSPF, but it's not.

So... before we start studying BGP and looking at some of its many features, let's take a look at what BGP is and what it's not.

What BGP Does and Who Uses It?
If you're in a position where you're used to seeing or working with routing protocols such as EIGRP and OSPF, you're probably now working with BGP. BGP is not a protocol you're going to configure at the office LAN. BGP is used to connect autonomous systems, which are very large "collections" of networks. (Those of you who have studied IGRP and EIGRP are familiar with the concepts of an AS.)

As opposed to routing protocols such as EIGRP and OSPF, BGP is an exterior routing protocol. BGP is not used to find a specific network instead, it's used to find the AS where that given network can be found.

A good way to look at it is that networks are contained in Autonomous Systems BGP helps you find the AS where a given network is found. Routing protocols such as OSPF and EIGRP take it from there.

BGP Terminology
You'll be happy to know that BGP has a terminology all its own. Well, you might not be happy about it, but it does! I'm not going to throw a bunch of terms at you right now, since these terms are better introduced to you when you can see what they do. Having said that, here are a couple of basic BGP terms that will help you make the transition from IGPs to BGP.

aggregation - This is just the BGP term for summarization. You're familiar with route summarization from your CCNA studies, and if not, it's time for a review. I told you you'd be using those skills for a long time!

IGP - Interior Gateway Protocol. These are routing protocols that run within an Autonomous System, such as OSPF and EIGRP.

EGP - Exterior Gateway Protocol. Remember from your CCNA studies that EIGRP routes are indicated by the letter "D" in your routing table? Ever wonder why? EGP, that's why. Run show ip route and take a look at the routing table key. EGP was BGP's predecessor, and is still in the routing table. EGP was there before EIGRP, so that's why "E" in the routing table doesn't indicate an EIGRP route.

We'll be adding to this list a great deal in the next few weeks.

Now that you've got a grasp on the differences between BGP and the IGPs you've been working with up to this point, it's time to start looking at some basic BGP concepts and configurations.

To your success,
Chris Bryant
CCIE #12933

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com), home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. Video courses and training, binary and subnetting help, and corporate training are also available. Pass the CCNA exam with Chris Bryant, CCIE #12933! For a copy of his FREE "How To Pass The CCNA" or "How To Pass The CCNP" ebook, write to [email protected]!