BGP, or Border Gateway Protocol, helps networks work out where internet traffic should go. It comes in two flavors. External BGP, or eBGP, connects different autonomous systems. Internal BGP, or iBGP, works between routers inside the same autonomous system. The basic difference sounds simple, and the details get more interesting once you see what each one is trying to do.<\/p>\n
An autonomous system, or AS, is a network run under one routing policy, and each one has an identifying number called an ASN. ISPs have them, and so do some large companies.<\/p>\n
eBGP is what routers use when they’re in separate autonomous systems. Say your company network connects to an ISP. Your router and the ISP’s router belong to different systems, so eBGP handles that relationship. iBGP stays inside one AS. A big network can have routers spread over several sites, and they still need to share BGP route information without treating each other like outsiders.<\/p>\n
The easiest way to keep them straight is to ask one question. Are the two BGP routers in the same autonomous system? If the ASNs are different, it’s eBGP, the normal choice for swapping routes with another organization. If they’re the same but BGP routes still need to travel between the routers, that’s iBGP.<\/p>\n
There are a few other differences too. eBGP adds the AS to the path as a route crosses into another system, which is a big part of how BGP avoids routing loops. iBGP doesn’t do that, since the route is staying inside the same AS. And eBGP peers are usually directly connected, while iBGP peers don’t have to sit next to each other.<\/p>\n
The AS path is one of the handy things BGP carries along with a route. A router that receives a route can see which systems it has already passed through. If it spots its own AS in that list, it rejects the route, because otherwise routes could go around in circles. Simple idea, very useful.<\/p>\n
A traditional iBGP setup needs every router to form a session with every other router in the group. Add more routers and the number of sessions balloons. That’s why bigger networks often use route reflectors, which cut down the direct sessions and make the whole design much easier to manage.<\/p>\n
A friend of mine ran a network with several routers and kept checking each morning which one was advertising a certain route. Once the BGP setup was organized properly, that little routine went away. Nothing exciting, just less annoying.<\/p>\n
If you’re looking at the link between your network and another autonomous system, think eBGP. If you’re looking at BGP between routers inside the same system, think iBGP. It’s easy to remember, but getting it wrong can cause real routing problems. Once you see the AS boundary as the dividing line, BGP feels a lot less mysterious. Funny how one little boundary explains so much.<\/p>","protected":false},"excerpt":{"rendered":"
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