As I understand it Service Providers used to use BGPs local preference for manipulating outbound traffic and AS Path for manipulating inbound traffic before they used communities. As the internet grew, Service Providers started to use Communities that allowed customers and peers to use community values to manipulate their routing policy (which included adjusting AS path using community values).

What were the limitations of manipulating inbound traffic using just AS Path?

What extra things can communities do to manipulate inbound traffic flow that cannot be done with AS path alone?

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The problem of manipulating paths in BGP is that your peer ASes are free to ignore what your AS wants. There are many steps to deciding which routes are preferred in BGP. You can try to influence your peer ASes with MED, but that doesn't mean your peer ASes will pay any attention to MED. Communities are the same. You need the cooperation of your peer ASes to use communities to influence routing. If you use AS_PATH to tack on a bunch of ASes, you neighbors will usually pay attention to that, and they will mostly send traffic for those prefixes to the inbound path that you are trying to set up as the inbound path, but any local preference you peer ASes use can override AS_PATH.

On the other hand, influencing your own AS routing is fairly simple. There are a few different things you can use to manipulate which path your traffic takes as the outbound path.

You really need to be careful about asymmetric routing when trying to influence paths, and you need to secure the cooperation of your peer ASes.

  • To add to this - AS path is a decent way to control overall preference for a given route but that's about it. Communities obviously require agreement between autonomous system but can communicate a great deal more information. Common usage of communities include indicating that a given prefix is not for export beyond the receiving AS, that said prefix is a customer route (vs received from an upstream), etc. Indeed, in many cases it's these communities that allow a carrier to use attributes like LP to control route selection. The ability to apply multiple communities makes this even richer.
    – rnxrx
    Nov 27, 2016 at 23:09

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