The Use of BGP Communities

Question

I am studying BGP communities at the moment.

I understand that communities are path attributes which are a way of tagging specific BGP route advertisements in order to ensure proper route selection policies/traffic engineering, etc.

I've seen examples of BGP communities like this that ISPs and their customers can use, for example, here's some from an ISP in the US

So if I understand this correctly, if a customer tags their prefixes with 64983:0, for example, then the ISP will prepend the AS number 3 times to all their BGP neighbors. If the customer tags their prefixes with 3356:90, then they will set the LOCAL_PREF to 90.

I understand how these communities work but the question is, when exactly would I want to use them in the real world? Couldn't all this traffic engineering and route selection be simply achieved by having the customer configure a route-map and set the LOCAL_PREF, MED, AS Path Prepending and the rest for their prefixes there? If so, why use a community?

If the customer wants to prepend its AS 3 times, they could just configure a route-map and configure it there instead of tagging their routes with a community.

Does anyone please have an example of where a community would be more suitable than what I've mentioned above?

Thank you in advance for your help.

Answer 1

The idea with communities is that customers can signal to their upstream what they want that upstream to do. Those customers of course cannot change the upstream’s configuration, but using communities they can let their upstream know how they would like their upstream to handle their routes. this allows customers to change routing preferences without asking upstreams, which allows for example automation and offers much more flexibility.