On Thu, 18 Apr 2002, Adam Atkinson wrote:

To what extent does setting communities on BGP routes work in the real world?

I can't help suspecting that in reality, routes would get aggregated, or communities would be dropped or replaced, somewhere between source and destination.

Most of the utility of communities comes from either what your own policy does with them or what your providers policy does with them. For internally significant communities, they are are very useful for setting up simple but effective export policies ... An ISP might use different communities for routes they learned from a Customer vs. a peer vs. a transit provider vs. Internal, then set up a rule to send routes marked with customer, peer, transit, and internal communities to their customers, but only customer, and internal routes are sent to peers and transit providers. Communities are also useful if you have a service provider that has implemented a rich policy regarding acceptance of communities from customers. For instance, some providers have implemented communities that adjust the local pref the provider sets on the route, or determines if and how the route will be sent outside of the providers network at all to its peers, customers and/or transit providers. For example provider 1234 could use community 1234:120 to mean routes sent from customers to them should be accepted with local pref of 120, instead of the default 100, or that 1234:666 means to send the routes learned with community to customers of the provider only and not to send it to any peers or transit providers. In addition to this, your provider may be sending communities to you that will help you make better routing decisions ... for instance they may be marking routes that originated in a specific region (Say US:SouthEast) with a community that allows you to treat this route differently in your Atlanta, GA Data Center then in your Palo Alto, CA Data Center. The communities a provider accepts and or sends will vary from provider to provider so obviously check with yours for more information about this. So you are correct that there is a possibility of communities being removed or the route aggregated or whatever, but that is OK because the meaning of a community is usually targeted at a specific application where you can be sure your community is in effect, like within your own network or between you and your provider or customer. Eric Eric Oosting eoosting@sockeye.com office:781-693-7041 Network Engineer Network Eng and Operations Sockeye Networks