BGP community settings in the real world

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 Oosting office:781-693-7041
Network Engineer Network Eng and Operations Sockeye Networks