  BGP communities are widely used to tag prefix aggregates for policy, traffic
engineering, and inter-AS signaling. Because individual ASes define their own
community semantics, many ASes blindly propagate communities they do not
recognize. Prior research has shown the potential security vulnerabilities when
communities are not filtered. This work sheds light on a second unintended
side-effect of communities and permissive propagation: an increase in
unnecessary BGP routing messages. Due to its transitive property, a change in
the community attribute induces update messages throughout established routes,
just updating communities. We ground our work by characterizing the handling of
updates with communities, including when filtered, on multiple real-world BGP
implementations in controlled laboratory experiments. We then examine 10 years
of BGP messages observed in the wild at two route collector systems. In 2020,
approximately 25% of all announcements modify the community attribute, but
retain the AS path of the most recent announcement; an additional 25% update
neither community nor AS path. Using predictable beacon prefixes, we
demonstrate that communities lead to an increase in update messages both at the
tagging AS and at neighboring ASes that neither add nor filter communities.
This effect is prominent for geolocation communities during path exploration:
on a single day, 63% of all unique community attributes are revealed
exclusively due to global withdrawals.
