|
BGP communities are widely used to tag prefix aggregates for policy, traffic
engineering, and inter-AS signaling. Because individual ASes de ne 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 updates 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 explo- ration: on a single day, 63% of
all unique community attributes are revealed exclusively due to global
withdrawals. |
Keep your Communities Clean: Exploring the Routing Message Impact of BGP Communities