Simple definition
An extended BGP community carries structured routing policy information, including route targets used in VPN routing.
Purpose and use
BGP extended communities carry structured labels, including route targets used to select routes imported into VPNs.
How does it work?
Each value contains eight octets with a type and fields interpreted according to the selected community format.
Example and verification
When a VPN route is absent from a VRF, compare its exported route targets with the targets permitted for import.
Points to watch
A route target is different from the route distinguisher that makes a VPN route identifiable. These fields serve different purposes.
In short
Extended communities express exchange and selection policy; they do not by themselves provide traffic security filtering.