- Enterasys Security Router User's Guide

Overview
XSR User’s Guide 6-9
Figure 6-6 MED Applied to Direct Ingress Traffic Flow to an AS
Community
A BGP community, as shown in Figure 6-7, is defined as a group of destinations that share some
common property and is not limited to one network or AS. Communities simplify routing policies
by identifying routes based on a logical property rather than an IP prefix or AS number. A BGP
speaker can then use this attribute along with others to control which routes to accept, prefer, and
relay to other BGP neighbors.
The XSR supports the following predefined communities based on the BGP COMMUNITIES
attribute:
• aa:nn - Advertise a particular AS and community number.
• internet - Specifies the entire Internet community.
• no-export - Do not advertise this route to an EBGP peer.
• no-advertise - Do not advertise this route to any peer (internal or external).
Based on one of the following values, the XSR supports BGP routing policies:
•IP address
•AS_PATH
• COMMUNITIES
Based on a shared common attribute, a community comprises multiple destinations. But, each
destination can also belong to multiple communities. While you can specify which communities
comprise a destination, all destinations belong to the default Internet community.
By creating a community list, you control which routing data to accept, prefer, or distribute to
other neighbors. A BGP speaker can set, append, or modify the community of a route when you