Juniper JUNOSE 11.2.X IP SERVICES, Configuration Manual

Page: 63 / 356

Community Lists
A community is a logical group of prefixes that share some common attribute. Community
members can reside on different networks and in different autonomous systems. BGP
enables you to define the community to which a prefix belongs. A prefix can belong to
more than one community. The community attribute lists the communities to which a
prefix belongs.
You can use communities to simplify routing policies by configuring the routing information
that a BGP device can accept, prefer, or distribute to other neighbors according to
community membership. When a route is learned, advertised, or redistributed, a BGP
device can set, append, or modify the community of a route. When routes are aggregated,
the resulting BGP update contains a community attribute that contains all communities
from all of the aggregated routes (if the aggregate is an AS-set aggregate).
Several well-known communities are predefined. Table 4 on page 37 describes how a
BGP device handles a route based on the setting of its community attribute.
Table 4: Action Based on Well-Known Community Membership
BGP Device Action Well-Known Community
Does not advertise the route beyond the BGP confederation
boundary no-export
Does not advertise the route to any peers, IBGP, or EBGP no-advertise
Does not advertise the route to any external peers local-as (also known as
no-export-subconfed)
Advertises this route to the Internet community; by default, all
prefixes are members of the Internet community internet
In addition to the well-known communities, you can define local-use communities, also
known as private communities or general communities. These communities serve as a
convenient way to categorize groups of routes to facilitate the use of routing policies.
The community attribute consists of four octets, but it is common practice to designate
communities in the
AA : NN format. The autonomous system number ( AA ) comprises the
higher two octets, and the community number ( NN ) comprises the lower two octets.
Both are expressed as decimal numbers. For example, if a prefix in AS 23 belongs to
community 411, the attribute could be expressed as 23:411. Use the
ip bgp-community
new-format command to specify that the show commands display communities in this
format. You can also use a regular expression to specify the community attribute.
Use the set community command in route maps to configure the community attributes.
You can add one or more communities to the attribute, or you can use the list keyword
to add a list of communities to the attribute. By default, the community attribute is not
sent to BGP peers. To send the community attribute to a neighbor, use the
neighbor send
community command.
37 Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring Routing Policy