Refer to the commands and guidelines in the section “Types of BGP Route Maps” on
page 71 for more information about configuring route maps.
Configuring the Community Attribute
A community is a logical group of prefixes that share some common attribute.
Community members can be on different networks and in different autonomous
systems. BGP allows 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 which routing
information a BGP speaker will accept, prefer, or distribute to other neighbors
according to community membership. When a route is learned, advertised, or
redistributed, a BGP speaker 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 have been predefined. Table 19 on page 92
describes how a BGP speaker handles a route based on the setting of its community
attribute.
Table 19: Action Based on Well-Known Community Membership
BGP Speaker ActionWell-Known Community
Does not advertise the route to any EBGP peers (does not
advertise the route beyond the local AS)
no-export
Does not advertise the route to any peers, IBGP or EBGPno-advertise
Advertises the route only to peers within the local
confederation
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 can be expressed as 23:411.
Use the ip bgp-community new-format command to specify that the show
commands display communities in this format.
92 ■ Configuring BGP Routing Policy
JUNOSe 11.1.x BGP and MPLS Configuration Guide
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