Configuring The Community Attribute; Table 19: Action Based On Well-Known Community Membership - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - BGP AND MPLS CONFIGURATION GUIDE 2010-10-12 Configuration Manual

JunosE 11.3.x BGP and MPLS Configuration Guide

Configuring the Community Attribute

90
BGP applies route map alpha to all routes learned from 10.5.5.2 (router NY). Instance 10
of route map alpha matches routes with access list dog1. This access list permits any
route whose AS-path attribute ends in 32 or 837—that is, routes that originate in AS 32
or AS 837. It sets their weight to 175, overriding the neighbor weight (50) set for updates
received from 10.5.5.2. Then, instance 20 of route map alpha permits all other routes
with no modification.
The result of this improved configuration is the following:
Router Chicago prefers routes learned from router Boston (weight 150) over routes
learned from router NY (weight 50), except that
Router Chicago prefers routes learned from router NY that originate in AS 837 or AS
32 (weight 175 as a result of route map alpha) over the same routes learned from router
Boston (weight 150).
Refer to the commands and guidelines in the section "Types of BGP Route Maps" on
page 70 for more information about configuring route maps.
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 90 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

Well-Known Community
no-export
no-advertise
local-as (also known as
no-export-subconfed)
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
BGP Speaker Action
Does not advertise the route to any EBGP peers (does not
advertise the route beyond the local AS)
Does not advertise the route to any peers, IBGP or EBGP
Advertises the route only to peers within the local
confederation
Advertises this route to the Internet community; by default,
all prefixes are members of the Internet community
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