•
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.
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 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
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.
Copyright © 2010, Juniper Networks, Inc.90
JunosE 11.2.x BGP and MPLS Configuration Guide
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