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Table 4: Action Based on Well-Known Community Membership (continued)
Well-Known Community
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 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.
A community list is a sequential collection of permit and deny conditions. Each
condition describes the community number to be matched. If you issued the ip
bgp-community new-format command, the community number is in AA:NN format;
otherwise, it is in decimal format (the hexadecimal octets converted to decimal).
The router tests the community attribute of a route against each condition in a
community list. The first match determines whether the router accepts (the route is
permitted) or rejects (the route is denied) a route that has the specified community.
Because the router stops testing conditions after the first match, the order of the
conditions is critical. If no conditions match, the router rejects the route.
Consider the network structure shown in Figure 5 on page 40.
BGP Device Action
Does not advertise the route to any external peers
Advertises this route to the Internet community; by default, all
prefixes are members of the Internet community
Chapter 1: Configuring Routing Policy
Community Lists
39
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