Community List Overview

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.

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 the aggregated routes (if the aggregate is an AS-set aggregate).

Several well-known communities are predefined. Table 4 describes how a BGP device handles a route on the basis of the setting of its community attribute.

Table 4: Action Based on Well-Known Community Membership

Well-Known Community

BGP Device Action

no-export

Does not advertise the route beyond the BGP confederation boundary

no-advertise

Does not advertise the route to any peers, IBGP, or EBGP

local-as (also known as no-export-subconfed)

Does not advertise the route to any external peers

internet

Advertises this route to the Internet community; by default, all prefixes are members of the Internet community

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.

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