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Community Lists

A community is a logical group of prefixes that share some common attribute. 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.

You can use communities to simplify routing policies by configuring the routing information that a BGP device can accept, prefer, or distribute to other neighbors according to community membership. 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 of the aggregated routes (if the aggregate is an AS-set aggregate).

Several well-known communities are predefined. Table 5 describes how a BGP device handles a route based on the setting of its community attribute.

Table 5: 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.

Consider the network structure shown in Figure 5.

Figure 5: Community Lists

Image g013111.gif

Suppose you want router Albany to set metrics for routes that it forwards to router Boston based on the communities to which the routes belong. You can create community lists and filter the routes with a route map that matches on the community list. The following example configures router Albany:



host1(config)#router bgp 293 

host1(config-router)#neighbor 10.5.5.2 remote-as
32 

host1(config-router)#neighbor 10.2.2.1 remote-as
451 

host1(config-router)#neighbor 10.2.2.4 remote-as
17 

host1(config-router)#neighbor 10.2.2.4 route-map
commtrc out 

host1(config-router)#exit 

host1(config)#route-map commtrc permit 1 

host1(config-route-map)#match community 1 

host1(config-route-map)#set metric 20 

host1(config-route-map)#exit 

host1(config)#route-map commtrc permit 2 

host1(config-route-map)#match community 2 

host1(config-route-map)#set metric 75 

host1(config-route-map)#exit 

host1(config)#route-map commtrc permit 3 

host1(config-route-map)#match community 3 

host1(config-route-map)#set metric 85 

host1(config-route-map)#exit 

host1(config)#ip community-list 1 permit 25 

host1(config)#ip community-list 2 permit 62 

host1(config)#ip community-list 3 permit internet

Community list 1 comprises routes with a community of 25; their metric is set to 20. Community list 2 comprises routes with a community of 62; their metric is set to 75. Community 3 catches all remaining routes by matching the Internet community; their metric is set to 85.

ip bgp-community new-format

Use to specify that communities must be displayed in AA:NN format, where AA is a number that identifies the autonomous system and NN is a number that identifies the community within the autonomous system.
Examplehost1(config)#ip bgp-community new-format
Use the no version to restore the default display.
See ip bgp-community new-format.

ip community-list

Use to create a community list for BGP and control access to it.
The list name can be up to 32 characters long.
A route can belong to any number of communities, so a community list can have many entries comprising many communities.
You can specify one or more community values when you create a community list. A clause in a route map that includes a list that has more than one value matches only a route that has all of the values; that is, the multiple values are logical ANDed.
You can specify community values with a number or a regular expression.
Examplehost1(config)#ip community-list 1 permit 100:2 100:3 100:4 host1(config)#route-map marengo permit 10 host1(config-route-map)#match community 1
A route matches this community list only if it belongs to at least all three communities in community list 1: communities 100:2, 100:3, and 100:4.
Use the no version to remove the specified community list, including all list entries.
See ip community-list.

neighbor send-community

Use to specify that a community attribute be sent to a BGP neighbor.
If you specify a BGP peer group by using the peer-group-name argument, all the members of the peer group inherit the characteristic configured with this command.
Examplehost1:vr1(config-router)#neighbor 192.3.4.5 send-community standard
Use the no version to specify that common attributes not be sent to a BGP neighbor.
See neighbor send-community.

set community

Use to set the community attribute in BGP updates.
You can specify a community list number in the range 1–4294967295, or in the new community format of AA:NN, or you can specify one of the following well-known communities:
- local-as—Prevents advertisement outside the local AS
- no-advertise—Prevents advertisement to any peer
- no-export—Prevents advertisement beyond the BGP confederation boundary
Alternatively, you can use the list keyword to specify the name of a community list that you previously created with the ip community-list command.
You can use this command with inbound, outbound, and redistribution route maps.
Use the none keyword to remove the community attribute from a route.
Examplehost1(config)#route-map 1 host1(config-route-map)#set community no-advertise
Use the no version to remove the set clause from a route map.
See set community.

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