Troubleshooting guide

Additional BGP Configuration

BGP Communities

Within BGP, the community is an optional attribute that can be used for identification, security, or to signal

a BGP peer that it should take a particular action. When used for identification and security, the attribute

adds another layer of complexity that requires special configuration to bring the BGP connection up. When

used to signal a peer, the attribute is commonly used when that peer is ignoring other attributes of the BGP

advertisement (which is often the case in MPLS networks).

A route can be a member of one or more BGP communities. A community is simply a way of grouping

routes together and applying a consistent policy to the group. A route can be placed into a community

according to any attribute in that route. One of the most common ways of grouping routes is by IPv4

network address and prefix length, which is defined in a prefix list and ultimately referenced in a route map

(refer to

Prefix List on page 21 and Filtering Routes According to Network IPv4 Address on page 25). In

order for a route’s membership in a community to have significance, administrators must define policies

that apply to the community.

Several commands must be issued when configuring BGP communities on an AOS device. A route map

must be configured, followed by the appropriate commands for sending and/or receiving a community

string. Also, the send-community standard command must be enabled for any neighbor that will be

sending or receiving community attributes (refer to

Enabling an AOS Device to Send or Receive BGP

Communities on page 15).

Configuring a Community List

A community list is used when an AOS device receives a community string using BGP. A community

list can be used to:

• Select the communities to which the router will apply a specific policy, such as filtering advertised

routes to those communities or applying policies to inbound routes from those communities.

• Define the communities that the BGP interface will delete from routes.

Use the ip community-list command to create a community list for BGP route map use. The

communities defined in this command should match an existing community string that the AOS device

receives. This command is issued from the Global Configuration mode:

(config)#ip community-list <name>

<name> Specifies the community list name. This is an arbitrary name for the list that is referenced in

a BGP policy using community strings.

This command places the user in the Community List Configuration mode where one or more

well-defined communities can be specified. A value for a privately defined community might also be

specified.

The following command adds an entry to the community list that either permits or denies BGP routes

containing the specified community string in the community attribute:

<value> Specifies a privately defined community for routes that contain this value in their community

attribute. This is a numeric value that can be an integer from 1 to 4294967295 or string in the form

aa:nn, where aa is the AS number and nn is the community number. Multiple community number

parameters can be present in the command.

internet Specifies routes that contain the reserved community number for the Internet community.