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Configuring BGP
A community is a group of destinations that share some common attribute. Each destination can belong
to multiple communities. Autonomous-system administrators can define to which communities a
destination belongs. By default, all destinations belong to the general Internet community. The
community is identified by the COMMUNITIES attribute, an optional, transitive, global attribute in the
numerical range from 1 to 4294967200. These are some predefined, well-known communities:
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Based on the community, you can control the routing information to accept, prefer, or distribute to other
neighbors. A BGP speaker can set, append, or modify the community of a route when learning,
advertising, or redistributing routes. When routes are aggregated, the resulting aggregate has a
COMMUNITIES attribute that contains all communities from all the initial routes.
You can use community lists to create groups of communities to use in a match clause of a route map.
As with an access list, a series of community lists can be created. Statements are evaluated until a match
is found. As soon as one statement is met, the test stops.
To set the COMMUNITIES attribute and match clauses based on communities, see the match
community-list and set community route-map configuration commands in the
Redistribute Routing Information" section on page
By default, no COMMUNITIES attribute is sent to a neighbor. You can specify that the COMMUNITIES
attribute be sent to the neighbor at an IP address by using the neighbor send-community router
configuration command.
Beginning in privileged EXEC mode, follow these steps to create and to apply a community list:
Command
Step 1
configure terminal
Step 2
ip community-list community-list-number
{permit | deny} community-number
Step 3
router bgp autonomous-system
Step 4
neighbor {ip-address | peer-group name}
send-community
Step 5
set comm-list list-num delete
Step 6
exit
Step 7
ip bgp-community new-format
Step 8
end
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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internet—Advertise this route to the Internet community. All routers belong to it.
no-export—Do not advertise this route to EBGP peers.
no-advertise—Do not advertise this route to any peer (internal or external).
local-as—Do not advertise this route to peers outside the local autonomous system.
38-96.
Purpose
Enter global configuration mode.
Create a community list, and assign it a number.
The community-list-number is an integer from 1 to 99 that
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identifies one or more permit or deny groups of communities.
The community-number is the number configured by a set
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community route-map configuration command.
Enter BGP router configuration mode.
Specify that the COMMUNITIES attribute is sent to the neighbor at this
IP address.
(Optional) Remove communities from the community attribute of an
inbound or outbound update that match a standard or extended
community list specified by a route map.
Return to global configuration mode.
(Optional) Display and parse BGP communities in the format AA:NN.
A BGP community appears in a two-part format 2 bytes long. The
Cisco-default community format is NNAA. In the most recent RFC for
BGP, a community takes the form AA:NN, where the first part is the
autonomous-system number and the second part is a 2-byte number.
Return to privileged EXEC mode.
Chapter 38
Configuring IP Unicast Routing
"Using Route Maps to
OL-12247-04
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