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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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Chapter 38 Configuring IP Unicast Routing
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:
• 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.
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 “Using Route Maps to
Redistribute Routing Information” section on page 38-96.
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 Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip community-list community-list-number
{permit | deny} community-number
Create a community list, and assign it a number.
• The community-list-number is an integer from 1 to 99 that
identifies one or more permit or deny groups of communities.
• The community-number is the number configured by a set
community route-map configuration command.
Step 3
router bgp autonomous-system Enter BGP router configuration mode.
Step 4
neighbor {ip-address | peer-group name}
send-community
Specify that the COMMUNITIES attribute is sent to the neighbor at this
IP address.
Step 5
set comm-list list-num delete (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.
Step 6
exit Return to global configuration mode.
Step 7
ip bgp-community new-format (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.
Step 8
end Return to privileged EXEC mode.
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