35-57
Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 35 Configuring IP Unicast Routing
Configuring BGP
Based on the community, you can control which 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 checked until a match
is found. As soon as one statement is satisfied, the test is concluded.
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 35-79.
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:
Configuring BGP Neighbors and Peer Groups
Often many BGP neighbors are configured with the same update policies (that is, the same outbound
route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update
policies can be grouped into peer groups to simplify configuration and to make updating more efficient.
When you have configured many peers, we recommend this approach.
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 be 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 is displayed in a two-part format 2 bytes long. The
Cisco default community format is in the format NNAA. In the most
recent RFC for BGP, a community takes the form AA:NN, where the
first part is the AS number and the second part is a 2-byte number.
Step 8
end Return to privileged EXEC mode.
Step 9
show ip bgp community Verify the configuration.
Step 10
copy running-config startup-config (Optional) Save your entries in the configuration file.
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