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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-08
Chapter 40 Configuring IP Unicast Routing
Configuring BGP
sequence number
command; to reenable automatic generation, use the
ip prefix-list sequence number
command. To clear the hit-count table of prefix list entries, use the
clear ip prefix-list
privileged EXEC
command.
Configuring BGP Community Filtering
One way that BGP controls the distribution of routing information based on the value of the
COMMUNITIES attribute. The attribute is a way to groups destinations into communities and to apply
routing decisions based on the communities. This method simplifies configuration of a BGP speaker to
control distribution of routing information.
A
community
is a group of destinations that share some common attribute. Each destination can belong
to multiple communities. AS 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 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
Redistribute Routing Information” section on page 40-94
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 be sent to the neighbor at
this IP address.
Summary of Contents for Catalyst 3750-E Series
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