Chapter 7 Configuring the Switch Ports
Configuring Flooding Controls
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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
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Blocking Flooded Traffic on a Port
By default, the switch floods packets with unknown destination MAC addresses
to all ports. Some configurations do not require flooding. For example, a port that
has only manually assigned addresses has no unknown destinations, and flooding
serves no purpose. Therefore, you can disable the flooding of unicast and
multicast packets on a per-port basis. Ordinarily, flooded traffic does not cross
VLAN boundaries, but multi-VLAN ports flood traffic to all VLANs they belong
to.
Beginning in privileged EXEC mode, follow these steps to disable the flooding of
multicast and unicast packets to a port:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface Enter interface configuration mode, and enter the port to
configure.
Step 3
port block multicast Block unknown multicast forwarding to the port.
Step 4
port block unicast Block unknown unicast flooding to the port.
Step 5
end Return to privileged EXEC mode.
Step 6
show port block {multicast |
unicast} interface
Verify your entries, entering the appropriate command
once for the multicast option and once for the unicast
option.
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