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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
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Chapter 7      Configuring the Switch Ports
Configuring Flooding Controls
Resuming Normal Forwarding on a Port
Beginning in privileged EXEC mode, follow these steps to resume normal 
forwarding on a port:
Enabling a Network Port
Network ports are assigned per VLAN and can reduce flooded traffic on your 
network. The switch forwards all traffic with unknown destination addresses to 
the network port instead of flooding the traffic to all ports in the VLAN. 
When you configure a port as the network port, the switch deletes all associated 
addresses from the address table and disables learning on the port. If you 
configure other ports in the VLAN as secure ports, the addresses on those ports 
are not aged. If you move a network port to a VLAN without a network port, it 
becomes the network port for the new VLAN. 
You cannot change the settings for unicast and multicast flooding on a network 
port. You can assign only one network port per VLAN. For the restrictions that 
apply to a network port, see the “Changing the Password” section on page 6-15.
Caution A network port cannot link cluster members. 
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
no port block multicast Enable unknown multicast forwarding to the port.
Step 4
no port block unicast Enable 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.