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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 21 Configuring Port-Based Traffic Control
Configuring Port Blocking
Configuring Port Blocking
By default, the switch floods packets with unknown destination MAC addresses to all ports. If unknown
unicast and multicast traffic is forwarded to a protected port, there could be security issues.
To prevent unknown unicast or multicast traffic from being forwarded from one port to another, you can
configure a port (protected or nonprotected) to block unknown unicast or multicast packets.
Note Blocking unicast or multicast traffic is not automatically enabled on protected ports; you must explicitly
configure it.
The port blocking feature is only supported on these switches:
• Catalyst 2950 Long-Reach Ethernet (LRE) switches running Cisco IOS Release 12.1(14)EA1
or later
• Catalyst 2950G-12-EI, 2950G-24-EI, 2950G-24-EI-DC, 2950G-48-EI, and 2955 switches running
Cisco IOS Release 12.1(19)EA1 or later
Blocking Flooded Traffic on an Interface
Note The interface can be a physical interface or an EtherChannel group. When you block multicast or unicast
traffic for a port channel, it is blocked on all ports in the port channel group.
Beginning in privileged EXEC mode, follow these steps to disable the flooding of multicast and unicast
packets to an interface:
To return the interface to the default condition where no traffic is blocked, use the no switchport block
{multicast | unicast} interface configuration commands.
This example shows how to block unicast and multicast flooding on a port:
Switch# configure terminal
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# switchport block multicast
Switch(config-if)# switchport block unicast
Switch(config-if)# end
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify the interface to configure, and enter interface
configuration mode.
Step 3
switchport block multicast Block unknown multicast forwarding to the port.
Step 4
switchport block unicast Block unknown unicast forwarding to the port.
Step 5
end Return to privileged EXEC mode.
Step 6
show interfaces interface-id switchport Verify your entries.
Step 7
copy running-config startup-config (Optional) Save your entries in the configuration file.