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25-8
Catalyst 3750 Switch Software Configuration Guide
OL-8550-09
Chapter 25 Configuring Port-Based Traffic Control
Configuring Port Blocking
Configuring Port Blocking
By default, the switch floods packets with unknown destination MAC addresses out of 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
block a port (protected or nonprotected) from flooding unknown unicast or multicast packets to other
ports.
Note With multicast traffic, the port blocking feature blocks only pure Layer 2 packets. Multicast packets that
contain IPv4 or IPv6 information in the header are not blocked.
These sections contain this configuration information:
• Default Port Blocking Configuration, page 25-8
• Blocking Flooded Traffic on an Interface, page 25-8
Default Port Blocking Configuration
The default is to not block flooding of unknown multicast and unicast traffic out of a port, but to flood
these packets to all ports.
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 unicast packets and
Layer 2 multicast packets out of an interface:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify the interface to be configured, and enter interface
configuration mode.
Step 3
switchport block multicast Block unknown multicast forwarding out of the port.
Note Only pure Layer 2 multicast traffic is blocked. Multicast
packets that contain IPv4 or IPv6 information in the
header are not blocked.
Step 4
switchport block unicast Block unknown unicast forwarding out of 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.