Configuring Port Blocking; Default Port Blocking Configuration; Blocking Flooded Traffic On An Interface - Cisco Catalyst 2975 Software Configuration Manual

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.
With multicast traffic, the port blocking feature blocks only pure Layer 2 packets. Multicast packets that
Note
contain IPv4 or IPv6 information in the header are not blocked.
These sections contain this configuration information:
•
•

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

The interface can be a physical interface or an EtherChannel group. When you block multicast or unicast
Note
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
Step 1
configure terminal
Step 2 interface interface-id
Step 3 switchport block multicast
Step 4 switchport block unicast
Step 5 end
Step 6 show interfaces interface-id switchport
Step 7
copy running-config startup-config
Catalyst 2975 Switch Software Configuration Guide
23-8
Default Port Blocking Configuration, page 23-8
Blocking Flooded Traffic on an Interface, page 23-8
Chapter 23
Purpose
Enter global configuration mode.
Specify the interface to be configured, and enter interface
configuration mode.
Block unknown multicast forwarding out of the port.
Only pure Layer 2 multicast traffic is blocked. Multicast
Note
packets that contain IPv4 or IPv6 information in the
header are not blocked.
Block unknown unicast forwarding out of the port.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Configuring Port-Based Traffic Control
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