Switchport Block - Cisco Catalyst 3750-X Command Reference Manual

Cisco ios release 15.2(1)e and later

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Table of Contents

Chapter 2 Catalyst 3750-X and 3560-X Switch Cisco IOS Commands

switchport block

Use the switchport block interface configuration command on the switch stack or on a standalone switch

to prevent unknown multicast or unicast packets from being forwarded. Use the no form of this command

to allow forwarding unknown multicast or unicast packets.

Syntax Description

Unknown multicast and unicast traffic is not blocked.

Command Modes

Interface configuration

Command History

Usage Guidelines

By default, all traffic with unknown MAC addresses is sent to all ports. You can block unknown multicast

or unicast traffic on protected or nonprotected ports. If unknown multicast or unicast traffic is not

blocked on a protected port, there could be security issues.

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.

Blocking unknown multicast or unicast traffic is not automatically enabled on protected ports; you must

explicitly configure it.

For more information about blocking packets, see the software configuration guide for this release.

This example shows how to block unknown unicast traffic on an interface:

Switch(config-if)# switchport block unicast

You can verify your setting by entering the show interfaces interface-id switchport privileged EXEC

switchport block {multicast | unicast}

no switchport block {multicast | unicast}

Specify that unknown multicast traffic should be blocked.

Specify that unknown unicast traffic should be blocked.

Modification

This command was introduced.

Only pure Layer 2 multicast traffic is blocked. Multicast packets that

contain IPv4 or IPv6 information in the header are not blocked.

Catalyst 3750-X and 3560-X Switch Command Reference

switchport block

Catalyst 3560-x