Unknown Unicast and Multicast Flood Blocking

This chapter describes how to configure the unknown unicast flood blocking (UUFB) and unknown multicast flood blocking (UMFB) features on the Catalyst 6500 series switches.

Note For complete syntax and usage information for the commands used in this chapter, refer to the Cisco IOS Master Command List, Release 12.2SX at this URL:

http://www.cisco.com/en/US/docs/ios/mcl/allreleasemcl/all_book.html

Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

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Understanding UUFB or UMFB

By default, unknown unicast and multicast traffic is flooded to all Layer 2 ports in a VLAN. You can prevent this behavior by using the UUFB and UMFB features to prevent or limit this traffic. The UUFB and UMFB features block unknown unicast and multicast traffic flooding at a specific port, only permitting egress traffic with MAC addresses that are known to exist on the port. The UUFB and UMFB features are supported on all ports that are configured with the switchport command, including private VLAN (PVLAN) ports.

Note Entering the switchport block multicast command on nonreceiver (router) ports of the VLAN could disrupt routing protocols. This command could also disrupt ARP functionality and other protocols, such as Network Time Protocol (NTP), that make use of local subnetwork multicast control groups in the 224.0.0.0/24 range.

Configuring UUFB

To configure UUFB or UFMB, perform this task:

 
Command
Purpose

Step 1 

Router# configure terminal

Enters global configuration mode.

Step 2 

Router(config)# interface {{type1  slot/port} | {port-channel number}}

Selects the interface to configure.

Step 3 

Router(config-if)# switchport

Configures the port for Layer 2 switching.

Step 4 

Router(config-if)# switchport block {unicast | multicast}

Enables unknown unicast or multicast flood blocking on the port.

Step 5 

Router(config-if)# do show interfaces [type1 slot/port] switchport | include Unknown

Verifies the configuration.

1 type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet

This example shows how to configure UUFB on Fast Ethernet port 5/12 and how to verify the configuration: 

Router# configure terminal 

Router(config)# interface fastethernet 5/12 

Router(config-if)# switchport 

Router(config-if)# switchport block unicast 

Router(config-if)# do show interface fastethernet 5/12 switchport | include Unknown

Unknown unicast blocked: enabled

Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

Participate in the Technical Documentation Ideas forum