Cisco Systems 3560 Manual De Usuario

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using 
the database created by the bridging STP. Using this feature also prevents loops. To support this 
capability, bridging must be configured on each interface that is to participate in the flooding. If bridging 
is not configured on an interface, it still can receive broadcasts. However, the interface never forwards 
broadcasts it receives, and the router never uses that interface to send broadcasts received on a 
different interface.

Packets that are forwarded to a single network address using the IP helper-address mechanism can be 
flooded. Only one copy of the packet is sent on each network segment. 

To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions 
used to consider packet forwarding using IP helper addresses.)

The packet must be a MAC-level broadcast.

The packet must be an IP-level broadcast.

The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by 
the ip forward-protocol udp global configuration command.

The time-to-live (TTL) value of the packet must be at least two.

A flooded UDP datagram is given the destination address specified with the ip broadcast-address 
interface configuration command on the output interface. The destination address can be set to any 
address. Thus, the destination address might change as the datagram propagates through the network. 
The source address is never changed. The TTL value is decremented. 

When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), 
the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they 
are present on the output interface.

Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to 
flood UDP datagrams:

Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding 
of IP broadcasts.

In the switch, the majority of packets are forwarded in hardware; most packets do not go through the 
switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP 
flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over 
Ethernet interfaces configured for ARP encapsulation.

Enter global configuration mode.

Use the bridging spanning-tree database to flood UDP datagrams.

end 

Return to privileged EXEC mode.

Verify your entry.

(Optional) Save your entry in the configuration file.