814
Configuring IP Unicast Routing
Configuring IP Addressing
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 so it 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.
BEFORE YOU BEGIN
Ensure that bridging is configured on each interface that is to participate in the flooding.
DETAILED STEPS
Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts.
EXAMPLE
The following example permits IP broadcasts to be flooded through the internetwork in a controlled fashion:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip forward-protocol spanning-tree
Switch(config)# end
Speeding up STP-Based UDP Flooding
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.
BEFORE YOU BEGIN
Enable the flooding of IP broadcasts as described in the Flooding IP Broadcasts, page 813.
Command Purpose
1. configure terminal Enter global configuration mode.
2. ip forward-protocol spanning-tree Use the bridging spanning-tree database to flood UDP
datagrams.
3. end Return to privileged EXEC mode.
4. show running-config Verify your entry.
5. copy running-config startup-config (Optional) Save your entry in the configuration file.
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