Cisco 3845 - Security Bundle Router Software Manual page 581

Chapter 29 Configuring IP Unicast Routing
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.)
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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.
Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to
flood UDP datagrams:
Command
Step 1
configure terminal
Step 2
ip forward-protocol spanning-tree
Step 3
end
Step 4
show running-config
Step 5
copy running-config startup-config
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.
Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:
Command
Step 1
configure terminal
Step 2
ip forward-protocol turbo-flood
Step 3
end
Step 4
show running-config
Step 5
copy running-config startup-config
To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.
OL-23400-01
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.
Purpose
Enter global configuration mode.
Use the bridging spanning-tree database to flood UDP datagrams.
Return to privileged EXEC mode.
Verify your entry.
(Optional) Save your entry in the configuration file.
Purpose
Enter global configuration mode
Use the spanning-tree database to speed up flooding of UDP datagrams.
Return to privileged EXEC mode.
Verify your entry.
(Optional) Save your entry in the configuration file.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Configuring IP Addressing
29-15