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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 32 Configuring IP Unicast Routing
Configuring IP Addressing
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.
Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:
To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip forward-protocol spanning-tree Use the bridging spanning-tree database to flood UDP datagrams.
Step 3
end Return to privileged EXEC mode.
Step 4
show running-config Verify your entry.
Step 5
copy running-config startup-config (Optional) Save your entry in the configuration file.
Command Purpose
Step 1
configure terminal Enter global configuration mode
Step 2
ip forward-protocol turbo-flood Use the spanning-tree database to speed up flooding of UDP datagrams.
Step 3
end Return to privileged EXEC mode.
Step 4
show running-config Verify your entry.
Step 5
copy running-config startup-config (Optional) Save your entry in the configuration file.