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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter32 Configuring IP Unicast Routing
Configuring IP Addressing
To be considered for flooding, packets mus t meet thes e cr iteria. (No te tha t thes e ar e the same cond itions
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 da ta gram pr opagat es t hrou gh the ne twork .
The source address is never changed. The TTL value is decremented.
When a flooded UDP datagram is sent out an interface (and the de stina tio n add re ss possi bly cha nged ),
the datagram is handed to the normal IP output routines and is, t h erefore, subject to access lists, if they
are present on the output interface.
Beginning in privileged EXEC mode, follow these steps to use the bridging span ning- tre e da tab ase t o
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 hard ware; mos t pa cke ts do no t go thr oug h th e
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- tr ee- bas ed fl o oding :
To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 ip forward-protocol spanning-tree Use the bridging spanning-tree database to flood UDP datagrams.
Step3 end Return to privileged EXEC mode.
Step4 show running-config Verify your entry.
Step5 copy running-config startup-config (Optional) Save your entry in the configuration file.
Command Purpose
Step1 configure terminal Enter global configuration mode
Step2 ip forward-protocol turbo-flood Use the sp anning-tree database to speed up flooding of UDP datagrams.
Step3 end Return to privileged EXEC mode.
Step4 show running-config Verify your entry.
Step5 copy running-config startup-config (Optional) Save your entry in the configuration file.