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
The packet must be an
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
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
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.
no ip forward-protocol spanning-tree global
configuration command to disable the flooding of IP broadcasts.
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.
To disable this feature, use
no ip forward-protocol turbo-flood global