Flooding Ip Broadcasts - Cisco WS-C3750-48PS-S Software Configuration Manual

Chapter 34 Configuring IP Unicast Routing

Flooding IP Broadcasts

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.)
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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. 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:
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 Catalyst 3750 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.
78-16180-02
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.
Catalyst 3750 Switch Software Configuration Guide
Configuring IP Addressing
34-17