Cisco Systems 78-11741-02 Establishing an IP Broadcast Address, Flooding IP Broadcasts, IPC-33

Configuring IP Addressing

Configuring Broadcast Packet Handling

Establishing an IP Broadcast Address

The Cisco IOS software supports IP broadcasts on both LANs and WANs. There are several ways to indicate an IP broadcast address. Currently, the most popular way, and the default, is an address consisting of all 1s (255.255.255.255), although the software can be configured to generate any form of IP broadcast address. Cisco software can receive and understand any form of IP broadcast.

To set the IP broadcast address, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#ip broadcast-address	Establishes a different broadcast address (other than 255.255.255.255).
[ip-address]

If the router does not have nonvolatile memory, and you need to specify the broadcast address to use before the software is configured, you must change the IP broadcast address by setting jumpers in the processor configuration register. Setting bit 10 causes the device to use all 0s. Bit 10 interacts with bit 14, which controls the network and subnet portions of the broadcast address. Setting bit 14 causes the device to include the network and subnet portions of its address in the broadcast address. Table 4 shows the combined effect of setting bits 10 and 14.

Table 4	Configuration Register Settings for Broadcast Address Destination
Bit 14		Bit 10	Address (<net><host>)
Out		Out	<ones><ones>
Out		In	<zeros><zeros>
In		In	<net><zeros>
In		Out	<net><ones>

Some router platforms allow the configuration register to be set through the software; see the “Rebooting” chapter of the Cisco IOS Configuration Fundamentals Configuration Guide for details. For other router platforms, the configuration register must be changed through hardware; see the appropriate hardware installation and maintenance manual for your system.

Flooding IP Broadcasts

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion using the database created by the bridging spanning-tree protocol. Turning on this feature also prevents loops. In order to support this capability, the routing software must include the transparent bridging, and bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still will be able to receive broadcasts. However, the interface will never forward broadcasts it receives, and the router will never use 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.

Cisco IOS IP Configuration Guide

IPC-33