User Guide:
ip directed-broadcast
Use this command to enable or disable IP directed broadcasts on an interface. By default,
interfaces on the A4 do not forward directed broadcasts. The no form of this command disables IP
directed broadcast on the interface.
Syntax
ip directed-broadcast
no ip directed-broadcast
Parameters
None.
Defaults
None.
Mode
Interface configuration: A4(su)->router(Config-if(Vlan 1))#
Usage
Directed broadcast is an efficient mechanism for communicating with multiple hosts on a network
while only transmitting a single datagram. A directed broadcast is a packet sent to all hosts on a
specific network or subnet. The directed broadcast address includes the network or subnet fields,
with the binary bits of the host portion of the address set to one. For example, for a network with
the address 192.168.0.0/16, the directed broadcast address would be 192.168.255.255. For a subnet
with the address 192.168.12.0/24, the directed broadcast address would be 192.168.12.255.
In order to minimize broadcast DoS attacks, forwarding of directed broadcasts is disabled by
default on the A4, as recommended by RFC 2644.
If the ability to send directed broadcasts to a network is required, you should enable directed
broadcasts only on the one interface that will be transmitting the datagrams. For example, if a A4
has five routed interfaces for the 10, 20, 30, 40, and 50 networks, enabling directed broadcast only
on the 30 network interface will allow anyone from any other networks (10, 20, 40, 50) to send
directed broadcast to the 30 network.
