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Configuring IP Addressing
Configuring Broadcast Packet Handling
IPC-32
Cisco IOS IP Configuration Guide
To enable forwarding of IP directed broadcasts, use the following command in interface configuration
mode:
Forwarding UDP Broadcast Packets and Protocols
Network hosts occasionally use User Datagram Protocol (UDP) broadcasts to determine address,
configuration, and name information. If such a host is on a network segment that does not include a
server, UDP broadcasts normally are not forwarded. You can remedy this situation by configuring the
interface of your router to forward certain classes of broadcasts to a helper address. You can use more
than one helper address per interface.
You can specify a UDP destination port to control which UDP services are forwarded. You can specify
multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older
diskless Sun workstations, and you can specify the network security protocol, Software Defined
Network Service (SDNS). By default, both UDP and ND forwarding are enabled if a helper address has
been defined for an interface. The description for the ip forward-protocol global configuration
command in the Cisco IOS IPCommand Reference, Volume 1 of 3: Addressing and Services publication
lists the ports that are forwarded by default if you do not specify any UDP ports.
If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are
configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry Dynamic Host
Configuration Protocol (DHCP) information, which means that the Cisco IOS software is compatible
with DHCP clients. (DHCP is defined in RFC 1531.)
To enable forwarding and to specify the destination address, use the following command in interface
configuration mode:
To specify which protocols will be forwarded, use the following command in global configuration mode:
See the “Helper Addresses Example” section at the end of this chapter for an example of how to
configure helper addresses.
Command Purpose
Router(config-if)# ip directed-broadcast
[access-list-number]
Enables directed broadcast-to-physical broadcast translation
on an interface.
Command Purpose
Router(config-if)# ip helper-address address
Enables forwarding and specifies the destination address for
forwarding UDP broadcast packets, such as BOOTP and
DHCP.
Command Purpose
Router(config)# ip forward-protocol {udp [port] | nd
| sdns}
Specifies which protocols will be forwarded over which ports.