IP Broadcast Settings
Fixed Switch Configuration Guide 20-9
The no form of the ip forward-protocol command removes a UDP port or protocol, disabling
forwarding.
DHCP and BOOTP Relay
DHCP/BOOTP relay functionality is applied with the help of UDP broadcast forwarding. A
typical situation occurs when a host requests an IP address with no DHCP server located on that
segment. A routing device can forward the DHCP request to a server located on another network
if:
• UDP broadcast forwarding is enabled
• The address of the DHCP server is configured as a helper address on the receiving interface
The DHCP/BOOTP relay agent function will detect the DHCP request and make the necessary
changes to the packet, including:
• Replacing the destination IP address with the address of the DHCP server,
• Replacing the source IP address with its own address (that is, the IP address of the local
routed interface), and
• Within the BOOTP part of the packet, changing the Relay Agent IP address from 0.0.0.0 to the
address of the local routed interface.
The last change to the BOOTP packet “tells” the DHCP server that it needs to assign an IP address
that is in the same subnet as the Relay Agent IP. When the response comes from the server, the
DHCP/BOOTP relay agent sends it to the host.
Use the ip helper-address command in conjunction with the ip forward-protocol command to
configure DHCP/BOOTP relay functionality to the specified server(s). Up to 6 IP helper addresses
may be configured per interface.
IP Broadcast Configuration
Procedure 20-5 describes how to configure IP broadcast.
4011 Alternate Service Boot
Table 20-2 UDP Broadcast Forwarding Port Default (continued)
Port Number Protocol
Procedure 20-5 Configuring IP Broadcast
Step Task Command(s)
1. In interface configuration command mode,
enable IP directed broadcasts on an interface.
ip directed-broadcast
2. In interface configuration command mode,
enable UDP broadcast forwarding and specify
the destination port number that controls the
forwarding protocol. Refer to Table 20-2 on
page 20-8 for a list of default port numbers.
ip forward-protocol udp [port]
3. In interface configuration command mode,
optionally, enable DHCP/BOOTP relay and the
forwarding of local UDP broadcasts, specifying a
new destination address.
ip helper-address address
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