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Other IP Commands
DHCP Relay Functionality
The DHCP Relay functionality is applied with the help of IP broadcast forwarding. A typical
situation, as shown in Figure
located on that segment.
Router 1 can forward the DHCP request (1) to the server located on N2, if IP forward‐protocol is
enabled for UDP, and the address of the DHCP server is configured as a helper address on the
receiving interface of Router 1. The DHCP Relay function will detect the DHCP request and make
the necessary changes to the header, replacing the destination address with the address of the
server, and the source with its own address, and send it further (2) to the server. When the
response (3) comes from the server, the DHCP Relay function sends it to the host (4).
ip helper-address
This command enables forwarding of local broadcasts specifying the new destination address. It is
one of two commands used for UDP broadcast forwarding. Also refer to the
protocol
than one helper address per interface. The command is also used to enable BOOTP Relay.
Syntax
ip helper-address address
address
Syntax of the "no" Form
The no form disables the forwarding of broadcast packets to the specified address:
no ip helper-address address
Mode
Interface configuration:
Example
In this example, with one server on network 191.168.1.255 and the other on network 192.24.1.255,
you permit UDP broadcasts from hosts on either network segment to reach both servers:
5-158 Configuring the Internet Protocol
Figure 5-7
N1
Router 1
DHCP
1
Host
eth 1
Function
4
Global Configuration
ip forward-protocol UDP
Destination
255.255.255.255
ip helper-address address1
Source: 0.0.0.x
command which defines the forward protocol and port number. You can add more
Destination broadcast or host address used when forwarding.
XSR(config-if<xx>)#
5‐7, occurs when a Host requests an IP address with no DHCP server
DHCP Functionality Example
INTERNET
Relay
interface ethernet 1
Router 2
2
addr1
3
Server
ip forward-
N2
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