ip helper-address

ip helper-address

To have the Cisco IOS software forward User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address interface configuration command. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.

ip helper-address address
no ip helper-address address

Syntax Description

address

Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.

Default

Disabled

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 10.0.

Combined with the ip forward-protocol global configuration command, the ip helper-address command allows you to control which broadcast packets and which protocols are forwarded.

One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP), which is defined in RFC 1531. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Since BOOTP packets are forwarded by default, DHCP information can now be forwarded by the router. The DHCP server now receives broadcasts from the DHCP clients.

Note    The ip helper-address command does not work on an X.25 interface on a destination router because the router cannot tell if the packet was intended as a physical broadcast.

Example

The following example defines an address that acts as a helper address:

interface ethernet 1

ip helper-address 121.24.43.2

Related Command

ip forward-protocol