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Cisco IOS in a Nutshell, 2nd Edition by James Boney

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Name

ip helper-address — interface

Synopsis

ip helper-address address
no ip helper-address address

Configures

IP address to which certain broadcast UDP packets are forwarded

Default

Disabled

Description

This command sets the helper address to address. The helper address should be the address of a host that can answer UDP requests from other hosts. The router sees these requests broadcast on a LAN interface and forwards them to the helper address (generally a unicast address) if one is defined. A helper is particularly useful for DHCP requests; without some kind of forwarding, DHCP requires you to have a separate server on every subnet. By itself, this command forwards packets for the BOOTP (DHCP), DNS, TFTP, TACACS, TIME, and NetBIOS name and datagram services. The ip forward-protocol command can be used to forward additional UDP services.

Example

To configure interface ethernet0 to have a helper address:

interface ethernet0
  ip address 10.10.1.2 255.255.255.0
  ip helper address 10.10.2.5

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