If you aren't on the same subnet, you will need an 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. |
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
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. Because 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.
All of the following conditions must be met in order for a UDP or IP packet to be helpered by the ip helper-address command:
•
The MAC address of the received frame must be all-ones broadcast address (ffff.ffff.ffff).
•The IP destination address must be one of the following: all-ones broadcast (255.255.255.255), subnet broadcast for the receiving interface; or major-net broadcast for the receiving interface if the no ip classless command is also configured.
•The IP time-to-live (TTL) value must be at least 2.
•The IP protocol must be UDP (17).
•The UDP destination port must be for TFTP, Domain Name System (DNS), Time, NetBIOS, ND, BOOTP or DHCP packet, or a UDP port specified by the ip forward-protocol udp global configuration command.
Note The ip helper-address command does not work on an X.25 interface on a destination router because the router cannot determine if the packet was intended as a physical broadcast.
Examples
The following example defines an address that acts as a helper address:
interface ethernet 1
ip helper-address 121.24.43.2
Related Commands
