WOL Between Subnets: IP Helper-Address??

[graphictech3]

I'm trying to setup WOL between two different subnets in an environment using cisco switches. I'm following along with some cisco instructions on the matter.

However, one thing that I'm unclear with, in this example, and others I've found on the web, they all set the IP helper address. In this example, the ip helper-address is the same as the DHCP server, which is performing the WOL. I understand why you need an ip helper-address between vlans for a DHCP server. However, why do you need this for wake on lan?

[Charles Hill]

Hello,

Without the ip helper-address on vlan 3, which is where the WOL server resides, the WOL packets would not be forwarded to the clients on the other vlans.

"Enables forwarding of WoL packets to clients. "

Hopefully this answered your question.

Please rate helpful answers.

Thanks.

[graphictech3]

That's embarrasing. I didn't look at the example I linked close enough. I didn't realize it's using different helper-addresses for different purposes.

It uses ip helper-address for DHCP on two of the VLANs
L3(config-if)#ip address 172.16.4.1 255.255.255.0

But it also uses ip helper-address to broadcast the network traffic to another subnet for WOL L3(config-if)#ip helper-address 172.16.2.255 L3(config-if)#ip helper-address 172.16.4.255

[Rolf Fischer]

Brett,

apart from DHCP-broadcasts, the ip helper-address command also converts (local) UDP broadcasts of serveral other well-known protocols (TFTP, DNS, NetBIOS, TACACS) into unicasts and then forwards them to the helper-address.

Similar discussion:

https://supportforums.cisco.com/discussion/12423626/directed-broadcast-and-unicast

HTH

Rolf