WOL - why do i need .255 broadcast address in the vlan of the sccm 2012 server

Hi - I'm having trouble setting up WOL for scccm 2012.

It was suggested I require ip-helpers to in the VLAN config on the VLAN that the sccm server is in (sender) -  1 for each vlan broadcast address to) to the .255 broadcast address.

Can someone please explain why i need these (see example config below with ip-helpers in  quotes "") My understanding of ip-helper is along lines of client (dhcp for eg) finds ip-helper in vlan to allow broadcast to go to dhcp server accross vlan.

interface Vlan20
description 1st Floor Vlan Wake on Lan Client
ip address
ip directed-broadcast 100

interface Vlan30
description 2nd Floor Vlan Wake on Lan Client
ip address
ip directed-broadcast 100

interface vlan40
description Wake on Lan Server
ip address
"ip helper-address"
"ip helper-address"


Can someone please give me explanation.
this in 3750 cisco router
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A WOL frame is a broadcast frame, as such the sending host puts the IP broadcast address for the subnet the sender is on.

Since you are trying to send the WOL to a host in a different subnet, you use the ip helper command to change the IP broadcast address.

Say your source host is and mask is  The broadcast would be  If you want to send this to a host on or subnet, it would be ignored by all hosts because is not the broadcast address for those subnets.  The ip helper address will cause the router to send out the WOL frame using the dest IP address of and
philb19Author Commented:
ok great thanks clear explanation. I have a Cisco engineer who is saying Its not required - and he has setup  WOL without it. - stating it is is dangerous? - allowing broadcast

I initially put in as suggested config and - as it does not work without it i imagine i was right.

I imagine with each vlan having ip directed-broadcast 101 as an ACL it will only allow broadcast from the sccm server
I will double check, I have never had to set this up personally, just helped others do it.  However, I'm not sure how it would work without.

Without that the WOL magic packet will NOT be forwarded to the other VLAN's.

Allowing it can cause problems.  Thinks about it, in your setup L2/L3 broadcast frame on VLAN 40 is now forwarded to both VLAN 30 and VLAN 20 no matter what.  That could be a ton of traffic or next to nothing.

However, as you stated, you can reduce the possibility of problems by creating an ACL to limit the source IP address(es) that will trigger the process.

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If the packets need to be sent to broadcast address, there MUST be some kind of broadcast forwarding or relaying if the computer that sends the magic packets is on another subnet than the computers to be awaken. Broadcasts are not supposed to be transmitted from one subnet to another. I would not forward all broadcasts between 2 subnets, this is not desirable.
You should check what kind of packets are sent by your WOL sender and forward only said packets (for instance UDP port 7 or 9).
Use a network packet capture tool (WireShark or MS Netmon), record a trace when the sender and receiver are on the same subnet and make some assumptions from there that will allow you to set the correct rules so that you don't forward all broadcasts from sender's subnet to receivers' subnets.
Another way around is to use Subnet directed broadcasts or a WOL proxy/Relay such as http://www.mylanviewer.com/wake-on-lan-proxy-server.html .

There are several types of WOL packets:
Magic Packets (described in this article in particular):
Pattern Match:

Some useful links:
philb19Author Commented:
thanks to all posts -just to add here it does work with the .255 to all vlans. - I do have
an ACL allowing  only sccm to direct-broadcast - and  sh access-list has hit-count 4 on the udp port 9 - all other ACE's to other ports have hit of 0 - so i will prune the acl I guess
You can run a packet capture tool on one of the receivers subnets to check that you have not more forwarded broadcasts than what you want.
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