Showing results for 
Search instead for 
Did you mean: 

Directed broadcast and unicast

Hi all , 

below is an excerpt from the link

A device that is not directly connected to its destination subnet forwards an IP directed broadcast in the same way it would forward unicast IP packets destined to a host on that subnet. When a directed broadcast packet reaches a device that is directly connected to its destination subnet, that packet is broadcast on the destination subnet. The destination address in the IP header of the packet is rewritten to the configured IP broadcast address for the subnet, and the packet is sent as a link-layer broadcast.

here is my question 


When server on serverfarm switch sending a wol packet ( ip directed broadcast ), it would forward like a unicast packet . 
The unicast packet will be routed through the core to the distribution . So as i understand i don't need to do any configuration changes on the core .
The  changes required only on distribution and serverfarm .

Please correct me i am wrong . 

My configurations are below 

Interface vlan 10
Ip add


Interface vlan 100
Ip add


Serverfarm switch

interface vlan 10
ip helper-address


Distribution switch 

access-list 102 permit udp host any eq 7
ip forward-protocol udp 7

interface vlan 100 
ip directed broadcast 102


Richard Burts
Hall of Fame Guru

Assuming that the link between core and distribution is a routed link and that routing is enabled on the distribution switch then your config looks pretty good except that the command "ip forward-protocol udp 7" goes on the serverfarm and not on the distribution switch.







Thanks Rick


Serverfarm switch

interface vlan 10
ip helper-address

Do i really need ' ip helper-address '  on interface vlan 10 (wol server vlan in  serverfarm switch )  ? .
i did a test without   'ip helper-address' , it did work. 



Serverfarm switch

interface vlan 10
ip helper-address





If your DHCP client and server in same vlan then you dont need " ip helper-address" command. But if you client and server is in different subnet then you need to have this command.


*Plz rate the usefull posts *

Thanks for the reply 

I am talking about the ' wol' . Wol server and destinations are in different vlan . But without '  ip helper-address ' at destination side PC are getting WOL packets. This is confuisng me 


Thanks again

Most command-line tools to generate the WoL Magic Packets send them as UDP datagrams and set the destination IP to the limited broadcast address, thus those Magic Packets are never routed (Scope = local subnet).

Thats why you need in this case the ip-helper command, which converts (local) UDP broadcasts of serveral well-known protocols (DHCP, TFTP, DNS, NetBIOS, TACACS) into unicasts and then forwards them to the helper address.

Since the portnumbers typically used by WoL tools (7, 9, 12287 [0x2FFF], ...) are different from those used by the well-known protocols, you'd also need the 'ip forward-protocol udp <number>' command to make it work.


Some tools, like SolarWinds Wake-On-LAN, allow you to send IP-Packets immediately to a directed broadcast IP outside the local subnet. In this case it is sufficient to allow directed broadcasts on the client's SVI(s). For security reasons, you should limit this with an ACL to the WoL server IP and portnumber:

access-list 100 permit udp host <server-ip> any eq <port-number>
interface Vlan <client-vlan>
 ip directed-broadcast 100


If the tool doesn't allow to set the destination IP to a directed broadcast, you'll need some additional configuration:

ip forward-protocol udp <port-number>
interface Vlan <WoL-server>
 ip helper-address <directed BC-address client-vlan>






I agree with Rolf that the explanation for this depends on the capability of your server. Many WOL servers send their packets only to the local broadcast address. For these you do need the helper address. If your server is capable of sending to the remote broadcast address then you do not need the helper address. 







We have a similar setup as above where,

a) Server Farm switch is Nexus 5K/FEX,

b) Core Switches are a pair of Nexus 7K,

c) Distribution Switches are a pair of 4500X

d) and Access Switches are 3650.

Routing is only implemented on the Core Nexus 7K, the links between Core/Distribution and Distribution/Access are all Layer 2.

Does similar configs apply in this scenario and does Nexus 7K support forwarding of directed broadcast packets on 6.2 release.

Thanks and Regards,