UDP Broadcast across WAN...

Unanswered Question
Jul 8th, 2008

Recently our company has purchased a new ethernet capable time clock system. I've read the manual concering the time clock software and tested in a lab environment using a laptop and small switch, which works fine. However, upon real implementation, I am having some issues having the software see the clock over the WAN. Obviously the only variable here is the WAN. I have two Cisco 2621 routers running IOS 12.0 at each site connected via a T1. We are not doing any type of blocking as this is a point to point connection. I contacted the time clock tech support and they informed me the reason the software would not see the time clock over the WAN was a networking infrastructre issue as by default UDP broadcast are disabled over a WAN link. The manual states "The WAN must allow for UDP broadcasts (via on Port 30718, and TCP/IP on Port 10001. How would I go about allowing UDP broadcase (via across the WAN? That seems kinda crazy to me to even do that as it creates massive traffic but I have to do it initially to get the software to see the clock. Any help would be greatly appreciated.

I have this problem too.
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Richard Burts Tue, 07/08/2008 - 12:09


There is a solution for forwarding UDP broadcasts over a WAN. The solution has 3 commands. You would use the command ip forward-protocol to identify the UDP traffic by port which should be forwarded. So to forward UDP port 30718 you would configure:

ip forward-protocol udp 30718

the second command is ip helper-address which identifies the address to which you forward the broadcasts. If you want to forward to the broadcast address of subnet you would configure on the interface where the broadcast is received:

ip helper-address

and the third command is to permit directed broadcasts, which are not enabled by default. On the interface where the broadcasts are forwarded to you would configure:

ip directed-broadcast.

Note this issue is not just about sending broadcasts over a WAN. Broadcasts are a "local" traffic and routers do not forward broadcasts from one subnet into other subnets and this is the case for both LAN and WAN.



ccosper08 Tue, 07/08/2008 - 13:10

Ok, let me make sure I have this correct. The software is on network The clock is on network So on the router of the 3.0 network which is sending the UDP broadcast I would type ip forward-protocol udp 30718. I would also add ip helper-address to the Fastethernet0/0 which has an ip of I would not add it on Serial0/0 interface right? At the other side of the WAN, would I add ip directed-broadcast to the FastEthernet0/0 which has ip or the Serial0/0 interface? Thanks for all the help.

Richard Burts Tue, 07/08/2008 - 17:46


If the software on is sending the UDP broadcasts then that is the router on which you configure ip forward-protocol. And if the LAN interface FA0/0 is where the broadcasts are received that is where you configure ip helper-address and not on the serial interface.

Beyond that perhaps we can get some clarification and maybe I can simplify a bit what you need to configure. I gave my suggestions based on the assumption that you wanted to forward the UDP broadcasts as broadcasts on the destination network. But if you need to forward these packets to the clock and not necessarily to the entire subnet and if you know the IP address of the clock then we do not need to use directed broadcasts. In that case instead of using in the helper-address command use 192.168.1..

This will forward the requests as unicast. That means that you do not need ip directed-broadcast on FA0/0 of the router with the clock. And it would be better because you would not be forwarding broadcasts on that router. And reducing broadcast traffic on the router is a good thing.

But if you keep the directed broadcast then it goes on FA0/0 and not on the serial interface.




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