How do I send user broadcast traffic across 2 seperate networks

Unanswered Question
Jan 6th, 2009

I am very new to cisco routers about 1 week experience. I have been setting up a network for testing purposes and I am running into an issue with a single user broadcast message.

We have a software package that sends a broadcast message out on port 9198 and another software that when the message is received sends a message back saying who it is.

The network is set up as such

cisco switch(8 comps attached)

port 1 on switch--> cisco router port fa1/14

span port on fa1/15

comps on ports fa1/1-4

fa1/14 ip

fa1/2 ip

The computer sending the broadcast message is plugged into the switch and has ip, it has no problems with the broadcast getting to all computers on the switch but is unable to get it to the other network. All UDP/TCP/Multicast traffic is able to make it across to all the computers but not this broadcast message.

Again I am very new to cisco routers so if there is any other info needed please let me know.

FYI Wireshark on the spanport sees the broadcast message but wireshark on the fa1/2 port does not.

I have this problem too.
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Richard Burts Tue, 01/06/2009 - 13:16


There are a few aspects of your situation that are not clear in this post. Is the switch configured with multiple VLANs or just a single VLAN? Is the port from the switch to the router a trunk port or just an access port?

But since you say that UDP and TCP make it accross it seems that something is set up to handle this.

By default routers do not forward broadcasts outside of the subnet in which they originate. There is an exception to this which you can configure on the router. Assuming that the broadcast on 9198 is a UDP packet then you would configure this on the router:

ip forward-protocol udp 9198

ip helper-address

Give it a try and let us know if it works.



genzo1422 Tue, 01/06/2009 - 13:53


That worked for me but I have a question. I also want it to be able to go to computers on ports fa1/3-4 on the router. The IPs of them are and is there a way to have the helper address be to send to all of them or do the helpers have to all be added in manually.

The reason I ask is that the addresses on our network will be changing all the time and it would be nice to have a global change that would allow it.

Thank you a ton


Richard Burts Tue, 01/06/2009 - 14:08


I am glad that my suggestion worked for you. Unfortunately there is not a way to have the broadcast forwarded to every network. The closest that you can come would be to use a directed broadcast. To use this you would need a helper address for each subnet using the subnet broadcast address and you would also need a configuration statement on the router interface where that subnet was. The config might look something like this:

ip helper-address

ip helper-address

ip helper-address


and on the inteface where 192.168.x.1 is located you would configure ip directed-broadcast. Note that directed broadcast is not enabled by default since there is some security risk associated with it. If you want to use the feature it is something that you can enable.



carl_townshend Wed, 01/07/2009 - 04:54

Hi Rick

I thought the helper address allows the device to send a unicast to a dhcp server ?

Also why do yuo need to put in the

ip forward-protocol udp 9198 , command ?

what does it do ?



Giuseppe Larosa Wed, 01/07/2009 - 04:58

Hello Carl,

the ip helper-address allows to convert a broadcast to a unicast routable packet.

the second command is needed because udp port 9198 is not forwarded by default so that command instructs the router to perform that conversion

Hope to help


carl_townshend Wed, 01/07/2009 - 09:01

Hi there, can you explain a little more what the ip forward protocol does, why would we need to use it? are you saying when the router sees traffic from port 9198 it will send a broadcast out the other interface?

please explain

Giuseppe Larosa Thu, 01/08/2009 - 04:21

Hello Carl,

when you enable ip helper address x.x.x.x under a LAN interface the router looks for broadcast packets.

The default behaviuor is to relay to x.x.x.x if UDP ports are in a set of default enabled ports that include

TFTP udp 69

bootpc bootps udp 67 and 68

some ports for windows networking (136,137,139)

Relay means that the non-routable destination address is changed to x.x.x.x.

When there is the necessity to make ip helper address feauture to work on other UDP ports you need to use the ip forward-protocol command in global config.

The task of this command is to modify the set of ports for which ip helper-address commands will perform their relay function.

Hope to help



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