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Forward all broadcast traffic on cisco switch

Posted on 2010-01-11
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Last Modified: 2012-05-08
Hello everyone,

I have a 3650 switch that is configured with 3 vlans and we are having an issue with a server on vlan 5 being able to see broadcasts from vlan 10.  We have not been able to get accurate information from the vendor on which ports it is broadcasting on. For troubleshooting i want to just forward all broacast traffic to one particular ip on vlan 5.  I have looked at ip-forward protocol but it seems to just deal with UDP traffic and does not give you a way to forward all broadcast traffic.

Waht is the command to forward all broadcast traffic from one vlan to another?

Thanks in advance
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Question by:jdflory
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10 Comments
 
LVL 50

Expert Comment

by:Don Johnston
ID: 26288905
>Waht is the command to forward all broadcast traffic from one vlan to another?

You can't. That's one of the features of routing. The IP helper address will let you forward certain broadcasts. But as you've discovered not all broadcasts.

>we are having an issue with a server on vlan 5 being able to see broadcasts from vlan 10

This one is puzzling. Unless the "ip helper address" command is configured on the VLAN 10 SVI, there's no way for broadcast traffic to get from VLAN 5 to VLAN 10.

If this is really happening, I would look for a layer 2 link between the VLANs somewhere. Maybe a cable connected where it shouldn't be.
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Author Comment

by:jdflory
ID: 26288964
Currently i have an ip helper address configured on vlan 10 to point to a server on vlan 5.  This is what i am using for DHCP requests witch works great.  I was using this same ip helper address to configure the ip-forward protocol which i found only works for UDP boadcasts.  

So are you telling me there is no way for forward broadcasts to vlan other then UDP?
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LVL 20

Expert Comment

by:MightySW
ID: 26289184
Hi, you are correct.

The ip helper address command will by default only forward UDP broadcasts from:

69, 67, 68, 37, 49, 53, 137, and 138

here is a command you use in global configuration mode that the ip helper-address command will consult for more specific instructions.  It is the ip forward-protocol command.  So use it for specific ports that you want to forward UDP broacast over.

HTH
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Author Comment

by:jdflory
ID: 26289192
I tried that but the port in question is TCP so it does not work in this particular situation.
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Expert Comment

by:MightySW
ID: 26289233
Sorry, I completely missed that.

Try removing the ip helper address command and ensure that the broadcast cease.

If they don't then it HAS to be a physical issue from one VLAN to another.  Do you possibly have a workstation on a port that has an ALLOW statement in it?  Or a workstation with 2 NIC's connected to separate switches/VLANs?

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Author Comment

by:jdflory
ID: 26289381
Sorry i think i may be confusing the matter.

I am not trying to stop the broadcasts.  I am trying to enable forwarding of broadcasts from one vlan to another vlan.  Currently broadcasts are contained as they should be.   I want broadcasts for vlan 10 to reach a particular host on vlan 4.  I have an ip-helper address and have tried ip-forward protocol which only works for UDP traffic.  Is there a command that will forward all broadcast traffic to one particular host on another node?
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Accepted Solution

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Don Johnston earned 1000 total points
ID: 26289465
Other than the ip helper forwarded broadcasts (UDP 67, 68, 69, etc.), you can not forward broadcasts through a router. That's just the nature of routers.

No way around it.

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LVL 20

Assisted Solution

by:MightySW
MightySW earned 1000 total points
ID: 26289518
Agreed.  designed to forward UDP Broadcasts for BOOTP functionality, etc..  You cannot route a TCP broadcast.  There is no command because it literally is not developed to work like that.  As Dj stated, its the nature of routers and how TCP AND UDP protocols operate over layer 3.  

Sorry for the confusion.  I thought you were actually receiving TCP broadcasts from one of your other subnets.   This would have been pretty hard to swallow.
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LVL 20

Expert Comment

by:MightySW
ID: 26289566
I would say that the only thing that would work is a multicast, but this would be difficult and may not be what you want as this is application specific over IGMP.

Thanks
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Author Comment

by:jdflory
ID: 26289696
Thanks guys for clarifying this for me.  We have an application that seems to be broadcast dependant so my only choice maybe to drop the server into the workstation vlan.
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