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Disable udp on ports 137 and 138 on cisco 1700

Posted on 2004-03-22
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Last Modified: 2007-12-19
Hi all. I've have a problem with workstations thinking they are the masterbrowser for my Windows 2000 domain. This is happening because I'm using ip helper addresses to broadcast dhcp requests on the router that connects another network on the same subnet . As a result, netbios udp broadcasts on ports 137 and 138 are also being forwarded, causing the master browser issue. What is the command to disable udp or broadcast forwarding on specific ports?
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Question by:bluespringsit
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Expert Comment

by:Netman66
ID: 10653625
Instead of using a helper address why not use a DHCP Relay agent on the subnet without the DHCP server?

This would be a better option - stops broadcast storms.

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Expert Comment

by:parkerig
ID: 10653908
this is a real simple example I am giving so modify it as you see fit.

on cisco router
create an access list like

deny udp 137
deny udp 138
permit all

apply this access list to the ethernet port ( or whichever port connects lan / wan)

Cheers
Ian
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lrmoore earned 500 total points
ID: 10654010
The ip helper-address interface commands opens up those udp ports. You can close them back down easily enough without an access-list

router(config)#no ip forward-protocol udp 137
router(config)#no ip forward-protocol udp 138

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by:Netman66
ID: 10654047
lrmoore,

Perfect!

I still think a relay agent would work too, but your answer is simpler.

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Author Comment

by:bluespringsit
ID: 10659426
Perfect, thank you lrmoore and everyone else. I went with the no ip forward commands instead of a relay agent. Theres only two computers on this other segment. Thanks again!
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