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IP helper and Master browser election

We have ip helper-address enabled on our 6509 msfc so we can get DHCP. However I am seeing a lot of Master Browser elections. I have seen on Microsoft article to turn off other broadcast that is turn on by default by the IP helper-address command. The article said to add this command:

no ip forward-protocol udp netbios-ns

no ip forward-protocol udp netbios-dgm

IS this correct? Is there a ciso article about this?

thanks,

gilbert

1 ACCEPTED SOLUTION

Accepted Solutions
Gold

Re: IP helper and Master browser election

Gilbert,

Here's a link to some useful information on Cisco's web site:

Windows Networking Design Implementation Guide

http://www.cisco.com/en/US/tech/tk870/tk877/tk880/technologies_tech_note09186a00801aa01f.shtml

Basically, when you use "ip helper-address" on an interface, it forwards several UDP broadcasts by default. Among these are ports 137 and 138, which are used by Microsoft Networking.

Many people want their routers to forward just BOOTP which also takes care of DHCP requests. If you search the document at the link above for "no ip forward-protocol" you will find all the commands you need to stop forwarding everything except BOOTP.

CAUTION: In this document, the command line to stop UDP port 53 broadcasts is incorrect. Instead of reading "no ip forward-protocol udp dns" it should read "no ip forward-protocol udp domain".

Also, while that example shows the command to make sure BOOTP Client (bootpc) is forwarding, it does not mention that BOOTP Server (bootps) UDP broadcasts are also forwarded by default. However, as long as you do NOT issue a "no ip forward-protocol" command to prevent that, it will forward.

You can issue the commands to disable forwarding of UDP broadcasts by port number rather than by name; but if that number also has a "well-known" name, then that's what will show up in your configuration.

Hope this helps.

2 REPLIES
New Member

Re: IP helper and Master browser election

Not sure if there is any Cisco info but we had the same issues. After implementing those commands the problem was fixed.

Peter

Gold

Re: IP helper and Master browser election

Gilbert,

Here's a link to some useful information on Cisco's web site:

Windows Networking Design Implementation Guide

http://www.cisco.com/en/US/tech/tk870/tk877/tk880/technologies_tech_note09186a00801aa01f.shtml

Basically, when you use "ip helper-address" on an interface, it forwards several UDP broadcasts by default. Among these are ports 137 and 138, which are used by Microsoft Networking.

Many people want their routers to forward just BOOTP which also takes care of DHCP requests. If you search the document at the link above for "no ip forward-protocol" you will find all the commands you need to stop forwarding everything except BOOTP.

CAUTION: In this document, the command line to stop UDP port 53 broadcasts is incorrect. Instead of reading "no ip forward-protocol udp dns" it should read "no ip forward-protocol udp domain".

Also, while that example shows the command to make sure BOOTP Client (bootpc) is forwarding, it does not mention that BOOTP Server (bootps) UDP broadcasts are also forwarded by default. However, as long as you do NOT issue a "no ip forward-protocol" command to prevent that, it will forward.

You can issue the commands to disable forwarding of UDP broadcasts by port number rather than by name; but if that number also has a "well-known" name, then that's what will show up in your configuration.

Hope this helps.

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