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UDP broadcast forwarding by Cisco's IP Helper


View products that this article applies to.

Symptoms

User Datagram Protocol (UDP) broadcasts are transformed into directed datagrams. Upon inspection of the media access control address, network packets have been transformed from the expected broadcast address (FFFFFFFF) to a specific media access control address. An event that resembles the following may be logged in the System log of the Master Browser:
Event Type: Error
Event Source: MRxSmb
Event Category: None
Event ID: 8003
User: N/A
Computer: DC-01
Description: The master browser has received a server announcement from the computer <Browser-Server-in-other-subnet> that believes that it is the master browser for the domain on transport NetBT_Tcpip_{EADD435F-5125-4F5. The master browser is stopping or an election is being forced. For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Data: (shown in Words) 0000: 00000000 004e0003 00000000 c0001f43 0010: 00000000 00000000 00000147 00000000 0020: 00000000 00000000
Note Status code "c0001f43" translates to EVENT_BOWSER_OTHER_MASTER_ON_NET.

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Cause

IP Helper has been enabled on a Cisco switch.

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Resolution

Disable IP Help either completely or on a per UDP port basis.

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Status

This behavior is by design.

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More information

Typically, a broadcast should be confined within its IP subnet. However, notable exceptions occur such as a DHCP (BootP) broadcast. Cisco has provided functionality in some of its router products called IP Helper to forward specific broadcast to designated computers. For example, you could configure all DHCP broadcasts to be directed to the specified DHCP server. The thinking behind this is a directed datagram will decrease the network traffic when compared to a broadcast packet propagated to all subnets.

If an IP helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default.
   Time Service             Port 37
   TACACS                   Port 49
   Domain Name Services     Port 53
   Trivial File Transfer    Port 69
   DHCP (BootP)             Port 67 and Port 68
   NetBIOS Name Server      Port 137
   NetBIOS Datagram Server  Port 138
				
The forwarding of NetBIOS broadcasts on UDP port 138 can impact the ability of the browser service to properly elect a segment master browser or a domain master browser. Disabling the forwarding of broadcast packets on ports 137 and 138 enables the browser service to work as designed.

You can use the Browstat command prompt utility to investigate browsing errors. For more information about browser errors, click the following article number to view the article in the Microsoft Knowledge Base:
188305 Troubleshooting the Microsoft Computer Browser service
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
135464 8003 browsing errors with UDP forwarding
The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.

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Keywords: kbnetwork, kbprb, KB190930

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Article Info
Article ID : 190930
Revision : 7
Created on : 1/17/2008
Published on : 1/17/2008
Exists online : False
Views : 114