UDP broadcast forwarding by Cisco's IP Helper

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Article ID: 190930 - View products that this article applies to.
This article was previously published under Q190930
Notice
This article applies to Windows 2000. Support for Windows 2000 ends on July 13, 2010. The Windows 2000 End-of-Support Solution Center is a starting point for planning your migration strategy from Windows 2000. For more information see the Microsoft Support Lifecycle Policy.
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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.

CAUSE

IP Helper has been enabled on a Cisco switch.

RESOLUTION

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

STATUS

This behavior is by design.

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.

Properties

Article ID: 190930 - Last Review: January 17, 2008 - Revision: 5.2
APPLIES TO
  • Microsoft Windows NT Server 4.0, Terminal Server Edition
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows 2000 Datacenter Server
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Workstation 3.1
  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Server 4.0 Enterprise Edition
  • Microsoft Windows 95
  • Microsoft Windows 98 Standard Edition
  • Microsoft Windows Server 2003, Standard Edition (32-bit x86)
  • Microsoft Windows Server 2003, Enterprise Edition (32-bit x86)
  • Microsoft Windows Server 2003, Enterprise x64 Edition
  • Microsoft Windows Server 2003, Standard x64 Edition
  • Microsoft Windows XP Professional
  • Microsoft Windows XP Home Edition
Keywords: 
kbnetwork kbprb KB190930

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