KB190930 - UDP Broadcast Forwarding by Cisco's IP Helper

Article ID: 190930

Article Last Modified on 1/23/2007

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

This article was previously published under Q190930

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.

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.

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

135464 8003 Browsing Errors with UDP Forwarding

The third-party products that are discussed in this article are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, regarding the performance or reliability of these products.

Additional query words: ethernet helper forward 8003 missing event events

Keywords: kbnetwork kbprb KB190930