UDP Broadcast Forwarding by Cisco's IP Helper

ID: Q190930

The information in this article applies to:

Microsoft Windows NT operating system version 3.1
Microsoft Windows NT Advanced Server version 3.1
Microsoft Windows NT Server versions 3.5, 3.51, 4.0
Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0
Microsoft Windows NT Server 4.0, Terminal Server Edition
Microsoft Windows NT Server, Enterprise Edition version 4.0
Microsoft Windows versions 95, 98

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 will allow the browser service to work as designed.

For related information, please see the following article in the Microsoft Knowledge Base: 8003 Browsing Errors with UDP Forwarding

The third-party products discussed here are manufactured by vendors independent of Microsoft; we make no warranty, implied or otherwise, regarding these products' performance or reliability.

Additional query words: ethernet helper forward 8003 missing event events

Keywords : kbnetwork
Version : WINDOWS:95,98; winnt:3.5,3.51,4.0
Platform : WINDOWS winnt
Issue type : kbprb