"Path Not Found" When Using 3Com LANPLEX 2500 Router

• Microsoft Windows NT Server versions 3.5, 3.51, and 4.0
• Microsoft Windows NT Workstation versions 3.5, 3.51, and 4.0
• Microsoft Windows 95

SYMPTOMS

When you attempt to connect a Windows NT Server or Workstation computer from one IPX network across a 3Com LANplex 2500 Router (running 8.115 microcode) to another Windows NT Server or Workstation computer located on a remote IPX network you will encounter the error:

   Path not found.

RESOLUTION

Contact 3Com for its latest version of microcode for this router which includes a fix for the problem.

MORE INFORMATION

Windows 95 and Windows NT handle netbios over IPX or IPX packet type 20's differently. In the following example, the Windows 95 client places it's current network number in the Find Name netbios broadcast packet to discover the netbios name of the Microsoft networking client (that is 27202.FFFFFFFFFFFF.455). The Windows NT Server or Workstation places a zero (0) in front of the broadcast packet rather than the network number (0.FFFFFFFFFFFF.455).

The following example came from an actual trace using Netmon:

Windows 95

IPX: NetBIOS Packet - 27202.00C04FD7ACBC.455 -> 27202.FFFFFFFFFFFF.455 - 0 Hops

Windows NT 4.0

The Windows NT server/client precedes the Find Name netbios packet from the client to the network with a zero (0) followed by the broadcast.

IPX: NetBIOS Packet - 2B9659.000000000001.455 -> 0.FFFFFFFFFFFF.455 - 0 Hops

By definition of Novell's IPX routing specification, the router should not care whether the netbios broadcast packet is preceded by a network number or a zero. This should be true when you set up your router to bridge netbios packets over IPX (packet type 20's). Bridging, by definition, should take the packet from one network and propagate it on another network.

The 3Com LANplex 2500 router, with microcode version 8.115, kills the Find Name netbios broadcast packet from a Windows NT Server or Workstation because the packet is preceded by a zero (0) network number. In other words, the 3Com LANPlex 2500 router incorrectly handles a netbios packet from a Windows NT Server or client.

Here is an excerpt from Novell's IPX routing specification:

   When a Type 20 broadcast packet is received by a router (this is
   indicated by a Packet Type of 14h in the IPX header,) the following
   sequence of events should occur:

    * The Transport Control Field of the IPX header is examined; this value
      indicates the number of routers traversed by the packet so far. This
      field also indicates how many Network Number fields in the packet
      have been filled in. If this value is 8 or greater the packet is
      discarded, otherwise proceed to the next step.

    * The router compares each Network Number entry in the packet to the
      Network Number of the segment on which the router received the
      packet. If a match is found, the packet is discarded to prevent
      multiple traversals of the same network segment; if no match is
      found, proceed to the next step.

    * The router places the address of the network segment on which the
      packet arrived into the next available Network Number field. The
      offset of this field is easily calculated as 4 * n bytes past the end
      of the IPX header, where n is the value of the Transport Control
      Field.

    * The router increments the Transport Control Field of the IPX header
      and broadcasts the packet to all directly connected network segments
      that are NOT included in the Network Number fields.