How IPX Routing Works

NetWare routers interconnect different IPX network segments and receive instructions for addressing and routing packets between these segments from the IPX protocol. IPX accomplishes these and other Network-layer tasks with the help of RIP, SAP, and NLSP.

For more information about how IPX routing works, refer to:

• When a Workstation Sends an IPX Packet
• When a Router Receives an IPX Packet
• When a Router Forwards an IPX Packet

When a Workstation Sends an IPX Packet

Consider a NetWare workstation that wants to send data to another workstation. If both workstations share the same network number (both are on the same segment), the sending workstation addresses and sends packets directly to the destination workstation's physical address. If the two workstations have different network numbers (each is on a different segment), the sending workstation must first find a router on its own segment that can forward packets to the segment on which the destination workstation resides.

To find this router, the sending workstation broadcasts a RIP packet requesting the fastest route to the destination segment. The router on the sending segment with the shortest path to the destination segment responds to the request. In its response, the router includes its own network and node address in the IPX header.

NOTE:  If the sending node is a router instead of a workstation, the router does not need to broadcast a RIP request to obtain this information; the router obtains the information from its internal routing table.

When the sending workstation knows the router node address, it addresses and sends packets to the destination workstation as follows:

1. The sending workstation places the destination node IPX network address---network, node, and socket numbers---in the corresponding destination fields of the IPX header.
2. The sending workstation places its own IPX network address---network, node, and socket numbers---in the corresponding source fields of the IPX header. The sending workstation also fills out all other fields in the header.
3. The sending workstation places the node address of the router that responded to the RIP request in the Destination Address field of the MAC header.
4. The sending workstation places its own node address in the Source Address field of the MAC header.
5. The sending workstation sends the packet.

When a Router Receives an IPX Packet

When a router receives an IPX packet, it performs the following tasks:

1. The router checks the Transport Control field of the IPX packet header.

   A RIP router discards the packet if the value in this field is greater than 16.

   An NLSP router discards the packet if the value in this field is greater than the value of the Hop Count Limit parameter.

2. The router checks the IPX header Packet Type field.

   Note that if the packet type is 20 (0x14, NetBIOS), the packet is handled as follows:

   1. The router examines the Transport Control field of the IPX header. If this value is 8 or greater, the router discards the packet. (Propagation of a NetBIOS packet is limited to eight networks.)
   2. 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 the router finds a match, it discards the packet to prevent multiple traversals of the same network segment. If the router finds no match, it performs the next step.

   3. The router places the address of the network segment from which the packet arrived in the next available Network Number field.
   4. The router increments the Transport Control field of the IPX header and broadcasts the packet to all directly connected network segments that are not represented in the Network Number fields.
3. The router checks the IPX header Destination Address fields---network, node, and socket numbers---to determine how to route the packet.

   If the packet is addressed to the router, the appropriate socket process handles it internally; otherwise, the router forwards the packet.

   NOTE:  Packets that must be handled internally are those addressed directly to the router and those broadcast (destination node = 0xFFFFFFFFFFFF) to any network segment to which the router is directly connected. Usually, only RIP, SAP, and diagnostic packets fall into this category.

When a Router Forwards an IPX Packet

When forwarding packets, the router can take one of two possible actions. If the packet is destined for a network number to which the router is directly connected, the router performs the following tasks:

1. The router places the destination node address from the IPX header in the Destination Address field of the MAC header.
2. The router places its own node address in the Source Address field of the MAC header.
3. The router increments the Transport Control field of the IPX header and forwards the packet to the destination node segment.

   IMPORTANT:  If the Transport Control field equals the maximum allowed hop count before the field is incremented, the router discards the packet. For RIP routers, the hop count limit is 16; for NLSP routers, this limit can be set to any number between 8 and 127.

   Note also that broadcast packets are never rebroadcast onto the network segment from which they are received.

If the router is not directly connected to the segment on which the final destination node resides, it sends the packet to the next router in the path to the destination node, as follows:

1. The router places the node address of the next router in the Destination Address field of the MAC header.

   The router gets this information from its Routing Information Table.

2. The router places its own node address in the Source Address field of the MAC header.
3. The router increments the Transport Control field in the IPX header and forwards the packet to the next router.