Networking Guide
Chapter 4, Administering SCO IPX/SPX

Intersegment packet routing

Intersegment packet routing

The addressing method depicted in Figure 4-9 is used when the two nodes reside on the same physical segment (or ring) or if they reside on separate segments interconnected by repeaters or bridges. 

If the two nodes have different network numbers (reside on different network segments), the sending node must find a router on its own segment that can forward packets to the destination node's network segment. To find this router, the workstation broadcasts a RIP packet requesting the fastest route to the destination node's network number. This RIP request is responded to by the router residing on the sending node's segment with the shortest path to the desired segment; in the response, the router includes its node number. 

Sending the packet

Once the sending node receives the router's node number, it is prepared to send packets to the destination node. The sending node addresses these packets by placing: 


Figure 4-10 illustrates this process. 

Figure 4-10 Packet addressing through a router



Routing the packet

When a router receives a packet to be routed, it 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 will: 

  1. Place the destination node number from the IPX header in the destination address field of the MAC header.

  2. Place its own node number in the source address field of the MAC header.

  3. Transmit the packet.
Figure 4-10 illustrates this process. If the router is not directly connected to the segment that the final destination node resides on, however, it will send the packet to the next router in the path to the destination node. To forward the packet to another router, the router will:

  1. Place the node number of that other router in the destination address field of the MAC header.

  2. Place its own node number in the source address field of the MAC header.

  3. Leave the IPX header as initially set by the sending node and sends the packet.