Router Forwarding Decisions and the IP Routing Table

Earlier in this chapter, Figure 4-1 shows the network layer concepts of routing, while Figure 4-2 shows the data link encapsulation logic related to routing. This next topic dives a little deeper into that same process, using an example with three routers forwarding (routing) one packet. But before looking at the example, the text first summarizes how a router thinks about forwarding a packet.

A Summary of Router Forwarding Logic

First, when a router receives a data link frame addressed to that router’s data link address, the router needs to think about processing the contents of the frame. When such a frame arrives, the router uses the following logic on the data link frame:

Step 1. Use ...

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