Routing Path Selection

Routing focuses on the end-to-end logic of forwarding data. Figure 5-1 shows a simple example of how routing works. The logic seen in the figure is relatively simple. For PC1 to send data to PC2, it must send something to R1, when sends it to R2, then on to R3, and finally to PC2. However, the logic used by each device along the path varies slightly.

PC1's Logic: Sending Data to a Nearby Router

In this example, PC1 has some data to send data to PC2. Because PC2 is not on the same Ethernet as PC1, PC1 needs to send the packet to a router that is attached to the same Ethernet as PC1. The sender sends a data-link frame across the medium to the nearby router; this frame includes the packet in the data portion of the frame. That frame uses data link layer (Layer 2) addressing in the data-link header to ensure that the nearby router receives the frame.

Figure 5-1 Routing Logic: PC1 Sending to PC2

The main point here is that the originator of the data does not know much about the network—just how to get the data to some nearby router. In the post office analogy, it's like knowing how to get to the local post office, but nothing more. Likewise, PC1 needs to know only how to get the packet to R1.

R1 and R2's Logic: Routing Data Across the Network

R1 and R2 both use the same general process to route the packet. The routing table for any particular network layer protocol contains a list of network layer address groupings. Instead of a single entry in the routing table per individual destination address, there is one entry per group. The router compares the destination network layer address in the packet to the entries in the routing table, and a match is made. This matching entry in the routing table tells this router where to forward the packet next. The words in the bubbles in Figure 5-1 point out this basic logic.

The concept of network layer address grouping is similar to the U.S. ZIP code system. Everyone living in the same vicinity is in the same ZIP code, and the postal sorters just look for the ZIP codes, ignoring the rest of the address. Likewise, in Figure 5-1, everyone in this network whose IP address starts with 168.1 is on the Token Ring on which PC2 resides, so the routers can just have one routing table entry that means "all addresses that start with 168.1."

Any intervening routers repeat the same process. The destination network layer (Layer 3) address in the packet identifies the group in which the destination resides. The routing table is searched for a matching entry, which tells this router where to forward the packet next. Eventually, the packet is delivered to the router connected to the network or subnet of the destination host (R3), as previously shown in Figure 5-1.

R3's Logic: Delivering Data to the End Destination

The final router in the path, R3, uses almost the exact same logic as R1 and R2, but with one minor difference. R3 needs to forward the packet directly to PC2, not to some other router. On the surface, that difference seems insignificant. In the next section, when you read about how the network layer uses the data link layer, the significance of the difference will become obvious.

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