How Network Layer Routing Uses LANs and WANs
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While the network layer routing logic ignores the physical transmission details, the bits still
have to be transmitted. To do that work, the network layer logic in a host or router must
hand off the packet to the data link layer protocols, which, in turn, ask the physical layer
to actually send the data. And as was described in Chapter 2, “Fundamentals of Ethernet
LANs,” the data link layer adds the appropriate header and trailer to the packet, creating a
frame, before sending the frames over each physical network.
The routing process forwards the network layer packet from end to end through the network,
while each data-link frame only takes a smaller part of the trip. Each successive data
link layer frame moves the packet to the next device that thinks about network layer logic.
In short, the network layer thinks about the bigger view of the goal, like “Send this packet
to the specified next device…,” while the data link layer thinks about the specifics, like
“Encapsulate the packet in a data-link frame and transmit it.” Figure 4-2 points out the key
encapsulation logic on each device, using the same examples as shown in Figure 4-1.
Figure 4-2 Network Layer and Data Link Layer Encapsulation
Because the routers build new data-link headers and trailers, and because the new headers
contain data-link addresses, the PCs and routers must have some way to decide what datalink
addresses to use. An example of how the router determines which data-link address to
use is the IP Address Resolution Protocol (ARP). ARP dynamically learns the data-link
address of an IP host connected to a LAN. For example, at the last step, at the bottom of
Figure 4-2, Router R3 would use ARP once to learn PC2’s MAC address before sending any
packets to PC2.
Routing as covered so far has two main concepts:
■ The process of routing forwards Layer 3 packets, also called Layer 3 protocol data units
(L3 PDU), based on the destination Layer 3 address in the packet.
■ The routing process uses the data link layer to encapsulate the Layer 3 packets into Layer
2 frames for transmission across each successive data link.