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How Routers Use a WAN Data Link

Today, most leased lines connect to routers, and routers focus on delivering packets to a
destination host. However, routers physically connect to both LANs and WANs, with those
LANs and WANs requiring that data be sent inside data-link frames. So, now that you know
a little about HDLC, it helps to think about how routers use the HDLC protocol when sending
data.

First, the TCP/IP network layer focuses on forwarding IP packets from the sending host to
the destination host. The underlying LANs and WANs just act as a way to move the packets
to the next router or end-user device. Figure 3-7 shows that network layer perspective.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 3-7 IP Routing Logic over LANs and WANs
Following the steps in the figure, for a packet sent by PC1 to PC2’s IP address:
1. PC1’s network layer (IP) logic tells it to send the packet to a nearby router (R1).
2. Router R1’s network layer logic tells it to forward (route) the packet out the leased
line to Router R2 next.
3. Router R2’s network layer logic tells it to forward (route) the packet out the LAN link
to PC2 next.

While Figure 3-7 shows the network layer logic, the PCs and routers must rely on the LANs
and WANs in the figure to actually move the bits in the packet. Figure 3-8 shows the same
figure, with the same packet, but this time showing some of the data link layer logic used by
the hosts and routers. Basically, three separate data link layer steps encapsulate the packet,
inside a data-link frame, over three hops through the internetwork: from PC1 to R1, from
R1 to R2, and from R2 to PC2.

Free CISCO CCNA Routing and Switching ICND1 Study Guide

Figure 3-8 General Concept of Routers De-encapsulating and Re-encapsulating IP Packets
Following the steps in the figure, again for a packet sent by PC1 to PC2’s IP address:
1. To send the IP packet to Router R1 next, PC1 encapsulates the IP packet in an
Ethernet frame that has the destination MAC address of R1.
2. Router R1 de-encapsulates (removes) the IP packet from the Ethernet frame, encapsulates
the packet into an HDLC frame using an HDLC header and trailer, and forwards
the HDLC frame to Router R2 next.
3. Router R2 de-encapsulates (removes) the IP packet from the HDLC frame, encapsulates
the packet into an Ethernet frame that has the destination MAC address of PC2,
and forwards the Ethernet frame to PC2.

In summary, a leased line with HDLC creates a WAN link between two routers so that they
can forward packets for the devices on the attached LANs. The leased line itself provides
the physical means to transmit the bits, in both directions. The HDLC frames provide the
means to encapsulate the network layer packet correctly so that it crosses the link between
routers.

Leased lines have many benefits that have led to their relatively long life in the WAN marketplace.
These lines are simple for the customer, are widely available, are of high quality,
and are private. However, they do have some negatives as well compared to newer WAN
technologies, including a higher cost and typically longer lead times to get the service
installed. The next section looks at an alternative WAN technology used in some examples
in this book: Ethernet.

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