Routers and WANs: Still a Match Made in Heaven
In Chapter 14, you
learned some of the details of PPP and HDLC in the context of routers and
routing, because these days, most of the devices on the ends of leased lines are
routers. Similarly, most of the devices on the ends of Frame Relay PVCs are
routers, so it helps to discuss routing briefly with Frame Relay in the picture.
To hit the highlights, look at Figure
15-8, with IP addresses shown, three sites, and Frame Relay used for WAN
connectivity.
This internetwork uses three routers at three sites. Each has
an access link to a local CO, and there's no PVC between R2 and R3, making the
network a partial mesh of PVCs. The Frame Relay provider supplied some
information to this company when it installed the Frame Relay service, as shown
in the legend on the bottom right in the figure. The telco told the customer to
configure R1 so that it uses DLCI 102 to send frames to R2, and DLCI 103 to send
frames to R3.
For this internetwork to work correctly, each of the three
routers needs an IP address for its serial interfaces. Notice that the IP
addresses shown next to the routers' serial interfaces are in the same
subnetsubnet 150.1.2.0. If you study Frame Relay in more depth than is covered
here, you'll learn that there are many options for IP addressing over Frame
Relay networks; this is just one option. In this case, just like subnet
150.1.2.0 was used when there was Ethernet between R1 and R2 (Chapter 11, "Knowing Where to Turn at Each Intersection [Router]),
and just like subnet 150.1.2.0 was used on the WAN link between R1 and R2 (Chapter 14), this example uses that same
subnet between R1 and R2.
Now that you have an established internetwork to refer to, the
next sections review Frame Relay encapsulation and DLCIs and then close with a
discussion of Frame Relay Inverse Address Resolution Protocol (ARP).
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