Next, consider the hardware on the ends of the serial link, in particular where the channel
service unit/data service unit (CSU/DSU) hardware resides on each end of the serial link. In
a real serial link that runs through a service provider, the link terminates at a CSU/DSU. The
CSU/DSU can either sit outside the router as a separate device (as shown on the left at router
R1) or integrated into the router’s serial interface hardware (as shown on the right).
As for cabling, the service provider will run the cable into the enterprise’s wiring closet and
often put an RJ-48 connector (same size as an RJ-45 connector) on the end of the cable.
That cable should connect to the CSU/DSU. With an internal CSU/DSU (as with router R1
in Figure 15-2), the router serial port has an RJ-48 port to which the serial cable should
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352 CCNA 200-301 Official Cert Guide, Volume 1
connect. With an external CSU/DSU, the CSU/DSU must be connected to the router’s serial
card via a short serial cable.
Cisco Integrated Services Routers
Product vendors, including Cisco, typically provide several different types of router hard-
ware. Today, routers often do much more work than simply routing packets; in fact, they
serve as a device or platform from which to provide many network services. Cisco even
brands its enterprise routers not just as routers, but as “integrated services routers,” empha-
sizing the multipurpose nature of the products.
As an example, consider the networking functions needed at a typical branch office. A typi-
cal enterprise branch office needs a router for WAN/LAN connectivity, and a LAN switch to
provide a high-performance local network and connectivity into the router and WAN. Many
branches also need voice-over-IP (VoIP) services to support IP phones, and several security ser-
vices as well. Plus, it is hard to imagine a site with users that does not have Wi-Fi access today.
So, rather than require multiple separate devices at one site, as shown in Figure 15-2, Cisco
offers single devices that act as both router and switch and provide other functions as well.
For the sake of learning and understanding the different functions, this book focuses on
using a separate switch and separate router, which provides a much cleaner path for learning
the basics.
Figure 15-3 shows a photo of the Cisco 4321 ISR, with some of the more important fea-
tures highlighted. The top part of the figure shows a full view of the back of the router. This
model comes with two built-in Gigabit Ethernet interfaces and two modular slots that allow
you to add small cards called Network Interface Modules (NIMs). The bottom of the figure
shows one example NIM (a NIM that provides two serial interfaces). The router has other
items as well, including both an RJ-45 and USB console port.
2-Port Serial NIM
Gi0/1
Aux
2 NIM Slots
RS-45
Console
Gi0/0
(RJ-45 or SFP)
USB
On/Off
Figure 15-3
Photos of a Model 4321 Cisco Integrated Services Router (ISR)
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