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As you can imagine, there are a few things that you need to know before connecting your
WAN in order to make sure that everything goes well. For starters, you have to understand the
kind of WAN Physical layer implementation that Cisco provides as well as ensure that you’re
familiar with the various types of WAN serial connectors involved.
The good news is that Cisco serial connections support almost any type of WAN service.
Your typical WAN connection is a dedicated leased line using HDLC, PPP, and Frame Relay
with speeds that can kick it up to 45Mbps (T3).
HDLC, PPP, and Frame Relay can use the same Physical layer specifications, and I’ll go
over the various types of connections and then move on to telling you all about the WAN protocols
specified in the CCNA objectives.
Serial Transmission
WAN serial connectors use
serial transmission
, something that takes place 1 bit at a time over
a single channel.
Parallel transmission can pass at least 8 bits at a time, but all WANs use serial
transmission.
Cisco routers use a proprietary 60-pin serial connector that you have to get from Cisco or
a provider of Cisco equipment. Cisco also has a new, smaller proprietary serial connection
that’s about one-tenth the size of the 60-pin basic serial cable, called the “
smart-serial
”—I’m
not sure why. But I do know that you have to make sure that you have the right type of interface
in your router before using this cable connector.
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Implement and verify WAN links
The type of connector you have on the other end of the cable depends on your service provider
and its particular end-device requirements. There are several different types of ends
you’ll run into:

EIA/TIA-232

EIA/TIA-449

V.35 (used to connect to a CSU/DSU)

EIA-530
Make sure that you’re clear on these things: Serial links are described in frequency or cycles
per second (hertz). The amount of data that can be carried within these frequencies is called
bandwidth
. Bandwidth is the amount of data in bits per second that the serial channel can carry.
Data Terminal Equipment and
Data Communication Equipment
By default, router interfaces are
data terminal equipment (DTE)
, and they connect into
data
communication equipment (DCE)
like a
channel service unit/data service unit (CSU/DSU)
.
The CSU/DSU then plugs into a demarcation location (demarc) and is the service provider’s
last responsibility. Most of the time, the demarc is a jack that has an RJ-45 (8-pin modular)
female connector, located in a telecommunications closet.
Actually, you may already have heard of demarcs. If you’ve ever had the glorious experience
of reporting a problem to your service provider, they’ll usually tell you everything tests
out fine up to the demarc, so the problem must be the CPE, or customer premises equipment.
In other words, it’s your problem not theirs.
Figure 8.1 shows a typical DTE-DCE-DTE connection and the devices used in the network.
FIGURE 8 . 1
DTE-DCE-DTE WAN connection
Clocking typically provided by DCE network to routers.
In non-production environments, a DCE network is not always present.
DCE
DTE
DTE
CSU/DSU CSU/DSU
8.2 Configure and verify a basic WAN serial connection
357
The idea behind a WAN is to be able to connect two DTE networks through a DCE network.
The DCE network includes the CSU/DSU, through the provider’s wiring and switches,
all the way to the CSU/DSU at the other end. The network’s DCE device (CSU/DSU) provides
clocking to the DTE-connected interface (the router’s serial interface).
As mentioned, the DCE network provides clocking to the router; this is the CSU/DSU. If
you have a nonproduction network and you’re using a WAN crossover type of cable and do
not have a CSU/DSU, then you need to provide clocking on the DCE end of the cable by using
the
clock rate
command.
Terms such as
EIA/TIA-232
,
V.35
,
X.21
, and
HSSI (High-Speed Serial Interface)
describe the Physical layer between the DTE (router) and DCE device
(CSU/DSU).
High-Level Data-Link Control (HDLC) Protocol
The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented,
Data Link layer protocol. It specifies an encapsulation method for data on synchronous serial
data links using frame characters and checksums. HDLC is a point-to-point protocol used on
leased lines. No authentication can be used with HDLC.
In byte-oriented protocols, control information is encoded using entire bytes. On the other
hand, bit-oriented protocols use single bits to represent the control information. Some common
bit-oriented protocols include SDLC, LLC, HDLC, TCP, and IP.
HDLC is the default encapsulation used by Cisco routers over synchronous serial links.
And Cisco’s HDLC is proprietary—it won’t communicate with any other vendor’s HDLC
implementation. But don’t give Cisco grief for it—
everyone’s
HDLC implementation is proprietary.
Figure 8.2 shows the Cisco HDLC format.
FIGURE 8 . 2
Cisco HDLC frame format
Flag Address
• Each vendor’s HDLC has a proprietary data field to support multiprotocol environments.
• Supports only single-protocol environments.
Flag Address Control Data FCS Flag
Control Proprietary Data FCS Flag
Cisco HDLC
HDLC
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As shown in the figure, the reason that every vendor has a proprietary HDLC encapsulation
method is that each vendor has a different way for the HDLC protocol to encapsulate multiple
Network layer protocols. If the vendors didn’t have a way for HDLC to communicate the different
layer 3 protocols, then HDLC would only be able to carry one protocol. This proprietary
header is placed in the data field of the HDLC encapsulation.
Configuring HDLC on Cisco Routers
Configuring HDLC encapsulation on an interface is really pretty straightforward. To configure
it from the CLI, follow these simple router commands:
Router#
config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#
int s0
Router(config-if)#
encapsulation hdlc
Router(config-if)#
^Z
Router#
So, let’s say you only have one Cisco router, and you need to connect to a non-Cisco router
because your other Cisco router is on order. What would you do? You couldn’t use the default
HDLC serial encapsulation because it wouldn’t work. Instead, you would use something like
PPP, an ISO-standard way of identifying the upper-layer protocols. You can check out RFC
1661 for more information on the origins and standards of PPP. Let’s discuss PPP in more
detail and how to connect to routers using the PPP encapsulation.
Point-to-Point Protocol (PPP)
Let’s spend a little time on Point-to-Point Protocol (PPP). Remember that it’s a Data Link layer
protocol that can be used over either asynchronous serial (dial-up) or synchronous serial
(ISDN) media. It uses Link Control Protocol (LCP) to build and maintain data-link connections.
Network Control Protocol (NCP) is used to allow multiple Network layer protocols
(routed protocols) to be used on a point-to-point connection.
Since HDLC is the default serial encapsulation on Cisco serial links and it works great, why
and when would you choose to use PPP? Well, the basic purpose of PPP is to transport layer
3 packets across a Data Link layer point-to-point link, and it’s nonproprietary. So, unless you
have all Cisco routers, you need PPP on your serial interfaces—the HDLC encapsulation is
Cisco proprietary, remember? Plus, since PPP can encapsulate several layer 3 routed protocols
and provide authentication, dynamic addressing, and callback, PPP could be the best encapsulation
solution for you instead of HDLC.
Figure 8.3 shows the protocol stack compared to the OSI reference model.
8.2 Configure and verify a basic WAN serial connection
359
FIGURE 8 . 3
Point-to-Point Protocol stack
PPP contains four main components:
EIA/TIA-232-C, V.24, V.35, and ISDN
A Physical layer international standard for serial
communication.
HDLC
A method for encapsulating datagrams over serial links.
LCP
A method of establishing, configuring, maintaining, and terminating the point-to-point
connection.
NCP
A method of establishing and configuring different Network layer protocols. NCP is
designed to allow the simultaneous use of multiple Network layer protocols. Some examples
of protocols here are IPCP (Internet Protocol Control Protocol) and IPXCP (Internetwork
Packet Exchange Control Protocol).
Burn it into your mind that the PPP protocol stack is specified at the Physical and Data Link
layers only. NCP is used to allow communication of multiple Network layer protocols by
encapsulating the protocols across a PPP data link.
Remember that if you have a Cisco router and a non-Cisco router connected
with a serial connection, you must configure PPP or another encapsulation
method, such as Frame Relay, because the HDLC default just won’t work!
Configuring PPP on Cisco Routers
Configuring PPP encapsulation on an interface is the same as HDLC. To configure it from the
CLI, follow these simple router commands:
Router#
config t
Enter configuration commands, one per line. End with CNTL/Z.
Upper-layer Protocols
(such as IP, IPX, AppleTalk)
Physical layer
(such as EIA/TIA-232, V.24, V.35, ISDN)
Network Control Protocol (NCP)
(specific to each Network-layer protocol)
Link Control Protocol (LCP)
High-Level Data Link Control Protocol (HDLC)
OSI layer
3
2
1
360 Chapter 8 Implement and verify WAN links
Router(config)#int s0
Router(config-if)#encapsulation ppp
Router(config-if)#^Z
Router#
Exam Objectives
Remember the default serial encapsulation on Cisco routers. Cisco routers use a proprietary
High-Level Data Link Control (HDLC) encapsulation on all their serial links by default.
Remember the PPP Data Link layer protocols. The three Data Link layer protocols are:
Network Control Protocol (NCP), which defines the Network layer protocols; Link Control
Protocol (LCP), a method of establishing, configuring, maintaining, and terminating the
point-to-point connection; and High-Level Data Link Control (HDLC), the MAC layer
protocol that encapsulates the packets.