CCENT Notes Part 4 - Wide-Area Networks

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					                                  CCENT Notes

                       Part 4 – Wide-Area Networks

  Ref : CCENT/CCNA ICND1 Official Exam Certification Guide, Second Edition
                          by Wendell Odom

It is highly recommended that you read at least once the above study guide to make full
use of this notes, it is expected that there may be minor errors in this notes, please
always refer the study guide for accurate information.

                             (Jojo Jacob - CCENT)

Chapter 16 – WAN Concepts..............................................................................................3
Chapter 17 – WAN Configuration.....................................................................................15

Chapter 16 – WAN Concepts
WAN Technologies

Voice calls with PCM (Pulse Code Modulation) codec in use consumes 64 Kbps through
the digital part of the PSTN (when using links like T1 and T3s inside the Telco)

Analog Modems

A Telephone converts sound waves into analog electrical signals, a modem converts a
stream of binary digits on a computer to into representative analog electrical signal.

The modems modulates and de-modulates the analog signals at the sending and receiving

Modems create an asynchronous circuit, but a leased line creates a synchronous circuit
with CSU/DSUs synchronizing the speeds.

Analog Modems basically uses the dial-up technology and max. speed is around 100

Digital Subscriber Line (DSL)

Comparison between DSL and Modem…
Some of the key features of DSL service…

•   DSL allows analog voice signals, and digital data signals to be sent over the same
    local loop wiring at the same time

•   The local loop must be connected some thing besides the traditional voice switch at
    the local CO, in this case a device called DSL Access Multiplexer (DSLAM).

•   DSL allows for a concurrent voice call to be up at the same time as the data

•   Unlike modems, the DSL data component is always on, in other words you don’t
    have to signal or dial a phone number to set up a data circuit.

DSL Connection from Home to an ISP…Typical topology and devices used for DSL

                                                          TELCO - CO
                                                                                   IP Network
                                                                                  Owned by ISP



             Ethernet                                 IP Traffic
                                                     Split to ISP
                  Digital                              Router
                 > 4000 Hz
   DSL                               Local Loop
  Router /                                                                DSLAM

                                                  Analog Voice
                   DTMF Tones,
                                                  Split to Voice
                   Analog Voice,
                    0 – 4000 Hz


                                                           Voice Switch

Telephone wall jack in uses RJ-11 connectors.

RJ11 is a physical interface often used for terminating telephone wires. It is probably the
most familiar of the registered jacks, being used for single line POTS telephone jacks in
most homes across the world.

RJ14 is similar, but for two lines, and RJ25 is for three lines. RJ61 is a similar registered
jack for four lines. The telephone line cord and its plug are more often a true RJ11 with
only two conductors.

Phone generates analog signals at the rate of 0 – 4000 Hz, and the DSL modem uses
frequencies higher than 4000 Hz, so that the phone and the DSL signals interfere very
much, still need to use a filter.

The DSLAM directs (multiplexes) the analog voice signals – frequency range between 0
Hz and 4000 Hz, to a voice switch.

Two views of an RJ25 6P6C crimp-on style connector. .

RJ11 is a physical interface often used for terminating telephone wires. It is probably the
most familiar of the registered jacks, being used for single line POTS telephone jacks in

BS6312 431A plug; colloquially, a British Telecom plug. Used in NZ.

DSL Types

Symetric DSL means that link speed in both directions is the same, and Asymetric means
more download speed as compared to a relatively low upload speed.

Acronym                       Spelled Out                   Type
ADSL                          Asymetric DSL                 Asymmetric
CDSL (G.lite)                 Consumer DSL                  Asymmetric
VDSL                          Very-high-data-rate DSL       Asymmetric
SDSL                          Symmetric DSL                 Symmetric
HDSL                          High-data-rate DSL            Symmetric
IDSL                          ISDN DSL                      Symmetric

Factors that affect speed of DSL line….

•   The speed of a DSL line can vary based on many factors, including…
•   The distance between the CO and the consumer (the longer the distance, the slower
    the speed)
•   The quality of the local loop cabling (the worse the wiring, the slower the speed)
•   The type of DSL (each standard has different maximum theoretical speed)
•   The DSLAM used in the CO, (older equipment may not have, recent improvements
    that allow for faster speeds, on lower grade local loops)

Theoretical maximum speed of an ADSL line on local loop is 10 Mbps (8.192 Mbps).
Most ISPs quote it as 1.5 Mbps downstream, 384 kbps upstream.

ADSL support local loops upto 18,000 feets, 5 Kms.

CABLE Internet

                                                   CABLE Company Facitlity

               HOME                             SATELITE




                                                           ISP Router

The equipment at the Head End splits the channels used for internet over to an ISP

Comparison of Analog, DSL and Cable Modem…Comparison points for Internet Access
                             Analog Modem     DSL                Cable Modem
Transport                    Telco Local loop Telco Local loop CATV Cable
Supports symmetric Speeds    Yes              Yes                No
Supports asymmetric speed    Yes              Yes                Yes
Typical practical speed      Upto 100kbps     1.5 Mbps           3 – 6 Mpbs
                                              downstream         downstream
Allows concurrent voice and  No               Yes                Yes
Always-on Internet Service   No               Yes                Yes
Local loop distance issue    No               Yes                No
Throughput degrades under    No               No                 Yes
higher loads

ATM – Packet Switching Service

ATM supports much higher physical link speed compared to Frame relay, especially
when using specification called Synchronous Optical Network (SONET).

ATM does not forward frames as in Frame relay, but it forwards Cells. ATM cells are
always fixed 53 bytes in length. 48 bytes of payload (data) and 5 byte header.

ATM header contains two fields that act like Data link connection identifier (DLCI) in
frame relay, by identifying each VC (Virtual Channel) , these two fields are called,
Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI).

When a router need to convert an Ethernet frame into an ATM cell, it creates the ATM
cells by splitting the data link frame into 48 bytes segments, each segment is placed in a
cell with 5 byte header. The other end router does the reassembly of ATM cells to
Ethernet frames. The whole process of segmenting frame into cells and reassembling
them is called segmentation and reassembly (SAR).

Circuit Switching : is the physical ability to send or receive data and voice across two end

Packet Switching : The device interpret the bits, and decides forwarding decisions based
on some type of address fields in the header.

Comparison of circuit switching and packet switching….
Features                               Circuit switching      Packet switching
Service implemented as OSI Layer       1                      2
Point-to-Point (two devices) or more Point-to-Point           Multi point (more than two)

IP Services for Internet Access

Besides basic routing, the access router needs to perform three additional important
functions, assigning addresses, learning routes, and translate address. (NAT).

Internet access equipments – separate devices.


                   Fast Ethernet
                                                          CATV Cable
                  Fa0/0                                                     ISP/Internet

                       R1              Cable Modem                 ISP1

                                        IP Addresses
                                         are in same

 Router /

Address assignment on Internet Access Routers.
Factors that impact the IP Addresses used by internet access routers….

   •    The internet access router (R1) has two interfaces, one facing the internet, and
        other facing the devices at that site, the router needs IP addresses on each of these
        interface, the IP addresses are chooses as per the following rules…
   •    The internet facing interfaces need one public IP address so that the routers in the
        internet knows how to route packet to the access router
   •    The ISP typically assigns that public (globally routable) ip address dynamically
        using DHCP.
   •    The local PCs typically needs to dynamically learn IP addresses using DHCP, so
        the access router acts as a DHCP server for the local hosts.

   •    The router needs a statically configured ip address on the local subnet, using a
        private network number

   •   The local LAN subnet will use ip address in private network number.

DHCP Client and Server Function in an Access Router

             R1 as DHCP Server                R1 as DHCP Client

                 `   DNS
                                                                              DHCP Server

                                         R1                          ISP1

                                                  ISP’s DNS Server
             `        GW                                

Access router is part of the Integrated DLS/Cable Modem. And is a customer located

Access router R1 needs a statically configured IP address on its local LAN facing
interface, a DHCP server function enabled on that interface, and DHCP client function
enabled on its internet facing interface.

Routing for the Interface Access Router

 The access router creates a default route with its default gateway(ISP router interface) as
its next-hop router.

The default gateway settings on the locals PCs, along with the default route on the access
router allows PCs to send packets that reach internet.

However the traffic from internet to local PCs are incomplete, as the internet routes will
never have a route for the private ip address for hosts PC1 and PC2, this problem is
solved by NAT and PAT.


NAT : Network Address Translation
PAT : Port Address Translation

ISP try to conserve public IP address by assigning only minimum (one) public IP address
to each access router, not to each local hosts connected to it.

A key concept of PAT : there is no important difference between some number of TCP
connections from different hosts, versus the same number of TCP connections from the
same hosts.

Three connections from three PCs

               Port 1024
                                                   Port 80

           Port 1024                                      Port 80
                                                  Port 80

           Port 1033                                            Server                                                            


Three connections from ONE PC

               Port 1024

                                                   Port 80

           Port 1025                                     Port 80
                                                 Port 80

           Port 1026                                            Server

The top part of the figure shows a network with three different hosts connecting to a web
server using TCP. The bottom part shows three TCP connection from a single host. All
six connection connect to Server port 80. In each of these cases the server is

able to differentiate between each connection, because each has a unique combination of
IP address and port number.

PAT allows the local hosts to use private ip addresses, while the access router uses a
single public ip address. PAT takes advantage of the fact that a sever does not care if it
has one connection each to three different hosts, or three connections to a single host ip

PAT translates the local hosts private IP address to the access router’s public ip address,
also to tell which ip packet needs to be sent back which local hosts, router keeps track of
both ip address and TCP/UDP port number.

Shows how PAT translates IP Addresses in an Internet Access Router….


 SA S.Port : 1024

                                                 SA S.Port : 1024

 PC1                                                                                           Server

                                                               DA D.Port : 1024
       DA D.Port : 1024

                                                       NAT Translation Table
                                        Inside Local        Inside Global
                                 64 .100.1.1 : 1024
                                 64 .100.1.1 : 1025

The numbered steps in the figure follow the logic….

1. PC1 sends a packet to server and as per PC1’s default gateway setting
send the packet to access router R1.

2. R1 performs PAT, based on the on the router’s NAT translation table, changing the
local host’s private ip address to router’s public ip address, R1 then forwards the packet
based on its default route

3. When the server replies to the packet sent from PC1, server sends the packet to router
R1’s address, with a destination port 1024. Internet routers know how to forward the
packet to R1, because it uses a public ip address.

4. R1 changes the destination ip address and port based on the NAT table, switching
destination ip address/port from : 1024 to : 1024 and R1 knows
the route to reach because this address is in subnet connected to R1.

NAT uses the following key terms…

Inside Host : Refers to a host inside the enterprises network

Inside Local : Refers to an IP address in an IP header, with that address representing a
local host as the packet passes over the local enterprise network. In this case and 102 are inside local IP addresses, and packets at step 1, and 4 in the
above figure shows inside local ip addresses.

Inside Global : Refers to an IP address in an IP header, with that address representing a
local host, as the packet passes over the global internet. In this case, is the one
inside global IP address, and the packets in step 2 and 3 in the above figure show inside
global ip addresses.

Inside Interface : The router interface connected to the same LAN as the inside hosts

Outside Interface: The router interface connected to the internet


ADSL : Asymmetric Digital Subscriber Line, one of many DSL technologies, ADSL is
designed to deliver more bandwidth downstream, than upstream.

Asymmetric : A feature of many internet access technologies including DSL, cable and
modems, in which downstream transmission rate is higher than the upstream transmission

ATM : Asynchronous Transfer Mode, The international standard for cell relay, in which
multiple service types, such as voice, video and data, are conveyed in fixed length, 53
byte cells. Fixed length cells allow cell processing to occur in hardware, thereby reducing
transit delays.

DSL : Digital Subscriber Line, Public network technology that delivers high bandwidth
over conventional telco local loop copper wiring at limited distance. Typically used as an
internet access technology, connecting use to an ISP.

Inside Global : For packets send to and from host that resides in the trusted part of a
network that uses NAT, a term referring to the IP address used in the headers of that
packets, when those packets traverse the global (public) internet.

Inside Local : For packets sent to and from hosts that reside in the trusted part of a
network that uses NAT, a term referring to the IP addresses used in the headers of those
packets when they traverse enterprises (private) part of the network.

Modem : Modulator –demodulator. A device that converts between digital and analog
signals so that a computer can send data to another computer using analog telephone

NAT : Network Address Translation. A mechanism for reducing the need for globally
unique (public) IP addresses. NAT allows an organization with addresses that are not
globally unique to connect to the internet by translating those addresses into public
addresses, in the globally routable address space.

PAT : Port Address Translation. A NAT feature in which one Inside Global IP address
supports over 65,000 concurrent TCP and UDP connections.

PSTN : Public Switched Telephone Network, A general term referring to variety of
telephone networks and services in place worldwide. Some times called POTS, or Plain
Old Telephone Service.

Symmetric : A feature of many internet access technologies in which downstream
transmission rate is same as the upstream transmission rate.

Chapter 17 – WAN Configuration

Configuring Point-to-Point WAN

Configuring HDLC

No layer 1 or Layer 2 commands are required for a router’s Ethernet or serial interface,
by default assumes Ethernet and HDCL as the data link layer protocols respectively. Only
command that is required is to configure ip address on the interface, and a no shutdown
command if the interface is administratively down.

Optional and required configuration steps for a serial link between two
routers….Configuration steps on a serial link…..

Step 1. Configure Interface IP address using ip address interface subcommand

Step 2. The following tasks are required only when the specifically listed conditions are

a. if an encapsulation protocol interface subcommand lists a protocol besides HDLC
already exists on the interface, use encapsulation HDLC interface subcommand to
enable HDLC

b. if the interface status is administratively down, enable the interface using the
 no shutdown interface sub command

c. if the serial link is a back to back serial link in a lab, configure the clocking rate using
the clock rate speed interface sub command, but only on the one router with the DCE
cable attached, (show controllers serial number, command)

Step 3. The following steps are optional, and have no impact on whether the link works
and passes IP traffic.

a. configure the link speed using the bandwidth speed-in-kbps interface subcommand
b. configure the description text subcommand on the interface for documentation

Configure PPP

To migrate from a working HDLC link (default) to a working PPP link, the only
command needed is an encapsulation ppp command on each of the two routers serial

Configuring and Troubleshooting Internet Access Routers

Internet Access Routers : Configuration Steps

Step 1. Establish IP connectivity, plan and configure from CLI, IP address on the local
LAN, so that PC on the LAN can ping the router’s LAN interface.

Step 2 : Install and access SDM (Cisco Router and Security Device Manager) , install
SDM on the router, and access the router’s SDM interface using the PC that can ping
router’s ip address

Step 3. Configure DHCP and PAT. Use SDM to configure both DHCP client services
and PAT services on the router

Step 4. Plan for DHCP services, plan the IP address to be assigned by the router to the
hosts on the local LAN, along with the DNS ip address, Domain name, and default
gateway settings that the router will advertise

Step 5. Configure DHCP Server, Use SDM to configure DHCP server features on the

Step 1. Establish IP Connectivity
IP Address details planned and configured on the local LAN for an internet access router.

a. Choose any private IP network number (generally a class c – network)
b. choose a mask that allows for enough hosts (typically the default mask is fine)
c. choose the router ip address from the network number


                  Fast Ethernet
                                                       CATV Cable
                  Fa0/0                                                 ISP/Internet

                          R1           Cable Modem              ISP1

                                       IP Addresses
                                        are in same

 Router /

Step 2. Install and Access SDM

Installation and accessing SDM from a host requires ip connectivity between host and the
router, and additional steps and configuration as per the SDM installation manuals.

Step 3 Configuring DHCP and PAT

Pleas refer to page 549 through to 554 for SDM configuration wizards …. which asks for
the user inputs and configures the router DHCP client and PAT features.

Step 4. Plan for DHCP services

The following lists outlines the key items that you need to gather before you configure
the router as a DHCP server, the first two items related to planning on the local LAN, and
the last two items are values learned from the ISP, that just needs to be passed on to the
hosts in the local LAN.

1. Recall the private network number and the mask used on the local LAN, and choose a
   subset of that network, that can be assigned to hosts using DHCP

2. Make a note of the router’s ip address in that network, this address will be the local
   hosts default gateway

3. Find the DNS server ip address learned by the router using DHCP client services, use
   the show dhcp server exec command, the router will then be able to inform the DNS
   sever ip address to the hosts in the local LAN

4. Find the domain name, again with the show dhcp server exec command

Step 4. Configure DHCP server

Pleas refer to page 556 and 557 for SDM DHCP server configuration wizards …. which
asks for the user inputs DHCP pool and DHCP setting learned in the previous steps.

Internet Access Router Verification

Steps to perform basic verification of the installation of the access routers….common
items to check when troubleshooting access router installation…

Step 1. Go to a PC on the local LAN and open a web page, if it works fine, then the
access router configurations worked, otherwise go to step 2.

Step 2. Try the ipconfig/all , command prompt command to verify the host has the
correct IP address, dhcp settings, DNS setting as per the configured DHCP server
configurations on the router, otherwise try ipconfig/release and ipconfig/renew
commands to lease a new ip address.

Step 3. Check the cabling and inside interface and outside interface as per the PAT

Step 4. Test the PAT functionality by generating traffic from a local pc to a host in the

EXEC commands on the access router for configuration verification…

Show ip dhcp binding – lists ip addresses assigned on the local lan
Show ip nat translation – lists the NAT translation table entries
Clear ip nat translation * - clears the NAT translation table entries

Please refer to page 558 for a output listing of the above commands


Cisco Router and Security Device Manager : Administrative web based interface on a
router, that allows for configuration and monitoring of a router, including the
configuration of DHCP, and NAT/PAT.

Please refer to page…540 for the Chapter 17 WAN configuration quiz….