Implementing Enterprise WAN

dingdongboomNetworking and Communications

Oct 27, 2013 (3 years and 11 months ago)

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© 2006 Cisco Sy stems, Inc. All rights reserv ed.

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1

Implementing
Enterprise WAN
Links


Introducing Routing and Switching in the Enterprise



Chapter 7

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Objectives


Describe the features and benefits of common WAN
connectivity options.


Compare and configure common WAN
encapsulations.


Describe Frame Relay



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Describe the Features and Benefits of
Common WAN Connectivity Options


Serial transmissions

enable reliable, long
-
range
communications at slower speeds than a LAN


Within a LAN, a Network administrator has physical
control over all the cabling, devices, and services


LANs operate at layer 3


WANs operate at layer 1 and 2


In a WAN, most organizations purchase WAN services
from a service provider


Equipment used for WANs can be purchased or shared



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4

Describe the Features and Benefits of Common
WAN Connectivity Options


Translation device:
modem

prepares the data from transmission
across the service provider network for an analog connection


Preparing the data:
CSU/DSU

combines into a single piece of
equipment integrated into the interface card in the router


Central Office (CO) and Customer Premises Equipment (CPE)


Demarc

is the point at which the control and responsibility of the
customer ends and the control and responsibility of the service
provider begins

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Describe the Features and Benefits of Common
WAN Connectivity Options


Central Office (CO)

is where the service provider stores
equipment and accepts customer connections


Local loop (last mile)

physical line from the demarc of
a customer to the edge of the telephone company
central office


Clocking signal
is provided by the CSU/DSU

(DCE)


Router is DTE

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Describe the Features and Benefits of
Common WAN Connectivity Options

Physical Layer standards which establish codes and electrical
parameters that the router and CSU/DSU use to communicate


HSSI, V.35


Link speed:


North America


T1

is 1.544 Mbps


T3

is 44.736 Mbps


Europe:


E1


2.048


E3


34.064



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Describe the Features and Benefits of Common
WAN Connectivity Options

Data link Layer protocols:


LAPF

Link Access Procedure for Frame Relay


HDLC

High
-
level Data Link Control


PPP

Point
-
to
-
Point Protocol

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8

Describe the Features and Benefits of Common
WAN Connectivity Options


Modems enable POTS

(plain old telephone systems) to be used
for WAN connections


Analog signals modulate

a carrier wave in the speech band


The modulated carrier wave carries information to destination
across the telephone network (MODEM)


Destination demodulates the carrier and extracts information


Modems are also used for DSL and cable connections to ISP

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9

Describe the Features and Benefits of
Common WAN Connectivity Options


DSO channel

time slice, not separate physical entity


Fractional connections

such as fractional T1


Time Division Multiplexing (TDM)


Statistical Time Division Multiplexing (STDM)

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10

Describe the Features and Benefits of
Common WAN Connectivity Options


Time Division Multiplexing (TDM)

allocates bandwidth
based on pre
-
assigned time slots. Each time slices are
assigned to individual conversations representing a period
of time during which a conversation has complete use of the
physical media. Bandwidth is allocated to each channel
regardless of whether the station using the channel has data
to transmit. So if a sender has nothing to say, its time slice
goes unused,
wasting valuable bandwidth.



Statistical Time Division Multiplexing (STDM)

is similar to
TDM except that it keeps track of conversations that require
extra bandwidth. It then dynamically reassigns unused time
slices on an as
-
needed basis. In this way,
STDM minimizes
wasted bandwidth.

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11

Features and Benefits of Common WAN
Connectivity Options


Dedicated Leased Line


Point
-
to
-
point serial link between two routers


Enables a
one
-
to
-
one connection


Each link requires a separate physical interface and separate CSU/DSU


Gets very expensive with addition locations being added










Circuit switching:


Dedicated bandwidth while call is
in place


Circuit released when finished


Higher level of security


Higher cost


Telephone calls use this
connection


Costs are incurred only when
connection is active


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12

Features and Benefits of Common WAN
Connectivity Options


Packet switching:


Uses
Bandwidth more efficiently


Data segmented into packets with
identifier on each packet


Data is then released in the service provider network


Preconfigured, but non
-
exclusive, link


Switches packets from multiple organizations over the same link


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13

Features and Benefits of Common WAN
Connectivity Options


Cell switching


High
-
speed variation

packet switching


Capable of
transferring voice, video and data

through private and
public networks at speeds in excess of 155 Mbps


Asynchronous Transfer Mode (ATM) used 53 byte cells


Switched quickly and efficiently


Prevents small messages from being held up by large messages


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Features and Benefits of Common WAN
Connectivity Options


Virtual circuits share link between devices with traffic from other
sources so medium is not private during connection


Switched virtual circuits (SVC)

dynamically established when router
requests a transmission (on demand)


Requires call set
-
up before transmission
(which will cause delay)


Permanent virtual circuits (PVC)

provides a permanent path to forward
data between 2 points


Preconfigured by service provider


Seldom disconnected


Eliminating call set/up delay


More popular than SVC


Example of this technology is FRAME
RELAY

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Features and Benefits of Common WAN
Connectivity Options


Last mile and long range WAN technologies


Analog dialup


Integrated Services Digital Network (ISDN)


Leased line


Cable


Digital Subscriber Line (DSL)


Frame Relay


Wireless

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Last Mile and Long Range WAN Technologies


Synchronous Optical Network (SONET)


Synchronous Digital Hierarchy (SDH)


Standards that allow the movement of large amounts of data over
great distances through fiber
-
optic cables


Both encapsulate earlier digital transmission standards


Support either ATM or Packet


Are used for moving both voice and data


Dense Wavelength Division Multiplexing (DWDM)


Assigns incoming optical signals to specific frequencies or
wavelengths of light


Capable of amplifying these wavelengths to boost the signal strength


Each channel is capable of carrying a multiplexed signal at 2.5 Gbps


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7.2 Common WAN Encapsulations



Encapsulation occurs before data travels across a WAN


Layer 2 adds header information specific to type of physical
network transmission


Encapsulates the packet into Ethernet frames


Flag: marks beginning/end of frame


Address


Control: indicates type of frame


Protocol: specifies the type of encapsulation


Data: used as Layer 3 data and IP datagram


Frame Check Sequence (FCS) verifies frame
was not damaged in transit

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7.2 Common WAN Encapsulations


Standard bit
-
oriented Layer 2 encapsulation: HDLC


Error free communication between two points


Contains flow control and error control using acknowledgments and
windowing


Each frame has the same format


Cannot handle multiple protocols (no type field)




Cisco HDLC: additional Type field


Default WAN encapsulation on Cisco devices


Cisco proprietary


Allows multiple Network Layer protocols to share a link


contains a type field



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PPP



Point
-
to
-
Point Protocol) PPP

Data Link Layer encapsulation
for Serial links


Uses layered architecture to encapsulate and carry multi
-
protocol datagrams



Interfaces that support PPP

Asynchronous serial

Synchronous serial

High
-
Speed Serial Interface (HSSI)

Integrated Services Digital Network
(ISDN)


Sub
-
protocols: LCP, NCP



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PPP (Sub Protocols)


Link Control Protocol options:


authentication


compression


error detection


multilink


PPP callback


LCP also:


Handles varied packet sizes


detects common misconfiguration errors


Determines when a link is functioning properly or is failing



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21

PPP


Phases of PPP sessions:


Link
-
establishment
, maintaining and terminating the link


Authentication
(optional) provides password protection



occurs after the two routers agree to the set parameters but before the
NCP Phase begins


NCP negotiation

provides interaction with different network layer
protocols (such as IP or IPX)


Link remains active until


closed by LCP or NCP


until an inactivity timer expires


Or user
terminates (see activity 7.2.2.5)



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Configuring PPP


HDLC is default encapsulation on serial
links (you don’t
have to specify it…unless it has changed to PPP and you
need to change it back)


Change encapsulation from HDLC to PPP on both links


Must be same on both links


Configure desired optional features


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Compare and Configure Common WAN
Encapsulations


Verification and troubleshooting commands:


show interfaces
serial

Displays the encapsulation


show
controllers

Indicates status of interface


debug serial
interface

Verifies the
keepalive

packets


debug
ppp

Provides info on PPP stages



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PPP Authentication



Password Authentication Protocol (PAP)

Two way handshake

sends username/password
in plain
text repeatedly

Remote node is in control of the frequency and timing of the login
attempts

Vulnerable to hijacking or hacking by replay
attack and packet sniffing



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25

PPP Authentication

CHAP


Challenge Handshake Authentication Protocol (CHAP)


More secure

because
passwords not sent across the link


Authentication during link establishment and repeatedly during activity


Uses
Three
-
Way Handshake


Local router sends a challenge message


Remote router uses the challenge and a shared secret and sends back
one
-
way hash to local router


Local router checks the response against its own calculation and
acknowledges authentication if values match or terminates connection if
values don’t match


CHAP provides protection against playback through variable challenge
value






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7.3 Frame Relay


Nonbroadcast

multiaccess

network


Packet switching with variable length packets


Uses STDM (statistical time division multiplexing) for
optimul

use of the available bandwidth


Virtual circuit between two DTE
devices (established using
PVCs)


Router connects to
service provider via a
leased line



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7.3 Frame Relay


Data link connection identifier (DLCI
)
identivies

the VC


Local significance, can be different at each end of the VC


Mapping the DLCI to remote IP address (Manual or Dynamic)


Dynamically is called
Inverse
ARP (done by the router)


LMI

signaling standard between DTE/Frame Relay Switch


Active state

connection is active, routers can exchange data


Inactive state

FR switch is working but remote connection to the FR
switch is not


Deleted State

local connection receives no LMI messages



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7.3 Frame Relay


Committed information rate (CIR)

minimum bandwidth rate
guaranteed
by
the provider for the data on a VC


Excess information rate (EIR)

average rate above the CIR
that a VC can support when no congestion


Discard eligible (DE)

frames

frames transmitted above the
speed of the CIR are uncommitted but are forwarded in the
network supports it


If congestion occurs the provider first drops frames with the DE bit set

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29

7.3 Frame Relay


Forward explicit Congestion Notification (FECN)

when set to ‘1’ indicates
to
a
receiving device
that
congestion was experience


Backward
Explicit Congestion Notification (BECN)
when set to ‘1’ indicates to a
sending device
that
the network is
congested

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30

Summary


Circuit switching WAN technologies create a physical
circuit between end devices before sending data


Packet and cell switching WAN technologies use virtual
circuits to send data across the network


Layer 2 encapsulation changes as frames move across
a WAN


PPP permits many advanced features including
authentication, compression, and load balancing


Frame Relay is a packet switched technology using
switched or permanent virtual circuits


Frame Relay uses parameters such as CIR to establish
the bandwidth used on each VC



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