Implementing Enterprise WAN

uptightexampleNetworking and Communications

Oct 24, 2013 (3 years and 5 months ago)

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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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7.1 WAN Devices and Technology


WAN Services purchased


Serial transmissions verses Ethernet in LANS


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7.1 WAN Devices and Technology


Translation device


prepare data for transmission: modem
(analog) or CSU/DSU (digital)


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


Demarc
-

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

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7.1 WAN Devices and Technology


Local loop


last mile (first mile for customer)


CSU/DSU or modem


controls the rate data moves onto
the loop (DCE), also provides clocking signal to router
(DTE)


Various physical layer protocols used



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7.1 WAN Devices and Technology

Note: DS0


Digital Signal 0, T1 (DS1) = 24 DS0s

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Activity 7.1.1.5

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Activity 7.1.1.5

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7.1.2 WAN Standards

Layer 2 WAN protocols:


LAPF


HDLC


PPP

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7.1.2 WAN Standards

Activity 7.1.2.2

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7.1.2 WAN Standards

Activity 7.1.2.2

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7.1.3 Accessing the WAN


Modems enable POTS to be used for WAN connections


Modems are also used for DSL and cable connections to ISP


A modem encodes the information onto that carrier wave before
transmission and then decodes it at the receiving end


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


Destination demodulates the carrier and extracts information

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7.1.3 Accessing the WAN


DSO channel = time slice of the physical bandwidth


Fractional connections


part of a T1


Two techniques which information from multiple channels can be
allocated bandwidth on a single cable based on time:

Time Division Multiplexing (TDM)
-

if a sender has nothing to say, its time
slice goes unused, wasting valuable bandwidth.

Statistical
-
Time Division Multiplexing (STDM)
-

dynamically reassigns
unused time slices on an as
-
needed basis, minimizes wasted bandwidth


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Activity 7.1.3.4

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Activity 7.1.3.4

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7.1.4 Packet and Circuit Switching


Dedicated Leased Line


point
-
to
-
point serial


Circuit switching:

Dedicated bandwidth

Higher cost

Higher level of security




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7.1.4 Packet and Circuit Switching


Packet switching:

Bandwidth efficiency

Identifier on each packet

Preconfigured, but non
-
exclusive, link


Cell switching:

high
-
speed packet switching

ATM (fixed length cells)

Large amount of overhead



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7.1.4 Packet and Circuit Switching


Virtual circuits

Switched virtual circuits
-

dynamically established between two
points when a router requests a transmission

Permanent virtual circuits
-

provides a permanent path to
forward data between two points (Frame Relay)

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7.1.5
Last mile and long range WAN
technologies

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7.1.5
Last mile and long range WAN
technologies


Elements of Layer 2 encapsulations

Flag

Address

Control

Protocol

Data

FCS



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7.1.5
Last mile and long range WAN
technologies


Great Distances with Fiber

Synchronous Optical Network (SONET)

Synchronous Digital Hierarchy (SDH)

SDH and SONET are used for moving both voice and data.

New developments for extremely long
-

dense wavelength
division multiplexing (DWDM)
-

can carry IP, SONET, and ATM
data concurrently



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Activity 7.1.5.3



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Activity 7.1.5.3



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7.2.1 Ethernet and WAN Encapsulations


Encapsulation occurs before data travels across the
WAN


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

LAN


Ethernet

WAN


depends on link

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7.2.2 Comparing Common WAN Encapsulations


Data Link Layer encapsulation may change continuously
to match the technology in use


Network Layer encapsulation will not change


Packets exit the LAN by way of the default gateway
router


Router strips off the Ethernet frame and then re
-
encapsulates that data into the correct frame type for the
WAN
-

acts as a media converter, by adapting the Data
Link Layer frame format to a format that is appropriate to
the interface

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7.2.2 Comparing Common WAN Encapsulations


Standard bit
-
oriented Layer 2 encapsulation: HDLC


Cisco HDLC: additional Type field

Default WAN encapsulation on Cisco devices

Allows multiple Network Layer protocols to share a link



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7.2.2 HDLC and PPP

Point
-
to
-
Point Protocol (PPP):


Data Link Layer
encapsulation for serial links


Uses a layered architecture
to encapsulate & carry multi
-
protocol datagrams over a
point
-
to
-
point link


Standards
-
based


Support

Asynchronous serial

Synchronous serial

Support High
-
Speed Serial
Interface (HSSI)

Integrated Services Digital
Network (ISDN)


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7.2.2 HDLC and PPP


PPP has two sub
-
protocols:

Link Control Protocol
-

responsible for establishing, maintaining and
terminating the point
-
to
-
point link.

Network Control Protocol
-

provides interaction with different Network
layer protocols.

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7.2.2 HDLC and PPP


Link Control Protocol negotiates:

Authentication


PAP and CHAP

compression

error detection

multilink

PPP callback


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7.2.2 HDLC and PPP


Phases of PPP sessions:

Link
-
establishment
-

Receipt of the configuration
acknowledgement frame completes this phase

Authentication (optional)
-

provides password protection to
identify connecting routers

NCP negotiation
-

The show interfaces command reveals the
LCP and NCP states.

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Activity 7.2.2.5

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Activity 7.2.2.5

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7.2.3 Configuring PPP


Change encapsulation from HDLC to PPP on both ends
of link


Configure desired optional features

ppp multilink


Configures load balancing across multiple links.


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7.2.3 Configuring PPP

Verification and troubleshooting commands:


show interfaces serial
-

Displays the encapsulation and
the states of the Link Control Protocol (LCP).


show controllers
-

Indicates the state of the interface
channels and whether a cable is attached to the
interface


debug serial interface
-

Verifies the incrementation of
keepalive packets.


debug ppp
-

Provides information about the various
stages of the PPP process, including negotiation and
authentication



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7.2.3.3 Configuring PPP

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7.2.4 PPP Authentication


Occurs after establishment of the link but before the
Network Layer protocol configuration


Password Authentication Protocol (PAP)

PAP sends the username/password pair across the link
repeatedly in clear text



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7.2.4 PPP Authentication


Challenge Authentication Protocol (CHAP)

uses a three
-
way handshake.

1.
PPP establishes the link phase.

2.
Local router sends a challenge message to the remote router.

3. Remote router uses the challenge and a shared secret password to generate a
one
-
way hash.

4. Remote router sends back one
-
way hash to the local router.

5. Local router checks the response against its own calculation, using the
challenge and the same shared secret.

6. Local router acknowledges authentication if values match.

7. Local router immediately terminates connection if the values do not match.


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7.2.4 PPP Authentication

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7.2.4 PPP Authentication

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7.2.5.3 LAB


Configuring PAP and CHAP

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Describe Frame Relay


Nonbroadcast multiaccess network


Packet switching with variable length packets


STDM


Virtual circuit between two DTE devices




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Describe Frame Relay


Data link connection identifier (DLCI)


Inverse ARP


LMI




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Describe Frame Relay

Service parameters:


Committed information rate (CIR)


Excess information rate (EIR)


Discard eligible (DE) frames

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Describe Frame Relay

Traffic management:


Forward explicit congestion notification (FECN)


Backward explicit congestion notification (BECN)

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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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