© 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 1 Implementing Enterprise WAN Links Introducing Routing and Switching in the Enterprise – Chapter

© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 1

Implementing Enterprise WAN Links

Introducing Routing and Switching in the Enterprise – Chapter 7


Objectives

Describe the features and benefits of common WAN connectivity options.

Compare and configure common WAN encapsulations.

Describe Frame Relay


7.1 WAN Devices and Technology WAN Services purchased

Serial transmissions verses Ethernet in LANS


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


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


7.1 WAN Devices and Technology

Note: DS0 – Digital Signal 0, T1 (DS1) = 24 DS0s


Activity 7.1.1.5


Activity 7.1.1.5


7.1.2 WAN Standards

Layer 2 WAN protocols:

LAPF

HDLC

PPP


7.1.2 WAN Standards

Activity 7.1.2.2


7.1.2 WAN Standards

Activity 7.1.2.2


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


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


Activity 7.1.3.4


Activity 7.1.3.4


7.1.4 Packet and Circuit Switching

Dedicated Leased Line – point-to-point serial

Circuit switching: Dedicated bandwidth

Higher cost

Higher level of security



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



Virtual circuitsSwitched 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)


7.1.5 Last mile and long range WAN technologies



Elements of Layer 2 encapsulationsFlag

Address

Control

Protocol

Data

FCS



Great Distances with FiberSynchronous 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


Activity 7.1.5.3


Activity 7.1.5.3


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


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


7.2.2 Comparing Common WAN Encapsulations

Standard bit-oriented Layer 2 encapsulation: HDLC

Cisco HDLC: additional Type fieldDefault WAN encapsulation on Cisco devices

Allows multiple Network Layer protocols to share a link


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)


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.


7.2.2 HDLC and PPP

Link Control Protocol negotiates: Authentication – PAP and CHAP

compression

error detection

multilink

PPP callback


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.


Activity 7.2.2.5


Activity 7.2.2.5


7.2.3 Configuring PPP

Change encapsulation from HDLC to PPP on both ends of link

Configure desired optional featuresppp multilink

Configures load balancing across multiple links.


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




7.2.3.3 Configuring PPP


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



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.






7.2.5.3 LAB – Configuring PAP and CHAP


Describe Frame Relay Nonbroadcast multiaccess network

Packet switching with variable length packets

STDM

Virtual circuit between two DTE devices



Data link connection identifier (DLCI)

Inverse ARP

LMI



Service parameters:

Committed information rate (CIR)

Excess information rate (EIR)

Discard eligible (DE) frames



Traffic management:

Forward explicit congestion notification (FECN)

Backward explicit congestion notification (BECN)


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


Documents

© 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 1 Implementing Enterprise WAN Links Introducing Routing and Switching in the Enterprise – Chapter