Giáo trình CCNA - Chương 4

WAN data link layer• The data link layer protocols define how data is encapsulated for transmission to remote sites, and the mechanisms for transferring the resulting frames.. • The co

CCNA – Semester4

Module 1 Scaling IP Address

Objectives

• Scaling networks with NAT and PAT

• Dynamic Host Configuration Protocol

Scaling networks with

NAT and PAT

• Outside local address – The IP address of an outside host as it is known to the hosts

on the inside network

• Outside global address – The IP address assigned to a host on the outside network The owner of the host assigns this address

Major NAT and PAT features

• NAT translations can be used for a variety of

purposes and can be either dynamically or

• Realistically, the number of ports that can be assigned a single IP address is around 4000.

Major NAT and PAT features

NAT advantages

• Eliminates reassigning each host a new IP

address when changing to a new ISP

• Conserves addresses through application level multiplexing

port-• Protects network security

• Reduce address overlap occurrence.

NAT disadvantages

• Translation introduces switching path delays and processing overhead.

• Loss of IP end-to-end traceability.

• Certain application will not function with NAT enabled.

Cisco IOS NAT

Configure static NAT

Static translation

Configure dynamic NAT

Configure dynamic NAT

Configure dynamic NAT

Dynamic NAT example

Configure PAT

Clear NAT table

Show NAT translation

Troubleshooting NAT and PAT configuration

• Use the following steps to determine whether NAT is operating as expected:

– Based on the configuration, clearly define what NAT is supposed to achieve

– Verify that correct translations exist in the translation table.

– Verify the translation is occurring by using showand debug

commands

– Review in detail what is happening to the packet and verify that

routers have the correct routing information to move the packet along

Debug IP NAT

Dynamic Host Configuration

Protocol

• Less work is involved in managing an IP

network when DHCP is used

• The DHCP protocol is described in RFC 2131

Easy IP

• Cisco routers can use a Cisco IOS feature set, Easy IP, to offer an optional, full-featured DHCP server

• Easy IP leases configurations for 24 hours by default

• The DHCP service is enabled by default on

versions of Cisco IOS that support it

• To disable DHCP service, use the no service dhcp

command

BOOTP and DHCP

• Both protocols are client/server based and use UDP ports 67 and 68 Those ports are still known as BOOTP ports

• The four basic IP parameters:

DHCP Messages

• If the client detects that

the address is already

in use on the local

segment it will send a

DHCPDECLINE

message and the

process starts again

• If the client received a DHCPNACK from the server after

sending the DHCPREQUEST , then it will restart the process again

• If the client no longer needs the IP address, the client sends a DHCPRELEASE message to the server

CISCO IOS DHCP

• Cisco IOS DHCP server always checks to make sure that an address is not in use before the server offers it to a client

• By default, router will issue 2 ICMP echo

requests , or will ping, to a pool address before sending the DHCPOFFER to a client

Configuring DHCP

Excluding IP

Key DHCP Server Commands

Verifying DHCP operation

• To verify the operation of DHCP, the command show

ip dhcp binding can be used

• To verify that messages are being received or sent

by the router, use the command show ip dhcp server statistics

Troubleshooting DHCP

Network services

• Remote clients will broadcast to locate servers while routers, by default, will not forward client broadcasts beyond their subnet

IP helper-address

• Remote clients will broadcast to locate servers while routers, by default, will not forward client broadcasts beyond their subnet

• When possible, administrators should use the ip address command to relay broadcast requests for these key UDP services

helper-• By default, the ip helper-address command forwards the following eight UDP services:

– NetBIOS Name Service

– NetBIOS datagram Service

DHCP Relay

• NAT and PAT concepts and operation

• NAT and PAT configuration and

troubleshooting

• Comparison between DHCP and BOOTP

• Cisco Router DHCP configuration

• DHCP relay with ip helper-address

Module 2 WAN Technologies

circuit-• Describe equipment involved in the implementation

of various WAN services

• Compare and contrast WAN design models

WAN Technology Overview

Parts of a WAN service

• In order for the local loop to carry data, a device such as a modem is needed to prepare the data for transmission.

DTE and DCE

• Devices that put data

on the local loop are

called data

WAN Line Types and Bandwidth

WAN Devices

WAN Devices: Modem

• A modem is needed if the local loop is analog rather than digital

• Modems transmit data over voice-grade telephone lines by modulating and demodulating the signal

WAN Devices: CSU/DSU

• The communications link

needs signals in an

appropriate format

• For digital lines, a channel

service unit (CSU) and a

data service unit (DSU) are

required

• The CSU/DSU may also be

built into the interface card

in the router

WAN standard organizations

WAN physical layer

• Describe how to provide electrical, mechanical, operational, and functional connections to the services provided by a communications service provider

Physical Connectors

WAN data link layer

• The data link layer protocols define how data is

encapsulated for transmission to remote sites, and the mechanisms for transferring the resulting

frames

WAN encapsulation

• The choice of encapsulation protocols depends on the WAN

technology and the equipment

• Most framing is based on the HDLC standard

• The address field is not needed for WAN links, which are almost always point-to-point

• The control field indicates the frame type:

– Unnumbered frames carry line setup messages

– Information frames carry network layer data

– Supervisory frames control the flow of information frames and request data retransmission in the event of an error.

WAN Data-link Protocols

• The data bits are

automatically delivered to the far end because the circuit is already

established.

packet

• Connection-oriented

systems predetermine the route for a packet, and each packet need only carry an identifier.

WAN link options

WAN Technologies

Analog dialup

• When intermittent, low-volume data transfers are needed, modems and analog dialed telephone lines provide low capacity and

dedicated switched connections

• The advantages are simplicity, availability, and low implementation cost

• The upper limit is around 33 kbps The rate can be increased to around 56 kbps if the signal is coming directly through a digital connection

• The disadvantages are the low

data rates and a relatively long

connection time, not proper for

voice or video traffic

ISDN

• The connection uses 64 kbps bearer channels

(B) for carrying voice or data and a signaling,

delta channel (D) for call set-up

• ISDN is used to provide additional capacity or backup for leased-line.

• ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice

connections.

• The dedicated capacity gives no latency or jitter

between the endpoints.

• These dedicated circuits are generally priced based on

bandwidth required and distance between the two

connected points

• The simplified handling of frames leads to reduced latency

• Most Frame Relay connections are PVCs rather than SVCs

• It can operate at much higher bit rate that provides permanent shared medium bandwidth connectivity that carries both voice and data traffic

ATM

• ATM provide very low latency and jitter at much higher bandwidths

• ATM has data rates beyond 155 Mbps, capable of

transferring voice, video, and data

• It is built on a cell-based architecture rather than on a frame-based architecture ATM cells are always a fixed length of 53 bytes including 5 bytes header.

DSL

• Digital Subscriber Line (DSL) technology is a broadband technology

• Use existing twisted-pair telephone lines to transport high-bandwidth data

• Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data

• The transfer rates are dependent on the actual length of the local loop and the type and condition of its cabling.

Cable modem

• Network access is available from some cable television networks allows greater bandwidth than telephone local loop

• Enhanced cable modems enable two-way, speed data transmissions up to 6.5 times that of T1 leased lines

high-• It is an attractive medium for transferring large amounts of digital information quickly, including video clips, audio files, and large amounts of data

WAN Design

Steps in WAN design

• Several modifications may

be necessary before a

design is finalized

• Continued monitoring and

re-evaluation are also

required after installation of

the WAN to maintain

WAN topologies

• Knowing the various end points allows the

selection of a topology or layout for the WAN

• The topology will be influenced by geographic considerations but also by requirements such

as availability

WAN Technologies

• With the end points and the links chosen, the necessary bandwidth can be estimated

• Type of traffic on the links may have varying

requirements for latency and jitter

• With the bandwidth availability already determined,

suitable link technologies must be selected

Three-layer design model

• A systematic approach is needed when many locations must be joined A hierarchical solution with three layers offers many advantages

• This three-layer model follows the hierarchical design used in telephone systems

Advantages of the hierarchical approach

Advantages of the hierarchical approach

Three-layer design model

Other layered design models

• Many networks do not require the complexity of a full three-layer hierarchy Simpler hierarchies may

be used

Summary

• Identify the devices used in a WAN and list standards

• Packet-switched and circuit-switched WAN

technologies

• Overview of analog dialup, ISDN, leased line, X.25, Frame Relay, cable modem and ATM services

• Steps in WAN design and WAN design requirements

• Advantages offered with a three-layer hierarchical WAN design

Module 3 PPP

Objectives

• Explain serial communications

• PPP concepts, components, operations

• PAP, CHAP and PPP authentication process

• PPP configuration

Serial Point-to-Point Links

Time-division multiplexing

• Time-Division Multiplexing (TDM) is the transmission

of several sources of information using one common channel, or signal, and then the reconstruction of the original streams at the remote end

• In 1979, the ISO agreed on HDLC as a standard

bit-oriented data link layer protocol that encapsulates data

on synchronous serial data links

• The following examples of derivative protocols are

called link access protocols:

– Link Access Procedure, Balanced (LAPB) for X.25

– Link Access Procedure on the D channel (LAPD) for ISDN

– Link Access Procedure for Modems (LAPM) and PPP for modems – Link Access Procedure for Frame Relay (LAPF) for Frame Relay

• HDLC defines a Layer 2 framing structure that allows for

flow control and error control using acknowledgments

and a windowing scheme

Cisco HDLC

• Standard HDLC does not inherently support

multiple protocols on a single link

• Cisco HDLC frame uses a proprietary ‘type’ field that acts as a protocol field

• HDLC is the default Layer 2 protocol for Cisco router serial interfaces

piggybacked on an information frame

– Supervisory frames (S-frames)– Provide request/response

mechanisms when piggybacking is not used

– Unnumbered frames (U-frames)– Provide supplemental link control functions, such as connection setup The code field identifies the U- frame type

HDLC encapsulation

Troubleshooting a serial interface

Show controller

Debug serial interface

Troubleshooting a serial line status

Troubleshooting a serial line status

Troubleshooting a serial line status

Troubleshooting a serial line status

Troubleshooting a serial line status

Troubleshooting a serial line status

Troubleshooting a serial line status

PPP Authentication

PPP

• PPP uses a layered architecture including NCP, LCP and the lowest encapsulation protocol which is HDCL.

• PPP provides a method for encapsulating multi-protocol datagrams over a point-to- point link, and uses the data link layer for testing the connection

PPP Physical Layer

PPP Data-link Layer

PPP Network Layer

Frame format: Flag

Flag-Indicates the beginning or end of a frame and consists of the binary sequence 01111110

Flag-Indicates the beginning or end of a frame and consists of the binary sequence 01111110

Frame format: Address

Consists of the standard broadcast address, which is the binary sequence

11111111 PPP does not assign

individual station addresses

Consists of the standard broadcast address, which is the binary sequence

11111111 PPP does not assign

individual station addresses

Frame format: Control

1 byte: 00000011, which calls for transmission of user data

in an unsequenced frame

1 byte: 00000011, which calls for transmission of user data

in an unsequenced frame

Frame format: Protocol

2 bytes that identify the

protocol encapsulated in

the data field of the frame

2 bytes that identify the

protocol encapsulated in

the data field of the frame

PPP frame format: Protocol types

• 0021: Internet Protocol

• 0029: Appletalk

• 002B: Novell IPX

• C021: Link Control Protocol

• C023: Authentication Protocol

Frame format: Data

Contain the datagram for the protocol specified in the protocol field

The default maximum length of the data field is 1,500 bytes

Contain the datagram for the protocol specified in the protocol field

The default maximum length of the data field is 1,500 bytes

Frame format: FCS

Normally 16 bits (2 bytes) Refers

to the extra characters added to a frame for error control purposes.

Normally 16 bits (2 bytes) Refers

to the extra characters added to a frame for error control purposes.

4 phases of a PPP session

1 Link establishment and configuration negotiation

2 Link quality determination

3 Network layer protocol configuration negotiation

4 Link termination

LCP Frames

There are three classes of LCP frames:

• Link establishment frames : Used to

establish and configure a link.

a link

• Link maintenance frames : Used to manage and debug a link

Phase I: Link establishment

• An originating PPP node sends LCP

frames to configure and establish the data link

• LCP packets contain a configuration option fields: MTU , compression , and link

authentication protocol

• LCP must first open the connection and negotiate the configuration parameters

• This phase is complete when a

configuration acknowledgment frame has been sent and received.

LCP Configuration Options

Phase II (Optional): Authentication

• The link is tested to determine whether the link quality is good enough to bring up

network-layer protocols.

• Then, the client or user workstation can be authenticated.

• Authentication takes place before the

network-layer protocol configuration phase begins.

• PPP supports two authentication protocols:

PAP and CHAP

Phase III: Network layer protocol

• The PPP devices send NCP packets to

choose and configure one or more layer protocols (such as IP).

network-• When each of the chosen network-layer

protocols has been configured, datagrams from each network-layer protocol can be sent over the link.

• You can check LCP and NCP states by using the show interfaces command