CA Ex S1M05 OSI Network Layer

7/28/2019 CA Ex S1M05 OSI Network Layer

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Network Fundamentals Chapter 5

OSI Network Layer

CCNA Exploration version 4.0


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Hc vin mng Bch Khoa - Website: www.bkacad.com 2

Objectives

Identify the role of the Network Layer, as it describescommunication from one end device to another end

device Examine the most common Network Layer protocol,

Internet Protocol (IP), and its features for providingconnectionless and best-effort service

Understand the principles used to guide the division orgrouping of devices into networks

Understand the hierarchical addressing of devices and

how this allows communication between networks Understand the fundamentals of routes, next hop

addresses and packet forwarding to a destination

network


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Introduction

End-to-End

connections

Host-to-Host


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Introduction

The protocols of the OSI model Network

layer specify: addressing and processes

that enable Transport layer data to bepackaged and transported.

The Network layer encapsulation allows itscontents to be passed to the destination

within a network or on another network with

minimum overhead.


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Introduction

IPv4

IPv4 addresses are 32 bits long, written in dotteddecimal, and separated by periods.


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Network Layer Communication from Host to Host

The Network layer, or OSI Layer3, provides services to

exchange the individual piecesof data over the network

between identified end devices.

5.1.1.1


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1.Addressing2.Encapsulation3.Routing The role of the router is to select

paths for and direct packets toward

their destination. This process isknown as routing.

4.Decapsulation

Layer 3 uses 4 basic processes:

Communication between

networks is called routing


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Each route that a packet takes toreach the next device is called a hop. As the packet is forwarded, its

contents (the Transport layer PDU),remain intact until the destination hostis reached.


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Transport layer (OSI Layer 4): managesthe data transport between the processesrunning on each end host

Network layer (OSI Layer 3): specify the

packet structure and processing used tocarry the data from one host to another host

Compare role between Layer 3,4


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Network Layer Protocols

Internet Protocol version 4 (IPv4) Internet Protocol version 6 (IPv6) Novell Internetwork Packet

Exchange (IPX) AppleTalk Connectionless Network Service

(CLNS/DECNet)


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The IPv4 Protocol Example Network Layer Protocol

The Internet Protocol was designed

as a protocol with low overhead (?) It provides only the functions that arenecessary to deliver a packet from a

source to a destination over aninterconnected system of networks. The protocol was not designed to

track and manage the flow ofpackets.


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The IPv4 Protocol Example Network Layer Protocol

Basic characteristics

5.1.2


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The IPv4 Protocol Connectionless

5.1.3


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The IPv4 Protocol Connectionless

Connectionless vs. Connection-oriented

Protocols?


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The IPv4 Protocol Best Effort

.

5.1.4


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The IPv4 Protocol Best Effort

Unreliable means simply that IP does nothave the capability to manage, and recoverfrom, undelivered or corrupt packets.

Since protocols at other layers can managereliability, IP is allowed to function very

efficiently at the Network layer.



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In a reliableprotocol, thereceiverconfirms (ACK)the packetits received.Example: TCP.

In a besteffort ornon-reliable

protocol,the receiverkeeps silentinstead.Example: IP, UDP.

Extra: Reliable vs. Best-effort Protocols

Receiver:I confirm that

Ive receivedthe packet #n.

Sender:

Ive sent thepacket #n.

Protocol:I have the mechanism to knowif the packet is received.

Receiver:I keep silenceupon receivingpackets.

Sender:Ive sent thepacket #n.

Protocol:I do not have the mechanism to

know if the packet is received.

This is a classification of networking protocols.Non-reliable does not mean inaccurately data delivery.


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The IPv4 Protocol Media Independent

In some cases, an intermediary device - usually a router - will needto split up a packet when forwarding it from one media to a mediawith a smaller MTU. This process is called fragmenting the packetor fragmentation.

MTU (Token Ring, Ethernet,PPP) ?

5.1.5


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Packaging the Transport Layer PDU

The routing performed by these intermediary devices only considers thecontents of the packet header that encapsulates the segment.

5.1.6


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IPv4 Packet Header

5.1.7


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Extra: Type of Service

RFC 791


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Extra: Type of Service


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Extra: Flags


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Extra: Protocols


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Network Layer Fields

4 bits Indicates version of IP used IPv4: 0100; IPv6: 0110


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4 bits Indicates datagram header length in 32 bit words


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8 bits Specifies the level of importance that has been

assigned by upper-layer protocol


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16 bits Specifies the length of the entire packet in bytes,

including data and header


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16 bits Identifies the current datagram


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3 bits

The second bit specifies if the packet can be fragmented; the lastbit specifying whether the packet is the last fragment in a series

of fragmented packets.


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13 bits Used to help piece together datagram

fragments


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8 bits

Specifies the number of hops a packet may travel. Thisnumber is decreased by one as the packet travels through a

router

Routing Loop ?


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8 bits

Indicates which upper-layer protocol, such as TCP(6) orUDP(17), receives incoming packets after IP processing has

been completed


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16 bits

Helps ensure IP header integrity Not caculated for the encapsulation data

N k L Fi ld


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32 bits Specifies the sending node IP address

N t k L Fi ld


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32 bits Specifies the receiving node IP address

N t k L Fi ld


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Variable length

Allows IP to support various options, such as security

N t k L Fi ld


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Variable length Extra zeros are added to this field to ensure that the

IP header is always a multiple of 32 bits.



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Variable length up to 64 KB Contains upper-layer information



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Networks

Dividing Hosts into Groups

Separating Hosts into Common Groups


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Networks can be grouped based on factors that include:Geographic location

Purpose

Ownership Geographic

5.2.1



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Purpose: Users who have similar tasks typically usecommon software, common tools, and have common

traffic patterns.



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Purpose



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Ownership: To assists in controlling access to thedevices and data as well as the administration of the

networks.



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Ownership

Why separate hosts into networks ?


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Performance degradation

Security issues Address Management

5.2.2

Common issues with large networks are:



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Improving Performance

Broadcast domain ?

5.2.2

Lab 5.2.2


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Lab 5.2.2




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Increase network security

5.2.3



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y p




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y p




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y p

Address management: To expect each host to knowthe address of every other host would impose a

processing burden on these network devices that

would severely degrade their performance.

ARP table ?Gateway ?

5.2.4


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Dividing the networks - Networks from networks


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If a large network has to be divided, additional layers of addressing can becreated. Using hierarchical addressing means that the higher levels of theaddress are retained; with a subnetwork level and then the host level.

Subnet Mask ?

Prefix length ?

5.2.6

Dividing the networks - Networks from networks


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Extra: Classes of IP Addresses


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Extra: Classes of IP Addresses


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Routing How Our Data Packets are Handled

Routing Protocols


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Routing is an OSI Layer 3function.

Routing is the process of finding

the most efficient path from onedevice to another.

Routing and Layer 2 Switching


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Hc vin mng Bch khoa - Website: www.bkacad.com 63

Router and Switch


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Each computer and router interface maintains anARP table for Layer 2 communication. The ARPtable is only effective for the broadcast domain (or

LAN) that it is connected to

MAC addresses are not logically organized, but IP

addresses are organized in a hierarchical manner.

Routed versus Routing


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Routed protocol: used at the network layer that transfer

data from one host to another across a router.

Routing protocols: allow routers to choose the best pathfor data from source to destination.

Routing protocol


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Provides processes for sharing route information Examples:

Routing Information Protocol (RIP), Interior Gateway RoutingProtocol (IGRP), Open Shortest Path First (OSPF), BorderGateway Protocol (BGP), and Enhanced IGRP (EIGRP)

Supporting communication outside our network


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To communicate with a device on another

network, a host uses the address of thisgateway, or default gateway, to forward apacket outside the local network.

The router also needs a route that defineswhere to forward the packet next. This is

called the next-hop (?) address.

5.3.1

Supporting communication outside our network


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5.3.1

IP Packets Carrying Data End-to-End


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5.3.2



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A gateway The way out of our network


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5.3.3.1



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5.3.3.2

Route command ?



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5.3.3.3

A Route The Path to a Network


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5.3.4.1

Routing table


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The routing table stores information aboutconnected and remote networks.

Routes in a routing table have 3 main

features:Destination network

Next-hop

Metric

Default route ?

Host Routing Table


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C:/> netstat -r5.3.4.2

Host Routing Table


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Routing table entries


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5.3.5.1

Routing table entries


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5.3.6

Default route


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5.3.5.2

Packet forwarding


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Routing is done packet-by-packet and hop-

by-hop. Each packet is treatedindependently in each router along thepath.

The router will do one of 3 things with thepacket: Forward it to the next-hop router

Forward it to the destination host Drop it

5.3.7

Packet forwarding


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5.3.7.1

Packet forwarding

U i h D f l R


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Using the Default Route

5.3.7.2

The default route is also known as

the Gateway of Last Resort.

Packet forwarding


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5.3.7.3

Lab 5.3.7.4


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Routing Processes How Routes are Learned

Routing protocol Sharing the route


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5.4.1

Routing protocol Sharing the route


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manually configured on the router or learned dynamically from other

routers in the same internetwork


This route information can be

Static Routing


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5.4.2

Dynamic Routing


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5.4.3.1

Routing protocols are the set of

rules by which routers

dynamically share their routing

information

Lab 5.4.3.2


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Extra: IGP and EGP


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Link state and Distance Vector

Th di i h


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The distance-vector routing approachdetermines the distance and direction,

vector, to any link in the internetwork. Routers using distance-vector algorithms

send all or part of their routing table entries

to adjacent routers on a periodic basis. This happens even if there are no changes

in the network. Eg: RIP, IGRP, EIGRP

Link state and Distance Vector

Li k i l d i di


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Link state routing protocols send periodicupdate at longer time interval (30), Flood

update only when there is a change intopology.

Link state use their database to creat

routing table. Eg: OSPF, IS-IS

Q&A

In a connectionless system:


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In a connectionless system:

The destination is notcontacted before a packet is

sent.


Q&A

If the default gateway is configured


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If the default gateway is configuredincorrectly on the host, what is the impact on

communications?The host can communicate with other

hosts on the local network, but is unable

to communicate with hosts on remotenetworks.


Q&A

What type of routing uses


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What type of routing usesinformation that is manuallyentered into the routing table?

static


Q&A

When the destination network is not


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When the destination network is notlisted in the routing table of a Cisco

router, what are two possible actionsthat the router might take?

The router discards the packet.The router forwards the packet out

the interface indicated by the defaultroute entry.


Q&A

What are the key factors to


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What are the key factors toconsider when grouping hosts into

a common network?

purpose

geographic location

ownership


Q&A

What are three common problems


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What are three common problemswith a large network?

performance degradation

security issues

host identification


Q&A


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Refer to the exhibit. All devices shown in the exhibit have

factory default settings. How many broadcast domains are

represented in the topology that is shown?

Terminology

Broadcasts are contained within a network. In this context, a network is alsoknown as a broadcast domain.


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For all other destinations, the hosts only need to know the address of anintermediary device, to which they send packets for all other destinationsaddresses. This intermediary device is called a gateway. The gateway is a

router on a network that serves as an exit from that network. The number of bits of an address used as the network portion is called the

prefix length.

The router also needs a route that defines where to forward the packet next.This is called the next-hop address. If a route is available to the router, the

router will forward the packet to the next-hop router that offers a path to thedestination network.

The default route is used when the destination network is not represented byany other route in the routing table.

Hosts typically have an ARP table, which is a cache of IP/MAC address

mappings. Hosts typically have an ARP table, which is a cache of IP/MAC address

mappings. When you want to send a packet to a local host, your software looksup the IP in the ARP cache, gets the MAC address, constructs an Ethernetheader with the correct source/destination MAC addresses, and sends that.

Summary


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