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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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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Network Layer Communication from Host to Host
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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Network Layer Communication from Host to Host
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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Network Layer Communication from Host to Host
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 Communication from Host to Host
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Network Layer Communication from Host to Host
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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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Network Layer Fields
4 bits Indicates datagram header length in 32 bit words
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Network Layer Fields
8 bits Specifies the level of importance that has been
assigned by upper-layer protocol
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Network Layer Fields
16 bits Specifies the length of the entire packet in bytes,
including data and header
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Network Layer Fields
16 bits Identifies the current datagram
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Network Layer Fields
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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Network Layer Fields
13 bits Used to help piece together datagram
fragments
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Network Layer Fields
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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Network Layer Fields
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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Network Layer Fields
16 bits
Helps ensure IP header integrity Not caculated for the encapsulation data
N k L Fi ld
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Network Layer Fields
32 bits Specifies the sending node IP address
N t k L Fi ld
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Network Layer Fields
32 bits Specifies the receiving node IP address
N t k L Fi ld
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Network Layer Fields
Variable length
Allows IP to support various options, such as security
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Network Layer Fields
Variable length Extra zeros are added to this field to ensure that the
IP header is always a multiple of 32 bits.
Network Layer Fields
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Network Layer Fields
Variable length up to 64 KB Contains upper-layer information
Network Layer Fields
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Network Layer Fields
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Networks
Dividing Hosts into Groups
Separating Hosts into Common Groups
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Separating Hosts into Common Groups
Networks can be grouped based on factors that include:Geographic location
Purpose
Ownership Geographic
5.2.1
Separating Hosts into Common Groups
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Separating Hosts into Common Groups
Purpose: Users who have similar tasks typically usecommon software, common tools, and have common
traffic patterns.
Separating Hosts into Common Groups
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Separating Hosts into Common Groups
Purpose
Separating Hosts into Common Groups
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Separating Hosts into Common Groups
Ownership: To assists in controlling access to thedevices and data as well as the administration of the
networks.
Separating Hosts into Common Groups
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Separating Hosts into Common Groups
Ownership
Why separate hosts into networks ?
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Why separate hosts into networks ?
Performance degradation
Security issues Address Management
5.2.2
Common issues with large networks are:
Why separate hosts into networks ?
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Why separate hosts into networks ?
Improving Performance
Broadcast domain ?
5.2.2
Lab 5.2.2
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Lab 5.2.2
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Why separate hosts into networks ?
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Why separate hosts into networks ?
Increase network security
5.2.3
Why separate hosts into networks ?
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y p
Increase network security
Why separate hosts into networks ?
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y p
Increase network security
Why separate hosts into networks ?
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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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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
IP Packets Carrying Data End-to-End
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IP Packets Carrying Data End-to-End
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IP Packets Carrying Data End-to-End
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IP Packets Carrying Data End-to-End
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IP Packets Carrying Data End-to-End
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A gateway The way out of our network
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5.3.3.1
A gateway The way out of our network
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5.3.3.2
Route command ?
A gateway The way out of our network
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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
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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.
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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.
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Q&A
What type of routing uses
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What type of routing usesinformation that is manuallyentered into the routing table?
static
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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.
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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
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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
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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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