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Network layer. Chapter 6 Intro to Routing & Switching. objectives. Upon completion of this chapter, you should be able to: Describe the purpose of the network layer Explain why IPv4 uses other layers for reliability Explain how host devices use routing tables to direct packets

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network layer

Network layer

Chapter 6

Intro to Routing & Switching

  • Upon completion of this chapter, you should be able to:
    • Describe the purpose of the network layer
    • Explain why IPv4 uses other layers for reliability
    • Explain how host devices use routing tables to direct packets
    • Compare host and router routing tables
    • Describe components of a router
    • Describe the boot-up process of a Cisco router
    • Configure a Cisco router and interfaces
    • Configure a default gateway
  • Low overhead
    • Delivers packet to destination only
    • Does NOT track or manage flow
      • Other layers handle that
  • Connectionless
    • Doesn’t need a connection established w/ dest.
  • Best effort delivery
    • Unreliable, no guarantee (other layers)
  • Media independent
    • Doesn’t matter which type of cable being used
media independent
Media independent
  • Doesn’t matter what cables/media it travels over
    • What layer handles the prep for media?
  • Read each IP characteristic & decide if it describes connectionless, best effort, or media independent.
review 3q
Review- 3Q
  • IP is described as connectionless, or connection-oriented?
    • Connectionless
  • When using IP, what protocol would be used to acknowledge delivery of packets and retransmission of missing ones?
    • TCP
  • What layer does TCP operate at?
    • Transport
ipv4 packet1
ipV4 packet
  • Includes Header & Payload
  • Header includes:
    • Source & destination IP
    • DS (Differentiated Services)
      • Defines the priority of each packet
    • Protocol
      • TCP/UDP
    • TTL (Time-to-Live)
      • Hops until dropped
sample ipv4 header
Sample ipv4 header

Version, IP’s

DS, Total Length, Flags, TTL

  • Which field in the IPv4 header…
    • Tells the priority of a packet?
      • DS
    • Helps with fragmented packets when split to not exceed the MTU on media?
      • Flags
  • Name a connectionless layer 3 protocol that is popular & in use today?
    • IP
  • What helps IP with guaranteed delivery?
    • TCP
what s wrong with ipv4
What’s wrong with ipv4?
  • Running out of addresses
  • Routing tables growing
  • Lack of end to end connectivity
    • NAT is normally used
      • NAT allows certain private IP addresses to be used within a network and not shared with the outside world
      • The internal private IP gets translated to a public one to send data across Internet
ipv4 to ipv6
IPv4 to IPv6
  • 32-bit addresses
    • 4 billion available
  • Many header fields
  • Must use NAT
    • To conserve addresses
  • 128-bit addresses
    • 340 undecillion avail.
  • Less header fields
    • Better packet handling
  • No need for NAT
    • A lot of IPv6 addresses



sample ipv6 header
Sample ipv6 header

Version, Payload Length, Hop Limit, IPv6 addresses

  • Handout to compare IPv4 & 6 headers
    • Read each IPv6 header description & click which field it belongs to.
  • Explain NAT.
    • Private internal IP addresses. Can’t be seen outside network. Translate to a public IP for the entire internal network.
  • Why is NAT not needed with IPv6?
    • A ton of addresses!
  • What field in an IPv6 header…
    • Has routers use the same path for real-time packets?
      • Flow label
    • Is like the DS field for priority?
      • Traffic Class
  • Review the picture.
  • What kind of IP is being used?
    • IPv4
  • How many hops from PC1 to PC33?
    • 3
host ready to send
Host ready to send
  • Itself (loopback)
  • Local
    • Shares same network address
  • Remote
    • On a different network
host ipv4 routing table
Host ipv4 routing table
  • PC has a “mini” routing table
  • Identify the parts of a host routing table
  • How does a PC know where to send packets?
    • It has its own routing table
  • What 2 commands on a PC will view the routing table?
    • Netstat –r or route print
how to route the packet
How to route the packet
  • After packet reaches DG (router), it looks at the routing table to see where to send it
  • What’s in the table?
    • Directly connected routes
    • Remote routes
routing table
Routing table
  • Show ip route
    • Stored in RAM
  • How it was learned
  • When it was updated
  • Which interface to use to get to that network




.1 /30














R1#show ip route

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route

Gateway of last resort is not set is variably subnetted, 2 subnets, 2 masks

D [90/2170112] via, 00:00:05, Serial0/0/0

D [90/2170112] via, 00:00:05, Serial0/0/0 is variably subnetted, 2 subnets, 3 masks

C is directly connected, GigabitEthernet0/0

L is directly connected, GigabitEthernet0/0 is variably subnetted, 2 subnets, 3 masks

C is directly connected, GigabitEthernet0/1

L is directly connected, GigabitEthernet0/1 is variably subnetted, 2 subnets, 3 masks

C is directly connected, Serial0/0/0

L is directly connected, Serial0/0/0


    • Identify the elements of a routing table
  • Lab
    • Discover your PC’s routing table
  • A packet comes into the interface of a router. Which address does it check?
    • Destination IP
  • What does it then use that address to look at?
    • Routing table
  • What happens if a match is found for the destination network?
    • It sends it out that port towards that destination
  • What command displays the routing table on a router?
    • Show ip route
  • Where is the routing table stored?
    • RAM
  • What would happen if a destination network was not in the routing table and there was no default route configured?
    • Packet is dropped
router interfaces
Router interfaces

WAN Serial Ports in slot




Interfaces in slot


Port RJ45


Port RJ45

    • Match the router interface or function with its description
  • Lab
    • Explore the router externally & using show commands
  • Lab
    • Determine connectivity options on a router
review 5q
Review- 5Q
  • Where is the IOS stored?
    • Flash
  • Where is the running-config at?
    • RAM
  • Where is the startup-config stored?
    • NVRAM
  • Where is the limited IOS at?
    • ROM
  • Where is the routing table stored?
    • RAM
review 4q
Review- 4Q
  • Based on the last 5 questions, which ones will get “lost” if you lose power to the router & why?
    • Running-config & routing table because they are in RAM
  • Which ports on the router will allow out-of-band management?
    • Console & AUX
  • If you want to gain access to router configuration remotely through Telnet or SSH, which interfaces would you be “connecting” to?
    • Ethernet/LAN or Serial/WAN
  • What 2 things get copied into RAM upon a normal boot?
    • IOS & startup-config
boot summary
Boot summary
  • Similar to PC booting
  • POST
  • IOS in flash memory & loads into RAM
  • Startup-config loads into RAM
    • Becomes running-config
  • Changes made to config happen in RAM/running-config
step 1
Step 1
  • POST tests hardware (CPU, Memory)
  • ROM: Bootstrap
    • Boot Image
    • Begins search for IOS
step 2
Step 2
  • Locate & load IOS into RAM
    • Usually in Flash (default) or TFTP Server
step 3
Step 3
  • Configuration File or Setup
    • NVRAM (1st), TFTP Server (2nd), Console (3rd)
    • If found, copies into RAM
      • Becomes the running-config
    • If not found, enters Setup Mode
verify troubleshoot
Verify & troubleshoot
  • Show version
  • IOS version
  • Version of bootstrap
  • Location & name of IOS
  • CPU & RAM
  • Interfaces
  • Amount of NVRAM & Flash
  • Config-register
    • 0x2102

Model & IOS version

Bootstrap version

IOS locale & name

Amount of RAM



Amt. of NVRAM

Amt. of Flash


  • In Packet Tracer…
    • Add an 1841 router
    • Go in & answer questions based on show version output on handout
    • Add a 2960 switch and do the same
    • The router boot process
    • Put the steps in order
review 4q1
Review- 4q
  • When would a limited version of the IOS load?
    • If the full IOS can’t be found
  • What does this mean?Flash0: c1841-universal.152-4.bin
    • The IOS is in flash & that’s its file name
  • What happens first when booting a router?
    • POST
  • When loading the IOS, where does it look?
    • Flash, TFTP Server, ROM
initial settings
Initial settings
  • Hostname
  • Passwords
    • Enable, enable secret, console, vty
  • Banner motd
  • Service password-encryption
  • Save= copy run start
  • Show run/ show start
  • Complete #5
  • Lab
    • Configure initial settings
verify interfaces
Verify interfaces
  • Show interfaces
  • Show ipint brief
  • Show ip route- routing table
configure dg on switch
Configure DG on Switch
  • S1(config)#interface vlan1
  • S1(config-vlan)#ip address
  • S1(config-vlan)#no shutdown
  • #2
  • Configuring the Default Gateway
    • You will use various show commands to display the current state of the router. You will then use the Addressing Table to configure router Ethernet interfaces. Finally, you will use commands to verify and test your configurations.
  • Troubleshooting DG Issues
  • Build a network or routers and switches
review 4q2
Review- 4q
  • What command saves the config?
    • Copy run start
  • What command displays all interface statuses and IP addresses in summary?
    • Show ipint brief
  • What command displays router memory, IOS name, and config-register?
    • Show version
  • What does your PC need to have configured in order to get out of your network?
    • Default gateway
review 3q1
Review- 3q
  • When will a packet go through a router?
    • To go to another network if destined for it
  • When configuring an interface, what command turns it on?
    • No shutdown
  • 2 routers connect with a serial cable. You configured one of their S0/0 interfaces but the link still isn’t working. Why?
    • Both connected must be configured before it works
review study
Review & study
  • Complete the study guide handout
  • Take the quiz on
  • Jeopardy review

In this chapter, you learned:

  • The network layer, or OSI Layer 3, provides services to allow end devices to exchange data across the network.
  • The network layer uses four basic processes: IP addressing for end devices, encapsulation, routing, and de-encapsulation.
  • The Internet is largely based on IPv4, which is still the most widely-used network layer protocol.
  • An IPv4 packet contains the IP header and the payload.
  • The IPv6 simplified header offers several advantages over IPv4, including better routing efficiency, simplified extension headers, and capability for per-flow processing.
  • In addition to IP addressing, the network layer is also responsible for routing.
  • Hosts require a local routing table to ensure that packets are directed to the correct destination network.
  • The local default route is the route to the default gateway.
  • The default gateway is the IP address of a router interface connected to the local network.
  • When a router receives a packet, it examines the destination IP address to determine the destination network.
  • The routing table of a router stores information about directly-connected routes and remote routes to IP networks.
    • If the router has an entry in its routing table for the destination network, the router forwards the packet.
    • If no routing entry exists, the router may forward the packet to its own default route, if one is configured, or it will drop the packet.
  • Routing table entries can be configured manually on each router to provide static routing or the routers may communicate route information dynamically between each other using a routing protocol.
  • In order for routers to be reachable, the router interface must be configured.
network layer2

Network layer

Chapter 6

Intro to Routing & Switching