CCNA 1 v6.0 Study Material – Chapter 6: Network Layer

Chapter 6 – Sections & Objectives

  • 6.1 Network Layer Protocols
    • Describe the purpose of the network layer in data communication.
    • Explain why the IPv4 protocol requires other layers to provide reliability.
    • Explain the role of the major header fields in the IPv4 and IPv6 packet.
  • 6.2 Routing
    • Explain how a host device uses routing tables to direct packets to itself, a local destination, or a default gateway.
    • Compare a host routing table to a routing table in a router.
  • 6.3 Routers
    • Describe the common components and interfaces of a router.
    • Describe the boot-up process of a Cisco IOS router.
  • 6.4 Configure a Cisco Router
    • Configure initial settings on a Cisco IOS router.
    • Configure two active interfaces on a Cisco IOS router.
    • Configure devices to use the default gateway.

6.1 Network Layer Protocols

Network Layer in Communications

  • The Network Layer
    • End to End Transport processes
    • Addressing end devices
    • Encapsulation
    • Routing
    • De-encapsulating
  • Network Layer Protocols
    • IPv4
    • IPv6

Characteristics of the IP Protocol

  • Encapsulating IP
    • Segments are encapsulated into IP packets for transmission.
    • The network layer adds a header so packets can be routed to the destination.
  • IP – Connectionless
    • Sender doesn’t know if the receiver is listening or the message arrived on time.
    • Receiver doesn’t know data is coming.
  • IP – Best Effort Delivery
    No guarantees of delivery are made.
  • IP – Media Independent
    • IP can travel over different types of media.

IPv4 Packet

    • IPv4 Packet Header

  • Version = 0100
  • DS = Packet Priority
  • TTL = Limits life of Packet
  • Protocol = Upper layer protocol such as TCP
  • Source IP Address = source of packet
  • Destination IP Address = destination of packet

IPv6 Packet

  • Limitations of IPv4
    • IP address depletion
    • Internet routing table expansion
    • Lack of end-to-end connectivity
  • Introducing IPv6
    • Increased address space
    • Improved packet handling
    • Eliminates the need for NAT
  • EncapsulatingIPv6
    • Simplified header format
    • No checksum process requirement
    • More efficient Options Header mechanism
    • Flow Label field makes it more efficient.
  • IPv6 Packet Header
    – xx

IPv6 Packet (Cont.)

    • IPv6 Packet Header
      – xx

  • Version = 0110
  • Traffic Class = Priority
  • Flow Label = same flow will receive same handling
  • Payload Length = same as total length
  • Next Header = Layer 4 Protocol
  • Hop Limit = Replaces TTL field

6.2 Routing

How a Host Routes

  • Host Forwarding Decision
    • Three types of destination: itself, local host, remote host.
  • Default Gateway
    • Routes traffic to other networks
    • Has a local IP address in the same address range as other hosts on the network
    • Can take data in and forward data out
  • Using the Default Gateway
    • Hosts will use the default gateway when sending packets to remote networks.
  • Host Routing Tables
    • Use the netstat –r command to display the
    • host routing table on a Windows machine.

How a Host Routes Router Routing Tables

  • Router Packet Forwarding Decision
    • Routers and hosts forward packets in a similar fashion.
    • The main difference is that routers have more interfaces while hosts often have only one.
    • Devices on directly connected networks can be reached directly.
    • Devices on remote networks are reached through gateway.
  • IPv4 Router Routing Table
    • The router routing table stores network routes the router knows about.
    • Use the show ip route command to display the routing table on a Cisco router.
    • The router routing table also has information on: how the route was learned, its trustworthiness and rating.
    • It also contains which interface to use to reach that specifc destination.
  • Directly Connected Routing Table Entries
    • C – Identifies a directly-connected network, automatically created when an interface is configured with an IP address and activated.
    • L – Identifies that this is a local interface. This is the IPv4 address of the interface on the router.
  • Remote Network Routing Table Entries
  • Remote Network Routing Table Entries
    • Remote destinations can’t be reached directly.
    • Remote routes contain the address of the intermediate network device to be used to reach the destination.
  • Next-Hop Address
    Next-Hop address is the address of the intermediate device used to reach a specifc remote destination.

6.3 Routers

Anatomy of a Router

  • A Router is a Computer
    • Routers have CPU, memory and I/O devices
    • Cisco routers use IOS as their operating system.
  • Router Memory
    • Just as a computer, routers have memory.
    • Routers contain RAM, ROM, NVRAM and Flash memory.
  • Inside a Router
    • Routers have the same general structure.
  • Connect to a Router
    • Routers have may ports to support connections.
  • LAN and WAN Interfaces
    • Routers have LAN and WAN ports.
    • Different models ship with different ports.
    • Ethernet is very common on different router models.

Anatomy of a Router

  • Bootset Files
    • IOS image file, stored in the Flash, contains the IOS.
    • The Flash also stores other system files.
    • The NVRAM stores configuration parameters.
  • Router Bootup Process
    • Perform the POST and load the bootstrap program.
    • Locate and load the Cisco IOS software.
    • Locate and load the startup configuration file or enter setup mode
  • Show Version Output
    • The show version command is very useful.
    • It provides information on the amounts of memory installed, what IOS images was loaded during boot and more.

6.4 Configuring a Cisco Router

Configure Initial Settings

  • Basic Switch Configuration Steps
  • Configure device name
  • Secure EXEC mode
  • Secure VTY lines
  • Secure privilege EXEC mode
  • Secure all passwords
  • Provide legal notification
  • Configure the management SVI
  • Save the configuration

  • Basic Router Configuration Steps
  • Configure device name
  • Secure EXEC mode
  • Secure VTY lines
  • Secure privilege EXEC mode
  • Secure all passwords
  • Provide legal notification
  • Configure the management SVI
  • Save the configuration

Configure Interfaces

    • Configure Router Interfaces
    • Enter the interface sub-configuration mode.
    • Add a description to the Interface (optional)
    • Configure an IPv4 or IPv6 address.
    • Activate the interface with a no shutdown command
  • Verify Interface Configuration
    • show ip route – Displays the contents of the IPv4 routing table stored in RAM.
    • show interfaces – Displays statistics for all interfaces on the device.
    • show ip interface – Displays the IPv4 statistics for all interfaces on a router.

Configure the Default Gateway

    • Default Gateway for a Host

  • Default Gateway for a Switch
    • A default gateway is required for remote network communication.
    • If a switch is to be managed via its VTY lines, it needs a default gateway.
    • Use the ip default-gateway command to configure the default gateway for a switch.

6.5 Chapter Summary


  • Explain how network layer protocols and services support communications across data networks.
  • Explain how routers enable end-to-end connectivity in a small to medium-sized business network.
  • Explain how devices route traffic in a small to medium-sized business network.
  • Configure a router with basic configurations.

Section 6.1 New Terms and Commands

  • encapsulation
  • routing
  • de-encapsulation
  • data
  • packet
  • frame
  • Internet Protocol Version 4 (IPv4)
  • Internet Protocol Version 6 (IPv6)
  • Network Layer PDU = IP Packet
  • Transport Layer PDu
  • Segment Header
  • IP Header

  • Auxiliary port (AUX)
  • connectionless
  • best effort delivery
  • media independent
  • Connectionless
  • unreliable
  • maximum transmission unit (MTU)
  • Version
  • Differentiated Services (DS)
  • Time-to-Live (TTL)
  • Internet Control Message Protocol (ICMP)
  • data payload

  • Identification, Flags, Fragment Offset fields
  • keyword
  • Network Address Translation (NAT)
  • Traffic Class
  • Flow Label
  • Payload Length
  • Next Header
  • Hop Limit
  • local host
  • remote host
  • default gateway

Section 6.2 New Terms and Commands

  • netstat –r
  • route print
  • interface list
  • IPv4 Route Table
  • IPv6 Route Table
  • directly-connected routes
  • remote routes
  • default route
  • show ip route
  • route source
  • destination network
  • outgoing interface
  • administrative distance
  • metric

  • next-hop
  • route timestamp
  • branch routers
  • WAN routers
  • service provider routers

Section 6.3 New Terms and Commands

  • Power-on-self-test POST
  • RAM
  • ROM
  • Flash
  • Synchronous dynamic RAM (SDRAM)
  • WIC
  • high-speed WIC (HWIC)
  • Advanced Integration Module (AIM)
  • Enhanced high-speed WAN interface card (eHWIC)

  • Serial module
  • Ethernet interfaces
  • Auxiliary (AUX) RJ-45 port
  • In-band router interfaces
  • Console
  • Out-of-band
  • Secure Shell (SSH)
  • Telnet
  • startup-config
  • running-config
  • bootstrap program
  • Trivial File Transport Protocol (TFTP)

  • setup mode
  • show version

Section 6.4 New Terms and Commands

  • interface type-and-number
  • ip address ipv4-address subnet-mask
  • description description-text
  • no shutdown
  • show ip interface brief
  • ping ip address
  • show ip route
  • show interfaces
  • show ip interface brief
  • ip default-gateway ip-address

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