Wednesday, February 10, 2016

CCNAv2 Chapter 4 Notes - Routers

Chapter 4
CCNA 2 Chapter 4 - Routers
  • Routers are Computers
    • It has a CPU, OS (Routers use Cisco IOS), Memory & Storage (RAM/ROM/NVRAM/FLASH/HD)

Important Table - Router Components (of Memory)
  • Running IOS
  • Running Config File
  • IP Routing & ARP Table
  • Packet Buffer
  • Bookup Instructions
  • Basic Diagnostic Software
  • Limited IOS
  • Startup Config File
  • IOS
  • Other System Files

Routers use specialized ports and network interface cards to interconnect to other networks.
  • Routers connect multiple networks.
  • Routers have multiple interfaces, each on a different IP network.

> Routers Choose Best Paths
  • Routers use static routes and dynamic routing protocols to learn about remote networks and build their routing tables.
  • Routers use routing tables to determine the best path to send packets.
  • Routers encapsulates the packet and forwards it to the interface indicated in the routing table.

> Packet Forwarding Methods
  • Process Switching
    • An older packet forwarding mechanism still available for Cisco router.
  • Fast Switching
    • A common packet forwarding mechanism which uses a fast-switching cache to store next-hop information.
  • Cisco Express Forwarding (CEF)
    • The most recent, fastest, and preferred Cisco IOS packet-forwarding mechanism. Table entries are not packet-triggered like fast switching but change-triggered.

> Default Gateways
  • To enable network access devices must be configured with the following IP address information:
    • IP Address
      • Identifies unique host on local network.
    • Subnet Mask
      • Identifies the host’s network subnet.
    • Default Gateway
      • TLDR “How to get out of the network”.

> Enable IP on a Host
  • Statically Assigned IP Address
    • Manually assigned IP.
      • Used to identify specific network resources such as servers/printers.
      • Can be used in small networks with little amount of hosts.
  • Dynamically Assigned IP Address
    • Automatically obtained IP address.
      • From DHCP Server.
      • Most hosts acquire their IP address information through DHCP.

> Device LEDs
  • Lights on the back of the router provide information if needed.

> Console Access
  • Console Access requires…
    • A computer with proper software (see third item: terminal emu).
    • Console Cable (RJ-45 to DB9 Console Cable).
    • Terminal Emulation Software: TeraTerm/ PuTTY/ HyperTerminal.

> Enabling IP on a Switch
  • Infrastructure devices require IP addresses.
  • Management IP addresses is assigned on a virtual interface.

> Configure Basic Router Settings
  • Name the Device
    • Distinguish the router’s name from other routers.
  • Secure Management Access
    • Secure privileged exec, user exec, telnet access, and encrypted passwords.

> Configure IPv4 Router Interface
  • To Configure:
  • Configured with an IP & Subnet Address.
  • Must use ‘no shutdown’.
  • Serial Cable using ‘DCE’ must have a clock rate.
  • Provide a description.

> Configure IPv6 Router Interface
  • To Configure:
    • Use IPv6 address (link local / eui-64).
    • Activate using ‘no sh’.
  • IPv6 supports more than one address.
    • Configure a specified global unicast address (addr/length).
    • Configure a global IPv6 Address with an Interface ID in the low order 64 bits.
    • Configure a link-local address.

> Configure a Loopback Address
  • A loopback interface is a logical interface that is internal to the router.
    • It is not assigned to a physical port, it is considered a software interface.
    • A loopback interface is used for testing.
    • It is important in the OSPF routing process.

> Verify Interface Settings
  • Show commands are used to verify operation and configuration of interface:
    • show ip int brief
    • show ip route
    • show running-config
    • show interface …
    • show ip interfaces

  • You can filter show commands.
    • Use the ‘terminal length <number>’ command to specify the number of lines to be displayed.
    • To filter out specific output of commands, use the “|” pipe character.
    • Can use parameters to filter:
      • section, include, exclude, and begin

> Best Path
  • Best path is selected by a routing protocol based on the value or metric it uses to determine the distance of a network.
    • A metric is the value used to measure the distance to a given network.
    • Best path to a network is the path with the lowest network.
  • Dynamic routing protocols use their own rules and metrics to build and update tables:
    • Routing Information Protocol (RIP) - Hop Count
    • Open Shortest Path First (OSPF) - Cost-based on cumulative bandwidth from source to destination.
    • Enhanced Interior Gateway Routing Protocol (EIGRP) - Bandwidth, delay, load, reliability.

> Load Balancing
  • When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally:
    • Equal cost load balancing can improve network performance.
    • Equal cost load balancing can be configured to use both dynamic routing protocol and static routes.

> Administrative Distance
  • If multiple paths to a destination are configured on a router, the path installed in the routing table is the one with the lowest Administrative Distance (AD).
    • A static route with an AD of 1 is more reliable than an EIGRP discovered route with an AD of 90.
    • A directly connected route with an AD of 0 is more reliable than a static route with an AD of 1.
      • Diagram:

> Routing Table Sources
  • show ip route
    • Display the contents of the routing table.
  • local route interfaces
    • Added to the routing table when an interface is configured.
  • directly connected interfaces
    • Added to the routing table when an int is configured and active.
  • static routes
    • Added when a route is manually configured and the exit interface is active.
  • dynamic routing protocol
    • Added when EIGRP or OSPF are implemented networks are identified.
> Static Routes
  • Static routes and default static routes can be implemented after directly connected interfaces are added to the routing table.

> Dynamic Routing
  • Where the routers “do the work for us”.
    • Dynamic routing is used by routers to share information about the reachability and status of remote networks. It performs network discovery and maintains routing tables.


  • OSPF

  • IS-IS

  • etc...


  1. Replies
    1. Hey, no problem. Glad I could help.
      Also, thanks for being the first comment on this blog. :P

  2. this is really overwhelming, i cant thank you enough. thank you

  3. Hi Nicholas, thank you for this amazing Blog. It really helps me a lot in my CCNA studies.

    1. Hello Jay!

      You are most certainly welcome! Thanks for being an active supporter of Seeseenayy.

      Have a great day.


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