Routers Basics


  • It is an internetworking component that connects networks at different geographical locations.It has
  • Serial Port
  • Auxillary Port
  • Console Port
  • AUI (Attachment Unit Interface) or Ethernet(which uses RJ45 port)

Routers are of two types

  • Fixed -fixed number of slots

(series from 700 to 2500, exception 1750, 1751)

  • Modular – Upgradable slots and ports

(series from 2600 to 12000)

Cisco Hierarchical Model

  • Core – 700 to 2500
  • Distribution – 2600 to 5000
  • Access – 7000 to 12000

Router Memory Elements:

When a router boots up, it performs a series of steps, called the boot sequence, to test the hardware and load the necessary software. The boot sequence consists of the following steps:

  1. Power on self test (POST): tests the hardware to verify that all components of the device are operational and present.
  2. The bootstrap loader in ROM executes: The bootstrap loader is a program in ROM that is used to find where a valid Cisco IOS image is located.
  3. If a valid Cisco IOS image is located, it is loaded.
  4. IOS loads configuration file. Once the IOS image is loaded, it will search for a valid startup configuration in NVRAM.
  5. If a valid startup configuration file cannot be found, the router will load the System Configuration Dialog (sometimes called setup mode). This mode allows you to perform the initial configuration of the router.


RAM holds the running configuration of a router. It is a volatile memory and looses memory If rebooted. So make sure the running-configuration is saved frequently

Backup/Restore IOS image using TFTP

TFTP Server allows you to take backup, upload and save your IOS Image and current configuration on it. TFTP is a software program of 1.3Mb. This should be installed on that TFTP server i.e., (Personnel Computer).

Backup Sources:

  • Through Telnet session
  • Through console session
  • Through Auxillary Port (Remote session)

Backup IOS Image:

  • Router#Copy Flash Tftp
  • Address or name of the remote host [ ]? (Ip address of tftp)
  • Destination file name [ ] ? (Press enter key or a name)

Backup Of Configuration:

  • Router#Copy Startup-config Tftp
  • Address or name of the remote host [ ]? (Ip address of tftp)
  • Destination file name [ ] ? (Give a name)


  • This can be done only on Console session
  • Router#Copy tftp Flash:
  • Address or name of remote host []?
  • Source file name []?
  • Destination file name []?
  • Erase Flash: before copying [confirm]?
  • Erasing the Flash file system will remove all files: continue? [confirm]
  • Erasing device eeeeeeee………
  • Loading!!!!!!!!!!!!!!!!!!!!!!!!!!!!!…….!!!!…….!!!!!!

Modes of Router:

  • User Mode
  • Privilege Mode
  • Global Config Mode
  • Interface config mode
  • Line configuration mode
  • Router Mode
  • Sub-Interface Mode

The Clock command

This is the only command that can be performed in Privilege mode. Any rest configurations are done in global configuration mode only.

Router Password Configuration

  • Privilege passwordEnable password cisco – Plain text
  • Enable secret cisco – Encrypted form – highest priority

Virtual Terminal password

  • (Config)# line vty 0 4
  • (Config-line)#password cisco
  • (Config-line)#login
  • Console Password
  • (Config)# line console 0
  • (Config-line)#password cisco
  • (Config-line)#login
  • Auxillary password
  • (Config)# line aux 0
  • (Config-line)#password cisco
  • (Config-line)#login

Configuring Interfaces on a Network:

  • LAN Interface (Ethernet)
  • WAN Interface(Serial Port)

Interpreting the Interface Status

Router#sh serial S0

Serial0 is up, line protocol is up: The interface is operational

Serial0 is up, line protocol is down: Any connection problem

Serial0 is down, line protocol is down: Problem with the Interface

Serial0 is Administratively down, line protocol is down: The interface is shutdown.

Hint: The interface status plays a vital role in troubleshooting the network problems. Be sure you get with these


  • Cisco’s proprietary protocol
  • CDP is a L2 protocol
  • When an Cisco device boot up CDP is enabled by default
  • The CDP is limited to immediate neighbors only
  • The summary includes
  • Device Identifier (switch configured or domain name)
  • Port Identifier (E0 or S0)
  • Capabilities list (the device can act as source Route Bridge or router)

CDP commands:

  • (Config)# sh cdp neighbor
  • Gives remote port ID and device ID
  • (Config)# Sh cdp entry
  • (Config)# Sh cdp interface
  • Gives status of interface, encapsulation type, how often packets are sent and CDP hold time
  • (Config)#Sh cdp traffic
  • Shows the no of packets sent
  • Disable CDP:(Config)# # no cdp run
  • (Config)#int S0
  • (Config-lf)#no cdp enable

Here is a related post you maybe interested in: Networking Basics