Router Basics

Cisco routers have four basic types of memory: RAM/DRAM, NVRAM, ROM, and Flash (EPROM) memory.

Memory/Type Contents
RAM/DRAM Active program and operating system instructions, the running configuration file, routing tables
NVRAM Startup configuration file
ROM POST, bootstrap, and startup/power-up utilities (usually limited version of), Cisco IOS
Flash Cisco IOS

For the exam, remember that the router's RAM is volatile DRAM that holds the working data and files of the router, and when the power is turned off, the contents of the RAM are lost.

Where DRAM is volatile and must have a power source to hold its contents, nonvolatile RAM (NVRAM) can hold its contents when the main power source is lost. You may even know this type of memory as static RAM (SRAM), but on the CCNA exam, remember it as NVRAM. The Cisco router uses NVRAM to store its startup configuration file.

There's another type of memory, called ROM (Read-Only Memory), that's even more reliable than NVRAM. Like NVRAM, ROM does not lose its contents when the power is turned off. On the Cisco router, ROM contains the program instructions for the power-on self-test (POST) diagnostics, the bootstrap program, which is the startup program for the router, and the router's operating system.

Cisco routers contain two copies of the IOS, a stripped won version that is stored in ROM and the full up-to-date version stored in Flash. The IOS version in Flash memory is the one you should focus on for the exam.

The Cisco Discovery Protocol (CDP) is a proprietary Cisco protocol that allows you to get information about directly connected CIsco router, bridges, and switches.

Stuff to memorize about CDP

  • CDP uses SNAP at the data link leve, which makes it protocol independent.
  • CDP detects attached devices regardless of what protocol they're running.
  • CDP is enabled by default when the router is booted on all interfaces.
  • CDP update requests are multicast by default every 60 seconds using layer 2 multicast messages.
  • CDP has a default holdtime. The amount of time a device holds a CDP update before discarding it is 180 seconds.

  • The two Exec modes in the user interface are User Exec and Enable Exec (a.k.a. Privileged) modes.

    Setup Mode
    when a router is first configured from the console port, Setup Mode is invoked. Setup mode can also be invoked from the Enable Exec mode prompt with the setup command, or by rebooting the router after deleting its startup-config file through the erase startup-config command.

    Another way to restart the router is to issue the reload command.

    Configuration Mode
    Like the setup mode, you can enter the configuration mode by using a command, in this case config. To move into what is called global configuration mode, enter the following config command at the Enable Exec prompt (the one with the # symbol):

    ROUTERNAME#config terminal

    The parameter terminal or its abbreviation t is absolutely necessary.

    The prompt for Configuration Mode is:
    RouterA#(config)

    The prompt for Configuration Interface Mode is:
    RouterB#(config-if)
    The suffix -if means interface.

    Enhanced Editing Keyboard Commands

    Key(s) Action
    Ctrl+A Move to the beginning of the current line
    Ctrl+E Move to the end of the current line
    Ctrl+B (or left-arrow key) Move back one character
    Ctrl+F (or right-arrow key) Move forward on character
    Ctrl+N (or down-arrow key) Recall most recent command
    Ctrl+P (or up-arrow key) Recall previous command
    Esc+B Move back to beginning of previous word (or beginning of current word).
    Esc+F Move forward one word
    Tab Complete the current word (shortcut command)

    Other configuration commands used in editing the comand history are:

  • Show history: This command displays the contents of the command history.
  • Terminal history size: This command is used to change the default value of ho wmany lines of the command history are to be displayed by a show history command. The default is to show the last 10 commands.
  • Terminal no editing: This command turns off the enhanced editing feature and is used to exit enhanced editing.
  • Terminal editing: This command turns the enhanced editing feature back on. Enhanced editing is on by default and must be turned off with the terminal no editing command.
  • Naming Your Router

  • You must first enter Enable Exec (privileged), the one that displays the number or pound sign prompt, and then enter Global Configuration mode, the one that adds the word "config" in parentheses.
  • The command used to assign the hostname is hostname. - Router(config)#hostname Name


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