Overview
Now that you have an understanding of the router command line interface, it is time to examine the router components that ensure efficient and effective delivery of data on a network. In this chapter, you will learn the correct procedures and commands to access a router, examine and maintain its components, and test its network connectivity.

4.1	Router Components
		4.1.1	External router configuration sources
In this section, you will learn about the router components that play a key role in the configuration process. Knowing which components are involved in the configuration process gives you a better understanding of how the router stores and uses your configuration commands. Being aware of the steps that take place during router initialization will help you determine what and where problems may occur when you start up your router. You can configure a router from many external locations as shown in the Figure, including the following: from the console terminal (a computer connected to the router through a console port) during its installation via modem by using the auxiliary port from Virtual Terminals 0-4, after it has been installed on the network from a TFTP server on the network

4.1	Router Components
		4.1.2	Internal router's configuration components
The internal architecture of the Cisco router supports components that play an important role in the startup process, as shown in the Figure. Internal router configuration components are as follows: RAM/DRAM -- stores routing tables, ARP cache, fast-switching cache, packet buffering (shared RAM), and packet hold queues; RAM also provides temporary and/or running memory for a router's configuration file while the router is powered; RAM content is lost during a power down or restart NVRAM -- non-volatile RAM stores the router's backup/startup configuration file; NVRAM content is retained during power down or restart Flash -- erasable, reprogrammable ROM that holds the operating system image and microcode; Flash memory enables software updates without removing and replacing processor chips; Flash content is retained during power down or restart; Flash memory can store multiple versions of IOS software ROM -- contains power-on diagnostics, a bootstrap program, and operating system software; software upgrades in ROM require removing and replacing pluggable chips on the CPU Interfaces -- network connections on the motherboard or on separate interface modules, through which packets enter and exit a router

4.1	Router Components
		4.1.3	RAM for working storage in the router
RAM is the working storage area for a router. When you turn a router on, the ROM executes a bootstrap program. This program performs some tests, and then loads the Cisco IOS software into memory. The command executive, or EXEC, is one part of the Cisco IOS software. EXEC receives and executes commands you enter for the router. As shown in the Figure, a router also uses RAM to store an active configuration file and tables of network maps and routing address lists. You can display the configuration file on a remote or console terminal. A saved version of this file is stored in NVRAM. It is accessed and loaded into main memory each time a router initializes. The configuration file contains global, process, and interface information that directly affects the operation of a router and its interface ports. An operating system image cannot be displayed on a terminal screen. An image is usually executed from the main RAM and loaded from one of several input sources. The operating software is organized into routines that handle the tasks associated with different protocols, such as data movement, table and buffer management, routing updates, and user command execution.

4.1	Router Components
		4.1.4	Router modes
Whether accessed from the console or by a Telnet session through a TTY port, a router can be placed in several modes. (see Figure) Each mode provides different functions: user EXEC mode -- This is a look-only mode in which the user can view some information about the router, but cannot make changes. privileged EXEC mode -- This mode supports the debugging and testing commands, detailed examination of the router, manipulation of configuration files, and access to configuration modes. setup mode -- This mode presents an interactive prompted dialog at the console that helps the new user create a first-time basic configuration. global configuration mode -- This mode implements powerful one-line commands that perform simple configuration tasks. other configuration modes -- These modes provide more detailed multiple-line configurations. RXBOOT mode -- This is the maintenance mode that you can use, among other things, to recover from lost passwords.

	4.2	Router Show Commands
		4.2.1	Examining router status by using router status commands
In this section, you will learn basic commands that you can issue to determine the current status of a router. These commands help you obtain vital information you need when monitoring and troubleshooting router operations. It is important to be able to monitor the health and state of your router at any given time. As shown in the Figure, Cisco routers have a series of commands that allow you to determine whether the router is functionally correct or where problems have occurred. Router status commands and their descriptions are shown below. show version -- displays the configuration of the system hardware, the software version, the names and sources of configuration files, and the boot image show processes -- displays information about the active processes show protocols -- displays the configured protocols; shows the status of all configured Layer 3 protocols show memory -- shows statistics about the router's memory, including memory free pool statistics show stacks -- monitors the stack use of processes and interrupt routines and displays the reason for the last system reboot show buffers -- provides statistics for the buffer pools on the router show flash -- shows information about the Flash memory device show running-config (write term on Cisco IOS Release 10.3 or earlier) -- displays the active configuration file show startup-config (show config on Cisco IOS Release 10.3 or earlier) -- displays the backup configuration file show interfaces -- displays statistics for all interfaces configured on the router

4.2	Router Show Commands
		4.2.2	The show running-config and show startup-config commands
Among the most used Cisco IOS software EXEC commands are show running-config and show startup-config. They allow an administrator to see the current running configuration on the router or the startup configuration commands that the router will use on the next restart. NOTE: The commands, write term and show config, used with Cisco IOS Release 10.3 and earlier, have been replaced with new commands. The commands that have been replaced continue to perform their normal functions in the current release but are no longer documented. Support for these commands will cease in a future release. You can recognize an active configuration file by the words current configuration at the top. You can recognize a backup configuration file when you see a message at the top that tells you how much non-volatile memory you have used.

4.2	Router Show Commands
		4.2.3	The show interfaces, show version and show protocols commands
The show interfaces command displays configurable parameters and real-time statistics related to all interfaces configured on the router (see Figure ). The show version command displays information about the Cisco IOS software version that is currently running on the router (see Figure ). You use the show protocols command to display the protocols configured on the router. This command shows the global and interface-specific status of any configured Level 3 protocols (for example, IP, DECnet, IPX, and AppleTalk). (see Figure ).

4.2

Router Show Commands

4.2.4

Lab: router show commands

Lab Activity

This lab will help you become familiar with the router show commands. The show commands are the most important information gathering commands available for the router. The show running-config (or "show run") is probably the single most valuable command to help determine the current status of a router because it displays the active configuration file running in RAM. The show startup-config (or "show start") command displays the backup configuration file that is stored in non-volatile or NVRAM. This is the file that will be used to configure the router when it is first started or rebooted with the "reload" command. All of the detailed router interface settings are contained in this file. The show flash command is used to view the available and the amount used of flash memory. Flash is where the Cisco Internetwork Operating System (IOS) file or image is stored. The show arp command displays the router's IP to MAC to Interface address mapping. The show interface command displays statistics for all interfaces configured on the router. Show protocol command displays global and interface-specific status of configured layer 3 protocols (IP, IPX etc.).