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					WANs and Routers

Router Description

          • Specialized computer
          • Like a general purpose PC, a
            router has:
              CPU
              Memory
              System Bus Connecting Internal
               Router Components Together
              I/O Interfaces
              An Operating System
               • Internetwork Operating System (IOS)
Cisco IOS Configuration Files

            • Configuration files contain the
              instructions and parameters that
              control routers.

            • The configuration files specify all
              the information for the correct setup
              and use of the routing and routed
              protocols on a router.
Types of Router Memory

         •   Random Access Memory (RAM)
         •   Nonvolatile RAM (NVRAM)
         •   Flash Memory
         •   Read-Only Memory (ROM)
RAM Characteristics

         • Stores routing tables
         • Holds ARP cache
         • Holds fast-switching cache
         • Performs packet buffering as shared
         • Maintains packet-hold queues
         • Provides temporary memory for the
           configuration file of a router while the
           router is powered on
         • Loses content when a router is powered
           down or restarted
NVRAM Characteristics

         • Provides storage for the startup
           configuration file
         • Retains content when a router is
           powered down or restarted
Flash Memory Characteristics

           • Holds the IOS image
           • Allows software to be updated without
             removing and replacing chips on the
           • Retains content when a router is
             powered down or restarted
           • Can store multiple versions of IOS
           • Is a type of electrically erasable
             programmable read-only memory
ROM Characteristics

         • Maintains instructions for power-on
           self test (POST) diagnostics
         • Stores bootstrap program and
           basic operating system software
         • Requires replacing pluggable chips
           on the motherboard for software
Internal Router Components

           •   CPU
                  The Central Processing Unit executes instructions in the operating system
                     •   System initialization, routing functions, and network interface control
           •   RAM
                  Holds routing table information and running configurations
                  In most routers the RAM provides space for the executable Cisco IOS
                  RAM is generally dynamic random-access memory (DRAM) and can be
                   upgraded with the addition of dual in-line memory modules (DIMMs).
Internal Router Components

           •   Flash
                   Stores the full Cisco IOS software image
                   The router normally loads the IOS from flash.
                   The IOS may be in uncompressed or compressed form.
                   In most routers an executable copy of the IOS is transferred to RAM during
                    the boot process.
           •   NVRAM
                   Stores the startup configuration
                   Retains contents when power is removed.
Internal Router Components

           •   Buses
                  System Bus: Used to communicate between the CPU and the interfaces or
                   expansion slots.
                  CPU Bus: Used by the CPU to transfer instructions and data to or from
                   specified memory addresses
           •   ROM
                  Permanently stores the startup diagnostic code, which is called the ROM
                  The main tasks for ROM are hardware diagnostics during router bootup and
                   loading the Cisco IOS software from flash to RAM.
                  ROMs are not erasable.
Internal Router Components

           •   Interfaces
                   Router connections to the outside
                   The three types of interfaces: LANs, WANs, and console or auxiliary (AUX)
                   The LAN interfaces are usually one of several different varieties of Ethernet
                    or Token Ring
                   The WAN interfaces include serial, ISDN, and integrated CSUs
                   The console and AUX ports are serial ports that are used primarily for the
                    initial configuration of a router. They are used for terminal sessions from the
                    communication ports on the computer or through a modem.
Internal Router Components

           • Power Supply
               The power supply provides the necessary
                power to operate the internal components.
               Larger routers may use multiple or modular
                power supplies.
Inside a 2600 Router
Outside a 2600 Router
                               WAN Connections

          LAN Connections
                            Management Connections
Connecting a PC to a Router
for Configuration
Connecting to a Router

                         • Generally, a utility
                           such as
                           HyperTerminal is
                           used to connect
                           to the router for
                         • These settings
                           are used each
Router Initialization
           • A router initializes by loading the bootstrap
             (usually from ROM), the operating system
             (usually from Flash), and a configuration file
             (usually from NVRAM).
           • If the router cannot find a configuration file, it
             enters setup mode.
Router Initialization
Router POST

        • When a Cisco router powers up, it
          performs a power-on self test (POST).
        • During this self test, the router executes
          diagnostics from ROM on all hardware
               These diagnostics verify the basic operation
                of the CPU, memory, and network interface
        • After verifying the hardware functions,
          the router proceeds with software
After POST
         1. The generic bootstrap loader in ROM executes.
             • A bootstrap is a simple set of instructions that tests
               hardware and initializes the IOS for operation.
         2. The IOS is located.
             • The boot field of the configuration register determines the
               location that is used to load the IOS.
             • If the boot field indicates a flash or network load, boot
               system commands in the configuration file indicate the
               exact name and location of the image.
After POST (2)
         3.   The operating system image is loaded.
              •   When the IOS is loaded and operational, a listing of the available hardware
                  and software components is sent to the console terminal screen.
         4.   The configuration file saved in NVRAM is loaded into main memory
              and executed one line at a time.
              •   The configuration commands start routing processes, supply addresses for
                  interfaces, and define other operating characteristics of the router.
              •   If no valid configuration file exists in NVRAM, the operating system
                  searches for an available TFTP server.
                  •    If no TFTP server is found, the setup dialog is initiated.
Setup Mode

        • Setup mode is a good way to get a
          router functioning on an IP network
        • Setup mode should not be used by
          network administrators to configure
          routers as not all features can be
          set in Setup mode.
Router Boot Sequence

   • Test the router hardware
   • Find and load the Cisco IOS software
   • Find and apply configuration statements, including
     protocol functions and interface addresses
Router Boot Process (1)
                                            1, 2

         1.   ROM
              a) POST                      4
              b) Bootstrap code executed
              c) Check Configuration Register value (NVRAM)
                  0 = ROM Monitor mode
                  1 = ROM IOS
                  2 - 15 = startup-config in NVRAM

         2. Check for IOS boot system commands in startup-config
            file (NVRAM)
             If boot system commands in startup-config
              a) Run boot system commands in order they appear in startup-
                 config to locate the IOS
              b) If boot system commands fail, use default fallback sequence
                 to locate the IOS (Flash, TFTP, ROM)
                                                   1, 2

Router Boot Process (2)                              3


          3.    Locate and load IOS, Default fallback sequence: (No IOS
                boot system commands in startup-config)
               a)       Flash (sequential)
               b)       TFTP server (netboot) - The router uses the configuration
                        register value to form a filename from which to boot a default
                        system image stored on a network server.
               c)       ROM (partial IOS) or keep retrying TFTP depending upon
                        router model
                    •       If no IOS located, get partial IOS version from ROM

          4.    Locate and load startup-config
               a)       If startup-config found, copy to running-config
               b)       If startup-config not found, prompt for setup-mode
               c)       If setup-mode bypassed, create a “skeleton” default running-
                        config (no startup-config)
Locating the IOS

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