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WANs and Routers

WANs and Routers. 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.

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WANs and Routers

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  1. WANs and Routers Routers

  2. 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)

  3. 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.

  4. Types of Router Memory • Random Access Memory (RAM) • Nonvolatile RAM (NVRAM) • Flash Memory • Read-Only Memory (ROM)

  5. RAM Characteristics • Stores routing tables • Holds ARP cache • Holds fast-switching cache • Performs packet buffering as shared RAM • 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

  6. NVRAM Characteristics • Provides storage for the startup configuration file • Retains content when a router is powered down or restarted

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

  8. 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 upgrades

  9. 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 software. • RAM is generally dynamic random-access memory (DRAM) and can be upgraded with the addition of dual in-line memory modules (DIMMs).

  10. Internal Router Components (2) • 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.

  11. Internal Router Components (3) • 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 monitor. • 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.

  12. Internal Router Components (4) • 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.

  13. Internal Router Components (5) • Power Supply • The power supply provides the necessary power to operate the internal components. • Larger routers may use multiple or modular power supplies.

  14. Inside a 2600 Router

  15. Outside a 2600 Router WAN Connections LAN Connections Management Connections

  16. Connecting a PC to a Router for Configuration

  17. Connecting to a Router • Generally, a utility such as HyperTerminal is used to connect to the router for configuration. • These settings are used each time.

  18. 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.

  19. Router Initialization

  20. 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 modules. • These diagnostics verify the basic operation of the CPU, memory, and network interface ports. • After verifying the hardware functions, the router proceeds with software initialization.

  21. After POST • The generic bootstrap loader in ROM executes. • A bootstrap is a simple set of instructions that tests hardware and initializes the IOS for operation. • 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.

  22. After POST (2) • 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. • 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.

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

  24. 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

  25. Router Boot Process (1) 1, 2 1. ROM • POST • Bootstrap code executed • 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 • Run boot system commands in order they appear in startup-config to locate the IOS • If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM) 3 4

  26. 1, 2 3 4 Router Boot Process (2) • Locate and load IOS, Default fallback sequence: (No IOS boot system commands in startup-config) • Flash (sequential) • 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. • ROM (partial IOS) or keep retrying TFTP depending upon router model • If no IOS located, get partial IOS version from ROM • Locate and load startup-config • If startup-config found, copy to running-config • If startup-config not found, prompt for setup-mode • If setup-mode bypassed, create a “skeleton” default running-config (no startup-config)

  27. Locating the IOS

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