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

• 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

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

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 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)

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

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).

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.

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.

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.

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.

Inside a 2600 Router

Outside a 2600 Router

WAN Connections

LAN ConnectionsManagement 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 configuration.

• These settings are used each time.

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 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.

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 quickly.

• 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

1. ROMa) POSTb) Bootstrap code executedc) Check Configuration Register value (NVRAM)

0 = ROM Monitor mode1 = ROM IOS2 - 15 = startup-config in NVRAM

2. Check for IOS boot system commands in startup-config file (NVRAM)

If boot system commands in startup-configa) Run boot system commands in order they appear in startup-

config to locate the IOSb) If boot system commands fail, use default fallback sequence

to locate the IOS (Flash, TFTP, ROM)

Router Boot Process (1)1, 2

3

4

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-configb) If startup-config not found, prompt for setup-modec) If setup-mode bypassed, create a “skeleton” default running-

config (no startup-config)

Router Boot Process (2)

1, 2

3

4

Locating the IOS