What are the Basics of a Cisco Router?

Definition

A Router is a layer 3 network device that transmits data between different network segments and can use a packet header to determine the best path for the packet to travel. Routers can connect network sections that use different protocols. They also allow all users in a network to share a single connection to the Internet or a wide area network (WAN).

Internal Router Components

• Read-Only Memory (ROM)
  ROM stores the router’s bootstrap startup program, operating system software, and any power-on diagnostic tests programs.
• Flash Memory
  Flash Memory is reprogrammable, erasable ROM that stores the system image(s) of the operating system. Memory content is retained when the router is switched off or restarted.
• Random Access Memory (RAM)
  RAM stores operational information like routing tables while providing caching and packet buffering capabilities. Its contents are lost whenever the router is switched off or restarted.
• Nonvolatile RAM (NVRAM)
  NVRAM is used to store the router’s startup configuration file. The contents of the file are maintained whenever the router is switched on, off or restarted.
• Network Interfaces
  The router’s network interfaces are located on the motherboard or on separate interface modules. Ethernet or Token Ring interfaces can be configured to authorize connection to a LAN, while synchronous serial interfaces are configured to permit connection to WANs.

External Router Components

A router can be configured over any of its network interfaces. Trivial File Transfer Protocol (TFTP) servers can be used to provide configuration information to a router.
TFTP is a simplified version of FTP.

Router’s Startup Procedure

Every time a router is switched on; it goes through self-test diagnosis to verify the basic operation of the CPU, network interfaces, and memory.

The system bootstrap software then searches for a valid router operating system software (Cisco® IOS image). The Cisco® IOS image can be found in ROM, Flash memory, or a TSFT server on the network.

Cisco® CLI Command Modes

The Cisco® IOS software provides access to a variety of different command modes, each of which provides a different group of related commands. The Cisco® Command Line Interface (CLI) is called EXEC, and can be used in either User mode or Privileged mode. The commands available in Privileged mode are also available in User mode.

User EXEC commands allow you to:

• Connect to remote devices
• Make temporary changes to terminal settings
• Perform basic tests
• List system information

To access privileged mode, a password is required. Privileged EXEC commands can be used to:

• Set operating parameters
• Perform a detailed examination of the router’s status
• Test and debug router operation
• Access global and other included configuration modes

You can also enter global configuration mode from Privileged mode, which will give you access to configuration commands that affect the entire system.