Role of routers in a WAN

What does a System Bus do?

Flash – Flash memory is used for storage of a full Cisco IOS software image. The router normally acquires the default IOS from flash. These images can be upgraded by loading a new image into 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. In other routers the IOS may be run directly from flash. The flash single in-line memory modules (SIMMs) or PCMCIA cards can be added or replaced to upgrade the amount of flash.

NVRAM – NVRAM is used to store the startup configuration. In some devices, EEPROMs can be used to implement NVRAM. In other devices it is implemented in the same flash device from which the boot code is loaded. In either case these devices retain contents when power is removed.

Buses – Most routers contain a system bus and a CPU bus. The system bus is used to communicate between the CPU and the interfaces or expansion slots. This bus transfers the packets to and from the interfaces.

The CPU bus is used by the CPU for accessing components from router storage. This bus transfers instructions and data to or from specified memory addresses.


ROM – ROM is used to permanently store 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. Some routers also have a scaled down version of the IOS that can be used as an alternative boot source. ROMs are not erasable. They can only be upgraded by replacing the ROM chips in the sockets.

Interfaces – The interfaces are the router connections to the outside. The three types of interfaces are LANs, WANs, and console or auxiliary (AUX). The LAN interfaces are usually one of several different varieties of Ethernet or Token Ring. These interfaces have controller chips that provide the logic for connecting the system to the media. The LAN interfaces may be a fixed configuration or modular.

The WAN interfaces include serial, ISDN, and integrated CSUs. As with LAN interfaces, WAN interfaces also have special controller chips for the interfaces. The WAN interfaces may be a fixed configuration or modular.

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.