Flash memory, also called flash memory, is a type of memory commonly used in routers. It is a readable and writable memory that can save data even after the system is restarted or shut down. The IOS (Router Operating System) currently in use is stored in the Flash.

The function of read-only memory (ROM) in Cisco routers is similar to that of ROM in computers, and is mainly used for system initialization and other functions. The ROM mainly contains: (1) System power-on self-check code (POST), which is used to detect whether each hardware part of the router is intact; (2) System boot area code (BootStrap), which is used to start the router and load the IOS operating system (3) Backup IOS operating system for use when the original IOS operating system is deleted or destroyed. Usually, this IOS is lower than the current version of IOS, but it is enough to make the router start up and work. As the name implies, ROM is a read-only memory, and the code stored in it cannot be modified. If you want to upgrade, you need to replace the ROM chip

RAM is also a readable and writable memory, but its stored content will be cleared after the system is restarted or shut down. Like the RAM in a computer, the RAM in a Cisco router is also a memory that temporarily stores the operating system and data during operation, so that the router can quickly access this information. The access speed of RAM is better than the access speed of the aforementioned three types of memory. During operation, RAM contains routing table items, ARP buffer items, log items, and packets queued for transmission in the queue. In addition, it also includes the running configuration file (Running-config), the code being executed, the IOS operating system program and some temporary data information.

Nonvolatile RAM (Nonvolatile RAM) is a readable and writable memory that can save data after the system is restarted or shut down. Since NVRAM is only used to save startup configuration files (Startup-Config), its capacity is small. Usually, only 32KB~128KB of NVRAM is configured on the router. At the same time, NVRAM is faster and more expensive.