The difference between flash, Rom, RAM, and nvram in routers


Flash memory is a type of memory commonly used in routers. It is readable and writable memory that retains data even after the system is restarted or shut down. The current IOS (router operating system) in use is stored in the Flash.


The function of read-only memory (ROM) in routers is similar to that of ROM in computers, and it is mainly used for system initialization and other functions. The ROM mainly includes:

  • System power-on self-test code (POST), which is used to detect whether each hardware part of the router is in good condition;
  • 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 a bit older than the version of the current running IOS, but it is enough to get the router up and working. As the name suggests, 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 RAM in a computer, RAM in a router is memory that temporarily stores the operating system and data during operation, allowing the router to quickly access this information. The access speed of RAM is better than the access speed of the three kinds of memory mentioned above. During runtime, RAM contains routing table entries, ARP buffer entries, log entries, and queued packets waiting to be sent. 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 is a readable and writable memory that can still save data after the system is restarted or shut down. Because NVRAM is only used to save the startup configuration file (Startup-Config), its capacity is small, and usually only 32KB~128KB of NVRAM is configured on the router. At the same time, NVRAM is faster and more expensive.

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