The flash and memory of the router

Flash memory, also known as Flash, is a commonly used type of memory in routers. It is a read-write memory that can still be saved after the system restarts or shuts down. In Flash, the iOS (Router operating system) currently in use is stored.
There may be a variety of memory in the router, such as Flash (Flash), DRAM (dynamic memory), and so on. Memory is used as storage configuration, router operating system, routing protocol software and so on. In low-end routers, the routing table may be stored in memory. In general, the larger the router memory, the better (regardless of the price). But like CPU power, memory does not directly reflect the performance and capabilities of the router. Because efficient algorithms and good software can greatly conserve memory.

Routers employ several different types of memory, each of which assists the router in a different way.

* Read-only memory (ROM)
* Flash memory (Flash)
* Random access memory (RAM)
* Non-volatile RAM (NVRAM)

1. Read-only memory (ROM)
The function of read-only memory (ROM) in the Cisco router is similar to the ROM in the computer, and is mainly used for system initialization functions. The main ROM contains:
(1) The system power on the Self-Test Code (POST), used to detect the hardware parts of the router is intact;
(2) The System boot area code (BOOTSTRAP) for booting the router and loading the iOS operating system;
(3) Back up the iOS operating system for use when the original iOS operating system is deleted or damaged. Typically, this iOS is a bit lower than the version currently running iOS, but it's enough to make the router start and work.
As the name implies, ROM is read-only memory, can not modify the code stored therein. To upgrade, replace the ROM chip.

2. Flash memory (Flash)
Flash memory (Flash) is readable and writable, and can still hold data after the system restarts or shuts down. In Flash, the current iOS is stored in use. In fact, if the flash capacity is large enough to hold more than one operating system, this can be useful for iOS upgrades. When you do not know whether the new version of iOS is stable, you can retain the old version of iOS after the upgrade, and when there is a problem can quickly return to the old version of the operating system, so as to avoid a long network failure.

3. Non-volatile RAM (NVRAM)
Non-volatile RAM (nonvolatile RAM) is readable and writable, and can still hold data after the system restarts or shuts down. Because the NVRAM is used only to save the startup configuration file (Startup-config), it has a smaller capacity and typically configures only the 32kb~128kb size NVRAM on the router. At the same time, the NVRAM speed is faster, the cost is relatively high.

4. Random Memory (RAM)
RAM is also readable and writable, but its contents are purged when the system restarts or shuts down. As with RAM in the computer, the RAM in the Cisco router is the memory that temporarily stores the operating system and data during the run, allowing the router to access the information quickly. Ram's access speed is better than the 3 memory accesses mentioned earlier.
During run time, the RAM contains routing table entries, ARP buffer items, log entries, and groupings queued for sending in the queue. In addition, the Run configuration file (running-config), executing code, iOS operating system program, and some temporary data information are included.

     routers are of different types, and iOS code is read differently. A Cisco 2500-series Router reads part of iOS from Flash only when it is needed, and the Cisco 4000-Series Router's entire iOS must be fully loaded into RAM before it can run. Therefore, the former is called the Flash running device (run from Flash), which is called the Ram running device (run from RAM).