Router Memory Components

Cisco routers utilize four different types of memory, with each type providing different functions. RAM, NVRAM, Flash memory, and ROM are the four types of memory used. Their functions are discussed in the following sections (see Figure 8.1).

Figure 8.1. Standard memory configuration for a Cisco router.

RAM

RAM serves as a working storage area for the router and contains data such as routing tables, various types of cache and buffers, as well as input and output queues. RAM also provides temporary memory for the router's active IOS and configuration file (the running configuration file). However, all the contents of RAM are lost if the router is powered down or restarted.

NVRAM

Unlike RAM, nonvolatile RAM (NVRAM) retains its contents when the router is powered down or restarted. NVRAM stores permanent information, such as the router's backup configuration file. The startup configuration file is retrieved from NVRAM during startup, at which time it is loaded into RAM.

Flash

Flash memory stores the Cisco IOS image and associated microcode . Flash memory is erasable, electronically reprogrammable ROM that retains its contents when the router is powered down or restarted. Flash memory allows software to be upgraded without chips being added, removed, or replaced .

ROM

Like Flash memory, ROM contains a version of IOS ”usually an older version with minimal functionality. It also stores the bootstrap program and power-on diagnostic programs. However, software upgrades can be performed only by replacing the ROM chip.