Types of Memory in a Router

1- RAM

• DRAM ;
• used by CPU to execute IOS and hold running-config, routing tables, and ARP cache ;
• information in RAM is cleared when the router is reloaded ; 

2- ROM

• referred to as EPROM ;
• contains POST diagnostics and the bootstrap or boot-loader software ;
• allows the router to boot from ROM Monitor mode when it cannot find a valid IOS image ;

 3- Flash

• available as EPROMs, SIMMs, or PCMCIA cards ;
• default location where a router finds and boots its IOS image ;
• contents of Flash are retained when the router is reloaded ;

4- NVRAM

• stores startup-config ;
• contains the software configuration register, a configurable setting in IOS that determines which image to use when booting the router ;
• contents of NVRAM are retained when the router is reloaded ;