An important part of your Cisco CCENT and CCNA certification studies is learning the differences between RAM, ROM, NVRAM, and Flash memory. You better know the differences when it comes to working in real-life networks as well, because vital Cisco files are found in these memory types - and since some of these files are lost on a router reload and some are not, we better know which is which!

The memory types and functions discussed in this section are the same for routers and switches, but to keep from saying "routers and switches" 500 times, I'll just say "routers". :)

Configuring the routers is a lot of fun, but we've got to know what's going on inside the router, too! Cisco routers have four different kinds of memory, and while some of the names are similar, their purpose is totally different.

The contents of some of these memory types is kept when the router is reloaded, and others are lost on a reload. We better know which is which!

It's a fair bet that these topics will come up on your CCENT and CCNA exams, and this is also information you've got to know to be a real network admin. Let's examine these four memory types closely and see what each one does!

ROM: Read-Only Memory. ROM stores the router’s bootstrap startup program, operating system software, and power-on diagnostic test programs (POST).

Flash Memory: Generally referred to simply as "flash", the IOS images are held here. Flash is erasable and reprogrammable ROM. Flash memory content is retained by the router on reload.

RAM: Random-Access Memory. Stores operational information such as routing tables and the running configuration file. RAM contents are lost when the router is powered down or reloaded. By default, routers look here first for an Internetwork Operating System (IOS) file during boot.

NVRAM: Non-volatile RAM. NVRAM holds the router’s startup configuration file. NVRAM contents are not lost when the router is powered down or reloaded.

Some important comparisons:

RAM contents are lost on reload, where NVRAM and Flash contents are not. NVRAM holds the startup configuration file, where RAM holds the running configuration file. Let's take a look at the boot process of a Cisco router, and then talk about the dreaded Setup Mode!

The Router Boot Process

When a Cisco router powers up, it first runs a series of POSTs (Power-On Self Test). A POST is a series of diagnostic tests designed to verify the basic operation of the network interfaces, memory, and the CPU.

Depending on the model or router of switch you're using, you can actually see some of these tests being passed. Here, I've reloaded a Cisco 2950 switch, and you can see some of the POSTs being run and passed at the very beginning of the bootup process.

Initializing flashfs...

flashfs[1]: 79 files, 3 directories

flashfs[1]: 0 orphaned files, 0 orphaned directori

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