Modular Many Cisco routers now come with modular interfaces. This is simply an empty slot, in which you can plug in any other type of interface that you purchase as an add-in card. Wide varieties of modular interfaces are available such as ISDN, frame relay, and voice interface cards. You can also add additional serial or Ethernet interfaces just like the ones discussed above. This eliminates the need for additional hardware between the router and the line.
Modular routers make it easier to build a solution that suits individual needs.
Unlike other network computers, Cisco routers use no disk-based storage. They also use ROM and RAM similar to personal computers. Let's take a look at each memory type and how it is used in a Cisco router. To upgrade the IOS, you had to replace the ROM chips with new ones. Newer routers, such as the 2500 series, no longer use ROM to store the operating system.