Routers

A router is a device that works at layer 3 of the OSI model, and hence routes IP packets between different subnets. The hardware architecture at a block level would be similar to that of a switch, with the difference that the switch processor is replaced by a route processor. Since the router operates at a higher OSI layer than that of a switch, all the functions of a layer 2 switch would also be inbuilt into the router.

The route processor is the brain of the router. The routing tables are built by the route processor by running various routing protocols that we will discuss in the next chapter. These routing tables can then be sent to specific ASICs controlling the different ports on a router for faster processing.

A router may have one or more Ethernet ports that connect to different LAN segments, one or more WAN ports that can connect to the WAN links over serial interfaces such as an E1 interface providing 2 Mbps bandwidth, or even ATM interfaces that can connect to legacy ATM backbones. The router will also have the console port and optional AUX ports for connecting to the device for configuration.

Like switches, routers also come in multiple form factors and could be modular or fixed configuration devices. Some of the commonly used routers in the Cisco family are the Integrated Services Routers (ISR) 3k & ISR4k series, or the highend ASR 9k series of routers. All of these routers would run a variant of the IOS software that implements the different features within the router, including the different routing protocols. IOS is the most common software, while IOS-XE is a modular build of the IOS software. IOS-XR is a modular software that runs on highend routers.