Routers are the brains of a network operating at layer 3 of the OSI model. Routers are the primary device you use to connect to the internet, Routers more packets between other routers. Routers come in all kinds of many different shapes and sizes; you have your small modem/router at home that connects to your ISP or your small office that normally used to provide wireless within your house. There are large enterprises and service providers routers that your ISP would use to connect to the world wide web, there is Software-defined routers for virtual instances in cloud environments and cloud-edge routers, there’s small business size routers for small office home office.

Routers have different interface connections such Fiber, Ethernet, Serial WAN links and even cellular links. Older routers had ISDN, ATM, ISDN interfaces, there also different virtual interfaces that you can find in a router, such as loopback, tunnel, Port-Channel and sub-interfaces.  There is also interface used to access the management of the router and configure the router using the Console interface or the AUX interface. Console interfaces you need a console cable or USB cable and Terminal emulator Software.

As well as all the different types of interfaces the hardware inside of the router consists of different times of memory such as Flash that stores the router’s operating system, RAM that runs the running config, and where the ARP table is stored and active packet buffer just like a PC RAM is erased when the router reboots, and NVRAM which is were the startup configuration is stored and does not erase when the router is rebooted or loses power.

Traffic/packets moves between routers using IP addresses and subnet masks by examining the IP headers IP destination IP address and the packet is sent out the interface that matches the IP destination that is in the routing table. Routing tables are build manually by adding static routers that are entered manually or dynamically entered by configuring routing protocols. Most of the type a dynamic routing protocol is used there is also default routes that are configured. dynamic routing protocol RIP, EIGRP, OSPF, BGP. Each routing protocol has its own flavours of how traffic forwarding is determined.

Routers have “features” that you’re able to manage, allow, deny, police and isolate different traffic using such things as ACLs, (Access Control Lists), NAT (Network Address Translation), quality of Service (QoS), Virtual Routing Forwarding (VRF) and tunneling protocols. as well as other features, older cisco routers you could configure call manager express for a small office phone system, as well as adding expansion card to support voicemail services and some unique features such as using a router as an access console server by adding asynchronous network modules and an octal cable allowing access to the console port of 32 different devices.

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