Several networking exams (CCNA, CCNP, JNCIA, JNCIE) will test your ability to identify different types of network equipment. This network certification tutorial on routers focuses on identifying network routers, and explaining the functions of routers provide. This tutorial is not specific to any vendor’s technology.


How Do Routers Work?

Let’s use a home wireless router connected to a cable provider’s internet network in a very simplified example.

  1. The router powers on and loads it’s OS from flash
  2. The router loads the configuration file last saved to NVRAM and sets up the network interfaces and routing protocols it will run.
  3. The router adds the network address and subnet for each interface to it’s routing table along with the name of the interface itself.
  4. The router has a simple static default route to send all non-local data out the network port connected to the cable company.
  5. When the router receives a web page request from your computer, it checks the destination IP address against its routing table.
  6. The bits forming the destination IP address in the IP packet are used as a hash key to point to the correct route, which in turn points to the correct network interface that the packet should be forwarded out of.
  7. The router transmits the packet out the correct interface, to the next router, which repeats the process until the packet reaches the destination.

The process is mostly the same for any router.

CAVEAT:   A small complication here.  For  home cable and wifi routers, the router is also acting as a “proxy” so the router does one thing that is NOT normal for a router out on the Internet somwehere:  it changes the source IP address in the IP packet to it’s own address.


Why do I need a Router?

Routers are used to connect networks together and routers perform the following network functions, whish you should be able to identify and describe on the Network+, CCNA or JNCP exam:

FUNCTIONS OF A ROUTER (identify and describe)

  1. Restrict broadcasts to the LAN
  2. Act as the default gateway.
  3. Perform Protocol Translation (Wired Ethernet to Wireless/WiFi, or Ethernet to CATV)
  4. Move (route) data between networks
  5. Learn and advertise loop free paths
  6. Calculate ‘best paths’ to reach network destinations.

Restrict Broadcasts to the LAN

Networks (especially Ethernet networks) use broadcast communication at the physical, datalink and network layer. Network layer broadcasts are transmissions sent to all hosts using the network layer protocol (usually Internet Protocol [IP] or IPX). Network broadcast communication is used to communicate certain kinds of information that makes the network function (ARP, RARP, DHCP, IPX-SAP broadcasts etc.). Since several devices could attempt to transmit simultaneously and cause collisions, it is preferable to separate large sets of hosts into different broadcast domains using a switch, or router.

As the number of hosts on the network increases, the amount of broadcast traffic increases. If enough broadcast traffic is present on the network, then ordinary communication across the network becomes difficult.

To reduce broadcasts, a network administrator can break up a network with a large number of hosts into two smaller networks. Broadcasts are then restricted to each network, and the router performs as the ‘default gateway’ to reach the hosts on the other networks.

Act as the Default Gateway

Especially in today’s networks, people want to use their computer to connect to the Internet. When your computer wants to talk to a computer on another network, it does so by sending your data to the default gateway.  The default gateway is the local router connected to the same network your computer is connected to.  The router serving as the default gateway receives your data, looks for the remote address of that far-off computer and makes a routing decision. Based on that routing decision, it forwards your data out a different interface that is closer to that remote computer. There could be several routers between you and the remote computer, so several routers will take part in handing off the packet, much like a fireman’s bucket brigade.

Move (route) Data between Networks

Routers have the capability to move data from one network to another. This allows two networks managed by different organizations to exchange data. They create a network between them and exchange data between the routers on that network. Because a router can accept traffic from any kind of network it is attached to, and forward it to any other network, it can also allow networks that could not normally communicate with each other to exchange data. In technical terms, a token ring network and an ethernet network can communicate over a serial network. Routers make all this possible.

A router can take in an Ethernet frame, strip the ethernet data off, and then drop the IP data into a frame of another type such as SDH/SONET, PDH/T1, ATM, FDDI. In this way a router can also perform ‘protocol conversion’, provided it has the appropriate hardware and software to support such a function. The whole point, however, is to forward the data from the interface it receives data on, to another interface that retransmits the received data onto another interface serving another network.

Learn and Advertise Loop-Free Paths

Routers can only learn and advertise routes dynamically if they are using a routing protocol such as RIP, OSPF, EIGRP, IS-IS or BGP.   Otherwise, a human has to configure the routes by hand, which is called static routing.

Routing moves data on a hop-by-hop basis, what is often called ‘hot potato’ routing.  If a set of routers ends up passing the data around in a circle, without reaching the destination, it’s calleda a ‘routing loop’.  Packets get handed off around the loop until they die of old age:  their ‘Time To Live’ expires. Time To Live is a counter that is part of the IP datagram header. The Time To Live value is decremented as it passes through each router and eventually it reaches zero and is discarded.

Router Components & Parts

Since routers are just specialized computers, the have the same “parts” as other computers:

  • Central Processing Unit (CPU)
  • Flash Memory
  • Non-Volitile RAM
  • RAM
  • Network Interfaces
  • Console

Central Processing Unit:  Runs special software called an “operating system” such as JunOS on Juniper routers, or Cisco IOS (Nexus OS) for Cisco routers. The operating system manages the router’s components and provies all the logical networking functions of the router.

Flash Memory is where the operating system is stored, and in this respect, is like the hard disk drive in your computer. If you use a Solid State Disk Drive (SSD), then your computer uses Flash RAM, just like the router does.

Non-Volitile RAM:  This is additional memory for storing the backup or startup version of the operating system being used. The router will boot from this memory and load all it’s programs from here.

RAM: When the router starts up, the operating system is loaded into RAM. Once the router finishes starting up, it begins to calculate its own routes and, if configured to do so, learns network routes from other routers via RIP (v1 and v2), OSPF, EIGRP, IS-IS or BGP. RAM is also used for caching ARP tables, routing tables, routing metrics and other data that can speed up the process of forwarding of packets.

Network Interfaces:  Routers always have lots of network interfaces. The operating system contains ‘drivers’ that allow the operating sytem to access the network hardware in the interface modules. Routers will learn which networks are configured on which ports as they start up. After that, they will ‘learn’ routes from other routers they are connected to, and learn which interface to transmit packets on to reach a remote network destination.

Console:  Last, but not least, is the console. In “Ye Olden Days” managing and configuring a router was performed at the console of individual devices, as was most troubleshooting and diagnostics. Network certification exams will contain a large selection of questions on the configuration and troubleshooting commands you can issue from the console. However, manufacturers are rapidly doing away with a console on each device and building management systems for managing large numbers of network devices from a centralized location.

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