CompTIA Network+ Exam Training:

Introduction To The Routing Process

By Chris Bryant, CCIE #12933

Routers run at Layer Three of the OSI model.  These devices receive packets and examine their routing tables to see where to send the packet. Routers can be configured with static routes, but most likely they'll be using one of the following protocols to dynamically exchange routes with other routers.

You'll learn a lot more about these protocols when you choose to pursue the Cisco Certified Network Associate (CCNA) certification.  These protocols are very different from each other, but regardless of which protocol is used to obtain the routes, the routing process itself stays the same. Let's walk through an example.

The PC on the 10.0.0.0 /8 network wants to send a packet to the PC on the 20.0.0.0 /8 network.  In this example, the sending PC has no idea where that host is.  If the sending PC is configured to use the router as its default gateway, the PC will send the packet to the router and hope the router knows how to get the packet to the 20.0.0.0 /8 network.  (Devices that connect different or dissimilar networks are called gateways.)

The router knows where 20.0.0.0 /8 is because it's directly connected to that network.  Routers do not depend on routing protocols to tell them about networks that are directly connected to them.   When the router sees an incoming packet with a destination IP address of 20.1.1.2, the router will check its routing table, see that it's directly connected to that network on its interface Ethernet1, and forward the packet.

Routing protocols become necessary when packets need to be routed to networks the router is not directly connected to, and that's the case more often than not, as in the following network.

In this network, the first router is receiving a packet destined for 30.1.1.2.  The router is not directly connected to the 30.0.0.0 /8 network, so that router must receive a dynamic routing update from another router telling it how to get packets to the 30.1.1.0 /8 network.  Without that dynamic route, the router will simply drop the packet.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com