Routing: How You Get There from Here

Do you remember reading about TCP/IP as a four-layer protocol stack: application, transport, network, and link?

Some time was taken to explain what the application and transport layers do, but the explanation stopped at the network layer. Well, the network layer is concerned with routing and how to get from one host to another host regardless of the physical interconnection or the layout of the network. A better name for this layer might be the IP layer because this is the layer at which IP addresses are used and routing occurs. It is significant to understand that IP doesn't concern itself with the underlying physical link.

You have already learned about the mechanism used to direct traffic to a host that resides on a network with the same network ID and subnet mask as the sending host. ARP is used to broadcast a request to all hosts on the local network asking one to respond with a MAC address that matches the desired destination IP number. How then is traffic directed to other networks since ARP is broadcast only on the local network? That is where routing comes in.

Each host has a routing table that knows about a default router. When the destination host is not on the local network, the traffic to be sent is directed to the default router. The router is responsible for forwarding the traffic one hop closer to its destination. This hop can be to another router or to the destination host itself if it resides on a network directly connected to the router's interface. The question then becomes, how do routers know how to correctly direct the traffic and how do they receive updated information? After all, this has to be a dynamic process given that routes change because of problems and growth.

Routers maintain tables of routes that they know about. They use dynamic routing protocols to update their tables.

Routing protocols are divided into two major categories: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). The Interior Gateway Protocols support routing traffic within a network that is under the same administrative control, also known as an Autonomous System (AS). This is a fancy name for all the routers for which a site has responsibility. The Routing Information Protocol (RIP) is a widely deployed IGP. RIP is a simple protocol, which requires very little configuration and is supported by essentially every device. Another IGP is Open Shortest Path First (OSPF). These two protocols differ in the way that they receive routing updates and their perspective on finding best routes.

Exterior Gateway Protocols are required when packets must travel between different Autonomous Systems. These protocols bridge separate Autonomous Systems into a single network in which all of the computers on the network can interact seamlessly with each other. The Border Gateway Protocol (BGP) is a widely used Exterior Gateway Protocol. Currently, BGP provides the routing protocol that supports the Internet backbone. BGP servers on the Internet backbone must maintain routing tables that include all of the external addresses on the Internet—a pretty daunting task.