Network+ Exam Objectives 1.6

1.6 Explain the purpose and properties of routing

To keep it simple, think of routers as specialized computers that are designed for two main functions: the ability to deliver a packet to a network host and the task of consulting set routing tables that provide specific directions as to where a packet needs to go.

• IGP vs. EGP – Most routing protocols used today are IGP and are designed to work within a single autonomous system. When it comes to talking about EGP, it’s important to remember that BGP is the only EGP that is commonly used in today’s world.  Make note that BGP works between autonomous systems such as connecting to the internet.

• Static vs. dynamic – It’s important to remember that static notates that the routing configuration has been done by a system administrator. Statically configuring routers completely is possible, but it is definitely the hard way to do things because each and every route would have to be reevaluated and changed accordingly.

With dynamic routing, the routers and routing protocols do all of the work therefore a system administrator is not needed as much. Since all routers are configured to be able to exchange information regarding data packets and the needed routing paths, dynamic routing is suitable for all routing protocols but some are able to perform dynamic routing much better than others. In the end it’s much more effective and efficient when looking at static routing.

• Next hop – A packets origination is never a worry when dealing with routing protocols. When it comes to routing, the routers are only worried with where the packet has to go, also referred to as the destination header. With the information provided in a destination header, the router can decide whether or not it can deliver the packet or if it needs to be delivered to another router for further network movement.

In the event that a router cannot deliver a packet itself, it will consult its routing table to determine where the packet needs to go next. When looking at the routing table, the information given will tell the router the interface that the packet needs to go to next. This next interface is usually known as the next hop. The movement of the packet from one interface to another is a hop.

• Understanding routing tables and how they pertain to path selection – When consulting routing tables, routers automatically know where a packet needs to go. By virtue of the table, a router knows exactly where a packet needs to be delivered in order to reach its final destination. When looking at routing tables, it’s important to remember that a packet’s destination determines the path it travels. A packet needing to get to a 172.16.0.0 destination will not be sent on the same interface as a packet needing to get to a 192.16.1.0 destination.

• Explain convergence (steady state) – Convergence occurs after a change has been made and everything involved is in agreement after the change. An example is adding a new router to a steady state network. If a system administrator adds a new interface to a network router, all of the routing protocols have to examine the new path and decide if the newly created path is more efficient than the one that is already being used. Every router on the network will perform this action which of course disrupts the steady state of the network.

Once all is said and done, the network activity will calm and return to a steady state. This downing of activity is convergence. 

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