Routing is the process of forwarding packets from one network to another. All the information needed for a router to forward packets to a hop (router/relay device) can be found in the router’s routing table.

Static Routing
Static routing occurs when you manually add routes in each router’s routing table. Static routes are routes that are administratively configured in routers. They are typically used when dynamic protocols are either unnecessary or unwanted.

Default Routing
Default routing use to send packets with a remote destination network not in the routing table to the next-hop router. You can only use default routing on stub networks—those with only one exit path out of the network.

Dynamic Routing
Dynamic routing is the process of using protocols to find and update routing tables on routers and to maintain a loop-free, single path to each network. This is easier than using static routing but it will cost u in terms of router CPU processes and bandwidth on the network links.

There are two types of dynamic routing protocols used in internetwork. Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP). IGP routing protocol are used to exchange routing information with routers in the same autonomous system (AS). An AS is a collection of network under a common administrative domain. EGP’s are used to communicate between ASes. BGP is an example of EGP.

Link State versus Distance Vector Routing Protocols
In determining the best route to a destination, different routing protocols use a number of different measurements. These measurements are called metrics. Each routing protocol uses one or more metric to calculate the best route to a particular destination. The most common metrics include path length (hop count), reliability, delay, bandwidth, load, and financial cost of a link.

Another major difference between routing protocols is how they handle updating each other with current information. There are many methods of doing this. Given these major differences, routing protocols are broken into two main categories: Distance Vector and Link State.

Distance Vector protocols include RIP and IGRP. They send their entire routing tables out in all directions at regularly scheduled intervals.
Link State protocols are more advanced than distance vector protocols because, unlike distance vector, they do not send periodic routing updates. Link State protocols include OSPF, NLSP, BGP, and IS-IS.
They send partial routing tables (of their own networks) to everyone and then send updates when necessary.

Classful Versus Classless Routing
The basic definition of classful routing is that subnet mask information is not carried within the routine, periodic routing updates. This means that every interface and host on the network must use the same subnet mask. In other words, a classful routing protocol abides strictly to the bit boundaries of the IP address classes. For example, the 10.0.0.0 network—a Class A network—cannot be advertised as anything Other than a route to 10.0.0.0, since the default network mask of a Class A network is 255.0.0.0. In other words, VLSMs are effectively useless. This is because the routing update packet has no field for subnet mask, so the default mask according to the class is assumed. Classful routing protocols include RIP v1 and IGRP.

Classless routing protocols include the subnet mask information when an update is sent. This allows different length subnet masks to be used on the network called Variable Length Subnet Masks (VLSM).

Default Administrative Distances
Administrative distances are used to rate the trustworthiness of routing information received on a router from a neighbor router. If a router learns of different types of routes to the same destination (statically configured
or advertised via a dynamic routing protocol), it must select which route to include in its routing table. Typically, only one route to a specific destination (same address and mask) is in a router’s routing table. One method of route “selection” is accomplished by comparing the administrative distance of all the routes to the same destination. Administrative Distance is a value, which rates the reliability of the source of the route. If the source that provides a route to a router is considered to be less reliable—less trustworthy—it receives a higher administrative distance value. The lowest administrative distance becomes the preferred route entered in the routing table. Administrative distance values range from 0 to 255. If desired, the administrator can configure administrative distances so that the default administrative distance is not used.