protocol

Routing Protocol is a term that refers to a method or set of rules which can used by a router to determine the appropriate or best path over which data should be transmitted. The routing protocol also determines how the routers within a network share information with each other and update changes.

Dynamic Routing	Static Routing
Default Routing	Bandwidth
Hops	Delay
Reliability	Load
Cost	References

The routing protocol enables a network to make dynamic changes to help suit its conditions, so routing decisions do not have to be static.

Dynamic routing defines the system of being able to make changes as and when required rather than in advance.

Static routing defines the processes or parts of a programme that are fixed and do not allow change.

These are manually defined by the system administrator and define the path that is taken if there is not a known route for the destination.

Back To Top

Metrics are cost based values used by routers to help determine the best path to a particular network. These factors help dynamic routing protocols determine which is the preferred or shortest path to a network destination. Some routing protocols metrics are static and may not be changed. Other routing protocols values may be assigned by a network administrator.

The most common metric values are hop, bandwidth, delay, reliability, load, and cost.

Bandwidth describes how much information can be passed in a wired or wireless communications link in a given time period (a second). A more correct techincal term or definition of bandwidth is the width of the range of frequencies that an electronic signal occupies on a given transmission medium.

Bandwidth is expressed as bits (of data) per second (bps).

Protocols that consider the capacity of a link. Links that support higher transfer rates like gigabit are preferred over lower capacity links like 56Kb. These protocols determine and consider the bandwidth capacity of each link along the path end to end. The path with the overall higher bandwidth is chosen as the best route

The lower path is longer (in terms of hops), but preferred because the bandwidth capacity end to end is greater than the upper path.

The upper path consists of three hops. The first and last are gigabit Ethernet, the second is low-speed 56Kb link. The lower path is four hops, with all hops offering gigabit capacity. Although the lower path is longer in terms of hops, it is much faster in terms of bandwidth capacity end to end.

Back To Top

A hop is a metric used to measure distance based on the number of networks a datagrams it passes. Each time a router forwards a datagram onto a segment this counts as a single hop. Routing protocols that observe hops as their primary metric value consider the best or preferred path (when multiple paths exist) to a destination to be the one with the least number of network hops

The upper path to network is preferred because it involves only three hops.

The upper path consists of three network hops, whereas the lower path has four hops. The assumption of the hop metric, is that the path with the least number of hops is the always the best route.

Routing protocols that only reference hops as their metric do not always select the best path through a network. Just because a path to a destination contains fewer network hops than another does not make it best. The upper path may contain a slower link, such as 56Kb dial-up link along the second hop, whereas the lower path may consist of more hops but faster links, such as gigabit Ethernet. If this were the case, the lower path would undoubtedly be faster than the upper.

Back To Top

Delay is measured in tens of microseconds (the symbol μ is used to indicate this). Delay represents the amount of time it takes for a router to process, queue, and transmit a datagram out an interface. Protocols that use this metric must determine the delay values for all links along the path end to end, considering the path with the lowest (cumulative) delay to be a better route

Although the lower path in is obviously longer in terms of hops, it is faster in terms of delay. The lower path has an overall delay time of 30 microseconds end to end, while the upper path has a delay of 60 microseconds. The lower the delay time the better the path.

The upper path has an end to end delay of 60μ. The lower path has a delay of 300μ, making it the preferred path.

Back To Top

This metric may be configured as a fixed value by an administrator, it is however generally measured dynamically over a specific time frame, such as five seconds. Routers observe attached links, reporting problems, such as link failures, interface errors, lost datagrams and so on. Links experiencing more problems would be considered less reliable than others making them less desirable paths—the higher the reliability the better the path. Because network conditions are constantly changing, link reliability will change. This value is generally measured as a percentage of 255, with 255 being the most reliable and 1 being least reliable.

The lower path contains links that are less reliable than the upper path.

In two of the links along the lower path have been experiencing intermittent problems. Because of these problems, the reliability factor for this path are lower (200) than the upper path. The upper path has not experienced any link or datagram delivery problems and therefore has a history of providing more reliable data delivery.

Back To Top

The term Load within a network environments described to be the amount of data (traffic) being carried by the network, and is a variable value, It is measured over a short time period, which helps indicate traffic load over a specific link. Load measures the amount of traffic occupying the link over a time period as a percentage of the link's total capacity.

The upper path contains three links. The last two show a 128 load value, which represents 50% bandwidth utilisation. The lower path has lower load values and is the preferred path.

Back To Top