Why do routing protocols use metrics?

Metrics are used by routing protocol to determine the best route it is a measure of how the route is trustworthy, best path (high bandwidth), shortest every routing protocol have some algorithm to calculate metric example RIP use hope count, EIGRP use bandwidth and Delay so metric use as a tie breaker when a router discover multiple path to the same network. Only the best route is placed on the routing table and this best route is selected based on Metric

Routers are the core networking devices used to transport data over different interconnected networks. An autonomous system or a large enterprise network may contain many different networks, along with many routers working to enable communications among them. Similarly, a segment of a large network may contain many small networks. Thus, there are many network communication paths involved in transporting network traffic. As these small and large networks are interconnected, and a network router records several paths or routes leading up to any specific node or network. However, to decide upon the optimal path among them, a router uses a routing metric as a core selection process. Routing metrics are composed of several different parameters and operational environments to determine points for comparisons among the available paths. Generally, routing metrics are also known as the cost calculation for a given path, which varies depending on the routing protocol in use. For example, distance vector routing rotocols implement the Bellman-Ford algorithm to add the total number of hops, or intermediary routers, involved in reaching a destination. Path reliability, load, speed, latency, packet loss and a few other factors are inputs for calculating path cost in modern routing protocols. 

Routers use various metrics and calculations to determine the best route for a packet to reach its final network destination. Each routing protocol uses its own algorithm with varying weights to determine the best possible path.

The following are metrics used in determining the best path for a routing protocol:

• Bandwidth – Throughput speed in bits per second

• Cost – An arbitrary value assigned by an administrator for the intersecting of networks

• Delay – Network latency caused by such factors as distance or congestion

• Hop Count – The number of routers (hops) a packets passes through to its destination

• Load – Measurement of traffic that flows through a router

• MTU (Maximum Transmission Unit) – The largest unit size allowed to be transmitted on all routes from source to destination

• Reliability – Represents the amount of network downtime, that is, how reliable a network path is)

• Ticks – Measurement of delay, where is tick is1/18 of a second. A tick is used as part of the routing protocol IPX RIP