Routing Metric Value, Hop Count, Bandwidth, Delay, Reliability, Load and Cost

Routing Metric Value: Hop Count, Bandwidth, Delay, Reliability, Load and Cost

In this article you will learn about routing metric value that includes hop count, bandwidth, delay, reliability, load and cost. 

When two or more protocols are used to enable routing on a router, the routing protocol with least administrative distance is used as the best routing protocol. The path described by the protocol with least administrative distance is used to forward Internet Protocol (IP) packets.

 The value of administrative distance varies from 0 to 255. 0 is used to indicate directly connected network and 255 indicate an unknown network. Unknown networks are given least priority by the router as its administrative distance is high.

Consider a scenario where only one routing protocol is used and there are multiple paths to reach the destination. As only one routing protocol is configured, administrative distance for all the paths will be same. It is ‘1’ if static routing is used. How the router know which is the best path to select and forward IP packets.

 The router considers various metrics before selecting the best path. The metrics that are considered by the router are based on routing protocol configured on that particular router. Some of the metrics that router considers are:

a.            Hop count: Number of routers that a packet passes before reaching the destination is considered as Hop Count. Every time a packet passes the router, the hop count is incremented by 1.

b.            Bandwidth: The speed at which data is transmitted over a channel is called as bandwidth. The bandwidth is measured in terms of number of bits that are transferred over the channel per second.

c.             Delay: The time taken by the router to process and forward a packet out of an interface is called as delay. The distance between routers and congestion on the network also plays a vital role on delay.

d.            Reliability: Reliability indicates the time for which the network is available for forwarding the packets.

e.            Load: load defines the amount of traffic that can be flown through the router.

f.             Cost:  Inverse of bandwidth of an interface is considered as cost of that interface

Various routing protocols use any one of the metric or combination of two or more metrics that are mentioned above. Hop count is used as metric for RIP routing protocol. RIP selects the path with less number of routers to forward the packet to the destination. It does not care about other metrics like load, cost, delay, bandwidth etc.

Enhanced Interior Gateway Routing Protocol uses a complex equation to find the best path. The complex equation caters about Bandwidth, Reliability, Delay and Load of the available paths.

                256 * [(k1* {k2* bandwidth}/{bandwidth+256-load}+ k3 * delay)(k5/{k4+reliability})]

K1, k3 =1 and K2, k4 and k5 = 0 by default.

OSPF uses cost as metric to select the best path. The path with least cost is selected as best path by OSPF.

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