Routing Metrics For CCNA

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[edit section] Routing Metrics

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 algorithm determines the metric for routes throughout the network and the smaller the metric value the better the route is likely to be. Smaller metrics indicate faster, better and more trustworthy routes. Higher metrics are the opposite and reflect routes that are not positive for any of the 3 attributes listed above.

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 is 1/18 of a second. A tick is used as part of the routing protocol IPX RIP

There are 3 routing algorithms used by varying routing protocols to determine the metric for routing. These algorithms distance vector, link-state and a hybrid approach (which is a combination of distance vector and link-state).

Routing Protocol Table

Protocol Type	Characteristics	Examples
Distance Vector	Uses hop count	RIP, IGRP, IPX RIP
Link State	Uses Shortest Path First, Common View of Network	NLSP, OSPF, IS-IS
Hybrid	Distance vector with more accurate update metrics	IS-IS, EIGRP

[edit section] When are Routing Metrics Compared?

The Routing Metrics of routes are compared when the routes have the same Administrative Distance. The Administrative Distance (AD) is a measure of trust worthiness of the routing information received.

The default administrative distances of common routing protocols are listed below:

Default Administrative Distances

Route Source	Default Administrative Distance
Directly Connected	0 (Highest reliability)
Static Route	1
EIGRP	90
IGRP	100
OSPF	110
RIP	120
External EIGRP	170
Route	Default
Unknown	255 (Lowest Reliability)

CCNA Exam Tip 1: You should know the default administrative distance table listed above. Also know the metric used by common routing protocols such as RIP and IGRP.

CCNA Exam Tip 2: Understand the 3 different types of routing protocols for the CCNA and be able to provide examples for each one.

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