metric and routing protocol

Different routing protocols use different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol. Two different routing protocols might choose different paths to the same destination due to using different metrics.
Metrics used in IP routing protocols include:
Hop count - A simple metric that counts the number of routers a packet must traverse
Bandwidth - Influences path selection by preferring the path with the highest bandwidth
Load - Considers the traffic utilization of a certain link
Delay - Considers the time a packet takes to traverse a path
Reliability - Assesses the probability of a link failure, calculated from the interface error count or previous link failures
Cost - A value determined either by the IOS or by the network administrator to indicate preference for a route. Cost can represent a metric, a combination of metrics or a policy.

Note: At this point, it is not important to completely understand these metrics; they will be explained in later chapters.
The Metric Field in the Routing Table
The metric for each routing protocol is:
RIP: Hop count - Best path is chosen by the route with the lowest hop count.
IGRP and EIGRP: Bandwidth, Delay, Reliability, and Load - Best path is chosen by the route with the smallest composite metric value calculated from these multiple parameters. By default, only bandwidth and delay are used.
IS-IS and OSPF: Cost - Best path is chosen by the route with the lowest cost. . Cisco's implementation of OSPF uses bandwidth. IS-IS is discussed in CCNP.
Routing protocols determine best path based on the route with the lowest metric.
Refer to the example in the figure The routers are using the RIP routing protocol. The metric associated with a certain route can be best viewed using the show ip route command. The metric value is the second value in the brackets for a routing table entry. In the figure, R2 has a route to the network that is 2 hops away.
R [120/2] via, 00:00:26, Serial0/0/1
Note: More detailed information about specific routing protocol metrics and how to calculate them will be available in the later chapters describing the individual routing protocols.


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