Metrics (networking)

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Router metrics are metrics used by a router to make routing decisions. A metric is typically one of many fields in a routing table. Router metrics help the router choose the best route among multiple feasible routes to a destination. The route will go in the direction of the gateway with the lowest metric.

A router metric is typically based on information such as path length, bandwidth, load, hop count, path cost, delay, maximum transmission unit (MTU), reliability and communications cost.

Examples

A metric can include:

  • measuring link utilisation (using SNMP)
  • number of hops (hop count)
  • speed of the path
  • packet loss (router congestion/conditions)
  • latency (delay)
  • path reliability
  • path bandwidth
  • throughput [SNMP - query routers]
  • load
  • MTU
  • administrator configured value

In EIGRP, metrics is represented by an integer from 0 to 4,294,967,295 (The size of a 32-bit integer). In Microsoft Windows XP routing it ranges from 1 to 9999.

A metric can be considered as:[1]

  • additive - the total cost of a path is the sum of the costs of individual links along the path,
  • concave - the total cost of a path is the minimum of the costs of individual links along the path,
  • multiplicative - the total cost of a path is the product of the costs of individual links along the path.

Service level metrics

Router metrics are metrics used by a router to make routing decisions. It is typically one of many fields in a routing table.

Router metrics can contain any number of values that help the router determine the best route among multiple routes to a destination. A router metric typically based on information like path length, bandwidth, load, hop count, path cost, delay, Maximum Transmission Unit (MTU), reliability and communications cost.

Availability

The availability of a computer network (or an individual service) may be expressed using the notation hh/d/ww. For a 24-hour service, seven days a week, available all year around, this would be expressed 24/7/52 (where the 52 stands for the number of weeks in a year). Service providers usually express that a service will be available for a percentage of this time.

To calculate the availability of a service expressed in this format, one needs to do the following calculation:

98% availability on 24/7/52

  1. Multiply 24 hours per day by 7 days per week by 52 weeks per year = 8736 hours per year
  2. Find 98% of the hours per year = 8736 * 98 / 100 = 8561.28 hours guaranteed

One can then deduce how many full hours/days per year the service can be unavailable before the supplier is in breach of their Service Level Agreement. In this example, 8736 (hours) - 8561 (hours) = 175 hours (or around 7.3 days).

References

External links

  • Survey of routing metrics
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