Metrics (networking)
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- For other uses, see Metric.
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Metrics is a property of a route in computer networking, consisting of any value used by routing algorithms to determine whether one route should perform better than another (the route with the lowest metric is the preferred route). The routing table stores only the best possible routes, while link-state or topological databases may store all other information as well. For example, Routing Information Protocol uses hopcount (number of hops) to determine the best possible route.
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
In EIGRP, metrics is represented by an integer from 0 to 4294967295. In Microsoft Windows XP routing it ranges from 1 to 9999.
A Metric can be considered as :
- 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.
A survey of routing metrics can be found here.
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Service Level Metrics
Service Level Metrics are concerned with the end user's experience of using the product.
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, you need to do the following calculation:
98% availability on 24/7/52
- Multiply 24 hours by 7 days = 8736 hours per year
- Find 98% of the hours per year = (8736 / 100) * 98 = 8561.21 hours guaranteed
You 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) = 176 hours (or around 7.3 days).
Redistribution
Sometimes networks must connect different routing protocols either because of the available hardware or because you simply have no control over the connected router. To fix this each side can configure redistribution to share routes between, say, EIGRP and OSPF. The seed metric acts as as a translation so that both protocols understand the viability of the path even if it is through a different routing protocol. A basic configuration chooses a core protocol, the routing protocol other protocols with redistribute into. In this case EIGRP is the core.
routerA(config)# router eigrp 10
routerA (config-router)# redistribute ospf 1
routerA (config-router)# default-metric 10000 100 255 1 1500
Where 10000 = Bandwidth, 100 = Delay, 255 = Reliability, 1 = Loading, and 1500 = Maximum Transmission Unit.
Notes
- ^ S. D. Rao, C. S. R. Murthy: “Distributed dynamic QoS-aware routing in WDM optical networks”, Computer Networks, Volume 48, Issue 4, 15 July 2005, Pages 585-604
- ^ Configuring Seed Metrics, Cisco.