Definition of routing metric in The Network Encyclopedia.
How It Works
Routing metrics enable routers to make intelligent decisions about how to forward packets to ensure that
- Packets are delivered efficiently and quickly
- Congestion does not occur over links between networks
- Packets are not lost by being dropped by overloaded or dead routers
The simplest metric used by routers to calculate routing table entries is the number of hops to a given destination network. For example, this metric is used by the Routing Information Protocol (RIP), which allows dynamic routers to communicate with each other to share routing information and synchronize the entries of their routing tables. If you need more control over the paths that packets take, you can use protocols such as Open Shortest Path First (OSPF) Protocol and Interior Gateway Protocol (IGP), which can use a number of other metrics, including real-time metrics that routers determine dynamically, such as the following:
- Load:Generally, the number of packets being processed per second by the router or its CPU utilization. If the load on a router becomes high, the router can advise other routers to recalculate routing tables in order to divert traffic around it.
- Latency:The time interval needed to route a packet through the router or over a specific path through the internetwork. Latency can be increased by delays due to such factors as port congestion on the router, heavy router load, bandwidth utilization of links between networks, and physical distance between networks.
Other routing metrics are manually entered into the router configuration by network administrators who have a knowledge of the physical layout and performance of the network. Such metrics can include the following:
- Bandwidth:The total capacity of each network link to carry traffic between different networks in the internetwork.
- Reliability:The relative amount of anticipated downtime for a given link between two networks.
- Cost:A parameter roughly proportional to the actual cost in dollars of using each network link. Some wide area network (WAN) links might have more latency but cost much less.
- Maximum Transmission Unit (MTU):The largest size of packet that the router can forward without segmenting the packet into subpackets. Segmentation of network traffic by routers adds additional latency to network communication.