IGRP is a distance vector Interior Gateway Protocol (IGP). Distance vector routing protocols mathematically compare routes using some measurement of distance. This measurement is known as the distance vector. Routers using a distance vector protocol must send all or a portion of their routing table in a routing-update message at regular intervals to each of their neighboring routers. As routing information proliferates through the network, routers can identify new destinations as they are added to the network, learn of failures in the network, and, most importantly, calculate distances to all known destinations.
Distance vector routing protocols are often contrasted with link-state routing protocols, which send local connection information to all nodes in the inter network. For a discussion of Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS), two popular link-state routing algorithms, see (A7984A) “Open Shortest Path First,” , “Open System Interconnection Routing Protocol,” respectively.
IGRP uses a composite metric that is calculated by factoring weighted mathematical values for internetwork delay, bandwidth, reliability, and load. Network administrators can set the weighting factors for each of these metrics, although great care should be taken before any default values are manipulated. IGRP provides a wide range for its metrics. Reliability and load, for example, can take on any value between 1 and 255; bandwidth can take on values reflecting speeds from 1200 bps to 10 Gbps, while delay can take on any value from 1 to 224. These wide metric ranges are further complemented by a series of user-definable constants that enable a network administrator to influence route selection.
These constants are hashed against the metrics, and each other, in an algorithm that yields a single,composite metric. Thus, the network administrator can influence route selection by giving higher or lower weighting to specific metrics. This flexibility allows administrators to fine-tune IGRP’s automatic route selection.
To provide additional flexibility, IGRP permits multipath routing. Dual equal-bandwidth lines can run a single stream of traffic in round-robin fashion, with automatic switchover to the second line if one line goes down. Multiple paths can have unequal metrics yet still be valid multipath routes. For example, if one path is three times better than another path (its metric is three times lower), the better path will be used three times as often. Only routes with metrics that are within a certain range or variance of the best route are used as multiple paths. Variance is another value that can be established by the network
administrator.
Distance vector routing protocols are often contrasted with link-state routing protocols, which send local connection information to all nodes in the inter network. For a discussion of Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS), two popular link-state routing algorithms, see (A7984A) “Open Shortest Path First,” , “Open System Interconnection Routing Protocol,” respectively.
IGRP uses a composite metric that is calculated by factoring weighted mathematical values for internetwork delay, bandwidth, reliability, and load. Network administrators can set the weighting factors for each of these metrics, although great care should be taken before any default values are manipulated. IGRP provides a wide range for its metrics. Reliability and load, for example, can take on any value between 1 and 255; bandwidth can take on values reflecting speeds from 1200 bps to 10 Gbps, while delay can take on any value from 1 to 224. These wide metric ranges are further complemented by a series of user-definable constants that enable a network administrator to influence route selection.
These constants are hashed against the metrics, and each other, in an algorithm that yields a single,composite metric. Thus, the network administrator can influence route selection by giving higher or lower weighting to specific metrics. This flexibility allows administrators to fine-tune IGRP’s automatic route selection.
To provide additional flexibility, IGRP permits multipath routing. Dual equal-bandwidth lines can run a single stream of traffic in round-robin fashion, with automatic switchover to the second line if one line goes down. Multiple paths can have unequal metrics yet still be valid multipath routes. For example, if one path is three times better than another path (its metric is three times lower), the better path will be used three times as often. Only routes with metrics that are within a certain range or variance of the best route are used as multiple paths. Variance is another value that can be established by the network
administrator.
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