Interior Gateway Routing Protocol (IGRP) is a kind of IGP which is a distance-vector routing protocol invented by Cisco, used by routers to exchange routing data within an autonomous system.
IGRP is a proprietary protocol. IGRP was created in part to overcome the limitations of RIP (maximum hop count of only 15, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth, delay, load, MTU, and reliability; to compare two routes these metrics are combined together into a single metric, using a formula which can be adjusted through the use of pre-set constants. The maximum hop count of IGRP-routed packets is 255 (default 100).
IGRP is considered a classful routing protocol. Because the protocol has no field for a subnet mask, the router assumes that all interface addresses within the same Class A, Class B, or Class C network have the same subnet mask as the subnet mask configured for the interfaces in question. This contrasts with classless routing protocols that can use variable length subnet masks. Classful protocols have become less popular as they are wasteful of IP address space.
IGRP maintains a number of timers and variables containing time intervals. These include an update timer, an invalid timer, a hold-time period, and a flush timer. The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds. The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period. The hold-time variable specifies the holddown period. The IGRP default for this variable is three times the update timer period plus 10 seconds. Finally, the flush timer indicates how much time should pass before a route should be flushed from the routing table. The IGRP default is seven times the routing update period.
IGRP is a proprietary protocol. IGRP was created in part to overcome the limitations of RIP (maximum hop count of only 15, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth, delay, load, MTU, and reliability; to compare two routes these metrics are combined together into a single metric, using a formula which can be adjusted through the use of pre-set constants. The maximum hop count of IGRP-routed packets is 255 (default 100).
IGRP is considered a classful routing protocol. Because the protocol has no field for a subnet mask, the router assumes that all interface addresses within the same Class A, Class B, or Class C network have the same subnet mask as the subnet mask configured for the interfaces in question. This contrasts with classless routing protocols that can use variable length subnet masks. Classful protocols have become less popular as they are wasteful of IP address space.
IGRP maintains a number of timers and variables containing time intervals. These include an update timer, an invalid timer, a hold-time period, and a flush timer. The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds. The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period. The hold-time variable specifies the holddown period. The IGRP default for this variable is three times the update timer period plus 10 seconds. Finally, the flush timer indicates how much time should pass before a route should be flushed from the routing table. The IGRP default is seven times the routing update period.