Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary Distance-Vector routing protocol. This means that all your routers must be Cisco routers in order to use IGRP in your network, keep in mind that Windows 2000 now supports it as well because they have bought a licence from Cisco to use the protocol !
Cisco created this routing protocol to overcome the problems associated with RIP.
IGRP has a maximum hop count of 255 with a default of 100. This is helpful in larger networks and solves the problem of there being only 15 hops maximum possible in a RIP network. IGRP also uses a different metric from RIP. IGRP uses bandwidth and delay of the line by default as a metric for determining the best route to an internetwork. This is called a composite metric. Reliability, load and Maximum Transmission Unit (MTU) can also be used, although they are not used by default.
IGRP has a set of timers to enhance its performance and functionality:
Update Timer: These specify how frequently routing-update messages should be sent. The default is 90 seconds.
Invalid Timers: These specify how long a router should wait before declaring a route invalid if it doesn't receive a specific update about it. The default is three times the update period.
Hold-down Timers: These specify the hold-down period. The default is three times the update timer period plus 10 seconds.
Route Flush Timer:These indicate how much time should pass before a route should be flushed from the routing table. The default is seven times the routing period.