The Interior Gateway Routing Protocol (IGRP) is another distance vector protocol commonly used on Cisco networks. Developed by Cisco, IGRP is proprietary and provides a number of advantages over RIP on a network comprised of Cisco routers. Firstly, the maximum hop count allowed by IGRP is much greater than RIP – up to 255 hops are supported, although by default IGRP is set to allow only 100. Secondly, where RIP uses only the very basic metric of hop count to make routing decisions, IGRP actually supports a variety of metrics including bandwidth, delay, reliability, load, and MTU. The default metrics used by IGRP are bandwidth and delay.
The metrics used by IGRP almost immediately make it a better choice routing protocol than RIP, since decisions can be made based on the most efficient path to a network rather than the one with the fewest hops. IGRP uses what is known as the composite metric in determining the best path to a network. The composite metric is actually a calculation that takes into account both the minimum bandwidth between a router and a destination, and the delay value of an interface. While you don’t actually need to know how the composite metric is calculated, I think it’s still worth knowing where the numbers come from. Ultimately, the path with the lowest composite metric will be used by IGRP. To view the bandwidth and delay numbers used in the calculation of the composite metric, use the show interface command followed by the interface that the update is received on.
RouterA#show int s0
Serial0 is up, line protocol is up
Hardware is HD64570
Description: WAN link Toronto-Montreal
Internet address is 10.0.20.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Like RIP, IGRP routers also periodically broadcast their routing tables to exchange information with neighboring routers. The intervals involved in the exchanges are:
Route Update Timer. The route update timer controls how often a router will send out routing table updates. An IGRP router will send out updates every 90 seconds by default.
Route Invalid Timer. The route invalid timer specifies the amount of time that will pass before an IGRP router will mark a network as unavailable. By default this is set to three times the update timer, or 270 seconds.
Route Holddown Timer. The route holddown timer specifies the length of the holddown timer that will be used when IGRP receives information about an unreachable routing table entry. The default holddown timer for IGRP is 3 times the update interval plus 10 seconds, or 280 seconds total.
Route Flush Timer. The amount of time that will pass before IGRP completely removes an entry from the routing table. By default, this is set to seven times the update interval, or 630 seconds.