Timers

Because IGRP is a distance vector protocol in which routing updates are sent periodically, the different timers are especially important because they control how fast the routes are learned and deleted. Ultimately, these timers determine the network convergence time, which is the time that it takes for all the routers in the network to realize that a certain network has been added or deleted. The IGRP timers are the same as RIP; they are discussed in this list:

• Update— IGRP sends updates over the broadcast address of 255.255.255.255, with IP protocol number 9. The update timer is the periodic timer in which routing updates are sent; it is the time between each routing update interval. This value is set to 90 seconds, by default, and is configurable. In other words, the router sends its entire routing updates every 90 seconds, by default.

• Invalid— When the router stops receiving routing updates within the invalid timer, the routes become invalid. This is set to 270 seconds, by default.

• Hold-down— This is the time used to suppress the defective routes to be installed in the routing table. The default time is 280 seconds.

• Flush— This is the time when the route is removed from the routing table. This is set to 630 seconds, by default.

The default value for the IGRP update timer is 90 seconds, compared to the default of 30 seconds for the RIP update timer. This allows IGRP to use less bandwidth for periodic updates; however, the trade-off is that IGRP has a slower convergence time than RIP. All the timers mentioned here are configurable. The command to change the timer is timers basic update invalid holddown flush. However, changing the timer on only one router in the network could result in a network convergence problem. Changing the timers is not recommended.

If the network changes, such as after deleting or adding a network, IGRP and RIP use Flash updates. In other words, IGRP and RIP send instant updates to all their neighbors as soon as a network change is detected. For example, if a router's Ethernet interface goes down, the router immediately sends a Flash update to its neighbors that its Ethernet network is no longer available. After receiving the Flash update, the neighbors immediately put the Ethernet network into hold-down state.

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