IGRP Routing UpdatesA router runnin

IGRP Routing Updates
A router running IGRP sends an update broadcast every 90 seconds, by default. When an update from the originating router is not received within three update periods (270 seconds), it declares a route invalid. After seven update periods (630 seconds), which include the three update periods, the router removes the route from the routing table. IGRP advertises three types of routes:

Interior— Routes between subnets in the network attached to a router interface.
System— Routes to networks within an autonomous system. The router derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers.
Exterior— Routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The router chooses a gateway of last resort from the list of exterior routes that IGRP provides. The router uses the gateway (router) of last resort if it does not have a better route for a packet and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort