IGRP uses a combination of user-configurable metrics, including internetwork delay, bandwidth,
reliability, and load.
IGRP also advertises three types of routes: interior, system, and exterior, as shown in Figure 17.
Interior routes are routes between subnets in the network attached to a router interface. If the
network attached to a router is not subnetted, IGRP does not advertise interior routes.
System routes are routes to networks within an autonomous system. The Cisco IOS software derives
system routes from directly connected network interfaces and system route information provided by
other IGRP-speaking routers or access servers. System routes do not include subnet information.
Exterior routes are routes to networks outside the autonomous system that are considered when
identifying a gateway of last resort. The Cisco IOS software chooses a gateway of last resort from
the list of exterior routes that IGRP provides. The software 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.By default, a router running IGRP sends an update broadcast every 90 seconds. It declares a route
inaccessible if it does not receive an update from the first router in the route within 3 update periods
(270 seconds). After 7 update periods (630 seconds), the Cisco IOS software removes the route from
the routing table.
IGRP uses flash update and poison reverse updates to speed up the convergence of the routing
algorithm. Flash update is the sending of an update sooner than the standard periodic update interval
of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger
routing loops caused by increases in routing metrics. The poison reverse updates are sent to remove
a route and place it in holddown, which keeps new routing information from being used for a certain
reliability, and load.
IGRP also advertises three types of routes: interior, system, and exterior, as shown in Figure 17.
Interior routes are routes between subnets in the network attached to a router interface. If the
network attached to a router is not subnetted, IGRP does not advertise interior routes.
System routes are routes to networks within an autonomous system. The Cisco IOS software derives
system routes from directly connected network interfaces and system route information provided by
other IGRP-speaking routers or access servers. System routes do not include subnet information.
Exterior routes are routes to networks outside the autonomous system that are considered when
identifying a gateway of last resort. The Cisco IOS software chooses a gateway of last resort from
the list of exterior routes that IGRP provides. The software 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.By default, a router running IGRP sends an update broadcast every 90 seconds. It declares a route
inaccessible if it does not receive an update from the first router in the route within 3 update periods
(270 seconds). After 7 update periods (630 seconds), the Cisco IOS software removes the route from
the routing table.
IGRP uses flash update and poison reverse updates to speed up the convergence of the routing
algorithm. Flash update is the sending of an update sooner than the standard periodic update interval
of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger
routing loops caused by increases in routing metrics. The poison reverse updates are sent to remove
a route and place it in holddown, which keeps new routing information from being used for a certain
period of time.
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