Route Calculation

Route Calculation
EIGRP uses multicasts instead of broadcasts. Therefore, only devices running EIGRP are
affected by routing updates or queries. Where IGRP updates use a 24-bit format, EIGRP uses
a 32-bit format for greater granularity. The default IGRP metric, which includes only bandwidth
and delay, can simply be multiplied by 256 to obtain the corresponding EIGRP metric.
Only changes in the network topology are advertised, instead of the entire topology table.
EIGRP is called an advanced distance-vector protocol, because it contains properties of both
distance-vector and link-state routing protocols when calculating routes. DUAL is much faster
and calculates the shortest path to a destination when updates or Hello messages, or the lack
thereof, cause a change in the routing table. Recalculation occurs only when the changes directly
affect the routes contained in the topology table.
Enhanced Interior Gateway Routing Protocol 121
This last statement may be confusing. If a change occurs to a network that is directly connected
to a router, all of the relevant information is used to calculate a new metric and route entry for it.
If a link between two EIGRP peers becomes congested, both routers would have to calculate a new
route metric and then advertise the change to any other directly connected routers.
Now that you understand the difference between a route update and a route calculation, we can
summarize the steps that a router takes to calculate, learn, and propagate route update information.