Diffusing Update Algorithm

Diffusing Update Algorithm
One last topic must be discussed in order for us to fully understand the EIGRP route calculation.
That topic is the Diffusing Update Algorithm (DUAL). DUAL is the algorithm by which all computation
of routes for EIGRP occurs. A full understanding of DUAL is beyond the scope of the
BSCI exam, so we’re only going to briefly discuss it here.
One of the biggest advantages of DUAL is how it speeds up convergence. It accomplishes this
by not recalculating routes when it doesn’t need to. Let’s take a look at the way DUAL operates.
An EIGRP neighbor sends an update to a router informing the router that the metric for a
route has changed or that there has been a topology change. At this point, the router looks for
Enhanced Interior Gateway Routing Protocol 125
a better route. The router looks for a feasible successor to the destination that has a better metric
than the new metric for the route. If a feasible successor is found, the router will immediately
choose it as the new successor. The router will now increase its own metric and advertise it to
its neighbors. If a feasible successor is not found, then DUAL will start recalculating to find a
new successor.
There are three instances that causes DUAL to recalculate:
 An alternate route is not found.
 The new best route is still the original successor.
 The new best route is not a feasible successor.
When the recalculation begins, the router queries all its neighbors about the destination. The
router then tracks the queries it has sent so it knows when all neighbors have replied.
When a neighbor receives a query, it first marks the route as unreachable and queries each
of its neighbors—this is what is meant by diffusing. The neighbor then replies with one of the
following six responses:
 The neighbor will reply with an infinite metric if it doesn’t have any information about the
destination.
 The neighbor will reply with its current best route if the route is already active.
 If the query is not from the neighbor in the path of a successor, the neighbor will reply with
its current best route and the route will remain passive.
 If the neighbor in the path of a successor sent the query and the replying router doesn’t have
any other neighbors, it will reply with an infinite metric.
 The neighbor will reply with the new best route if it has multiple routes to the destination.
 If the neighbor has other neighbors besides the querying router, it will propagate the query
to its other neighbors. When it doesn’t have an alternate route, then the route is through the
router that sent the query, or the route is not through a neighbor in the path of a feasible
successor. The process then occurs on each of the routers receiving the propagated query.
Once the original querying router receives all replies from its neighbors, the router selects the
new best route. The router then sends out a routing update with the new information.
An issue will occur if any of the neighbors do not reply to the query. If the active timer expires—
which is set to three minutes by default—the router will declare the route stuck in active (SIA).
With all this in mind, you can now see how DUAL can cut down on the number of times it
must recalculate routes. What this means for you is a faster converging network.