| The centerpiece of EIGRP is the
Diffusing Update Algorithm (DUAL),
EIGRP's route-calculation engine. The full name of this technology is
DUAL finite-state machine (FSM). An FSM is an abstract machine, not
a mechanical device with moving parts. FSMs define a set of possible
states that something can go through, what events cause those
states, and what events result from those states. Designers use FSMs
to describe how a device, computer program, or routing algorithm
will react to a set of input events. The DUAL FSM contains all the
logic used to calculate and compare routes in an EIGRP network.
DUAL tracks all the routes advertised by neighbors and uses the
composite metric of each route to compare them. DUAL also guarantees
that each path is loop-free. Lowest-cost paths are then inserted by
the DUAL protocol into the routing table.
As noted earlier in the chapter,
EIGRP keeps important route and topology information readily
available, in a neighbor table and a topology table. These tables
supply DUAL with comprehensive route information in case of network
disruption. DUAL selects alternate routes quickly by using the
information in these tables. If a link goes down, DUAL looks for a
feasible successor in its neighbor and topology tables.
A successor is a neighboring router
that is currently being used for packet forwarding; it provides the
least-cost route to the destination and is not part of a routing
loop. Feasible successors provide the next lowest-cost path without
introducing routing loops. Feasible successor routes can be used in
case the existing route fails. Packets to the destination network
are immediately forwarded to the feasible successor, which at that
point is promoted to the status of successor as illustrated in Figures
- .
Note in the example that router D
does not have a feasible successor identified. The FD for router D
to router A is 2 and the AD via router C is 3. Because the AD is
smaller than the best-route metric but larger than the FD, no
feasible successor is placed in the topology table. Router C has a
feasible successor identified as well as router E because the route
is loop-free and because the AD for the next hop router is less than
the FD for the successor.
|