Nov 27,2008 00:00
Redundant Link Calculation
The topology database stores all known routes to a destination and the metrics used to calculate
the least-cost path. Once the best routes have been calculated, they are moved to the routing
table. The topology table can store up to six routes to a destination network, meaning that
EIGRP can calculate the best path for up to six redundant paths. Using the known metrics to
the destination, the router must make a decision as to which path to make its primary path and
which path to use as a standby or secondary path to a destination network. Once the decision
is made, the primary route—the one with the lowest metric—will become the successor and be
added to the routing table. Any route that has an advertised distance lower than the successor’s
feasible distance will become a feasible successor route.
The path-cost calculation decisions are made from information contained in the routing
table using the bandwidth and delay from both the local and adjacent routers. Using this information,
a composite metric is calculated. The local router adds its cost to the cost advertised by
the adjacent router. The total cost is the metric. Figure 4.4 shows how cost is used to select the
best route (successor) and the backup route (feasible successor).