Figure C4 Eigrp Loop Detection

Router B will receive three advertisements for 10.1.1.0/24 as follows:

• Through Router A with a metric of 2500096

• Through Router C with a metric of 281600

• Through Router D with a metric of 2500096

Normally, Router B receives only one of these advertisements—through Router C— because of split-horizon. Split-horizon is turned off in this example to explain how EIGRP finds invalid routes based only on the metrics.

Router B adds the metric through the interface that it receives the advertisements on, and now it has these paths:

• Through Router A with a metric of 2756096

• Through Router C with a metric of 1988096

• Through Router D with a metric of 2756096

Now, Router B chooses the best path (lowest metric) to 10.1.1.0/24, which is through Router C, and uses this as a "measuring stick." Because the distances advertised by Routers A and D (before Router B adds the metrics in through its interfaces) are both higher than the best path (after Router B adds in its interface metrics), neither of these paths are valid.

Remember from the previous example in Figure C-3 that it's mathematically impossible for the metric through A or D to be lower than the total distance to the destination if the path contains a loop (passes through B more than once).

To put this in EIGRP terms:

• The distance to the destination advertised by the neighbor is the reported distance.

• The best metric available to the network is the feasible distance.

• The neighbor with the best metric to a destination is the successor.

• Any neighbors whose reported distances are less than the feasible distance are feasible successors. (They are advertising a loop free route.)

This model is conservative. Sometimes, a route is determined to be a possible loop when it isn't.

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