Thursday, January 1, 2009

EIGRP metric calculation

If the standard bandwidth and delay parameters are used within EIGRP the formula for calculating the metric is as follows...

(10,000,000/bw(kbit) + delay/10) * 256

As an example examine the following entry from the eigrp topology table.

Router_1#s ip eigrp top
IP-EIGRP (AS 100): Topology entry for
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2684416
Routing Descriptor Blocks: (Serial2/1), from, Send flag is 0x0
Composite metric is (3026432/2514432), Route is Internal
Vector metric:
Minimum bandwidth is 1280 Kbit
Total delay is 40100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2

Using the formula....

(10,000,000/1280 + 40100/10) * 256 = (7812.5 + 4010) * 256
= 11822 * 256
= 3026432!!

N.B. Before moving figures outside of brackets truncate (not round) the figure to 0 decimal places.

Understanding this metric calculation may seem a bit like overkill - it did to me! However it could feasibly come into play if a question asked for EIGRP load balancing to be performed according to a certain ratio. In this instance it may be necessary to adjust the EIGRP metric to achieve the desired balancing.

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