EIGRP Route cost calculation

[geoff.hill]

Hello all

I am trying to work out why certain routes have certain costs within EIGRP.

All of my eigrp K values are default, so eigrp cost is equal to 256*(10^7/BW + delay)

However, in the example below, FD is 312832, yet advertised BW and delay from BGP are 8196 and 20ms

256*(10^7/8096 + 20) = 317467, which is greater than 312576 below.

How is the value 312576 calculated (the next hop router is the redistributing router)

switch#sh ip eigrp top 10.x.0.0/16
IP-EIGRP (AS 1): Topology entry for 10.x.0.0/16
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 312832
  Routing Descriptor Blocks:
  10.x.x.x (Vlan843), from 10.x.x.x, Send flag is 0x0
      Composite metric is (312832/312576), Route is External
      Vector metric:
        Minimum bandwidth is 8196 Kbit
        Total delay is 20 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 80.y.y.y

        AS number of route is 64577
        External protocol is BGP, external metric is 0
        Administrator tag is 1001 (0x000003E9)

[Peter Paluch · Accepted solution]

Hi Geoff,

Don't forget to do integer arithmetics, and the delay must be input in tens of microseconds, not in units of microseconds, therefore:

INT(10^7 / 8196) = 1220

256 * (1220 + 2) = 312832

Note the value 312832 is exactly the total distance computed through the neighbor, as displayed in your output. The feasible distance is not the current best metric to the distance, rather, it is the minimal distance to the destination since the last time the route went from Active to Pasive state - in fact, it is a record of the minimum distance, not the current best distance which may have increased during the time.

Best regards,

Peter