# The truth about EIGRP FD Calculation

**dmorrow**Feb 25, 2012 11:25 PM

**This is a contradiction I could not get my head around until now. The comment below was taken below from Diane Teare's latest CCNP route books for the 642-902, the same is said across a number other books. **

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**Basically the quote below says the FD is calculated based on the sum of the next hop routers AD and the cost between local router and next hop router. However as you read on through the book it says the metric is calculated using the EIGRP metric calculation. To simplifiy things for now lets just say metric= least cost bw + accumulative delay**

**CCNP Quote:**

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**Advertised distance and feasible distance—**DUAL uses distance information, known as a metric or cost, to select efficient, loop-free paths. The lowest-cost route is calculated by adding the cost between the next-hop router and the destination—referred to as the advertised distance (AD)—to the cost between the local router and the next-hop router. The sum of these costs is referred to as the feasible distance (FD).

The former appears not to work as I don't get the correct FD value by adding local metric to next hop routers AD, unless I also subtract 256. I do however get the correct FD value from the metric formula.

Here is my Topology which is taken from the CCNP Route LAB manual LAB 2-1

(R1)--fa0/0 <-- 100Mbps --> fa0/0-(R3)--loopback3 <--> 10Gbps

(R1)--fa0/0 = 10.1.100.1/24

(R3)--fa0/0 = 10.1.100.3/24

(R3)--Loopback3 = 10.1.3.1/24

I'm going to calculate the FD for 10.1.3.0/24 from R1 perspective using the first scenario FD = AD + Local Router to Next Hop Router metric

R1#sh ip eigrp topology

IP-EIGRP Topology Table for AS(1)/ID(10.1.1.1)

P 10.1.3.0/24, 1 successors, FD is 409600

via 10.1.100.3 (409600/128256), FastEthernet0/0

P 10.1.100.0/24, 1 successors, FD is 281600

via Connected, FastEthernet0/0

Ok so now we calculate the FD by adding the AD for 10.1.3.0/24 via 10.1.100.3 and FD for 10.1.100.0/24 connected local therefore FD = 128256 + 281600 = 409856.

hmm.. 409856 does not match the FD shown by the show ip eigrp topology output as this shows the FD as 409600, there is a difference of 256.

I tried this same calculation on other routes and seem to get a difference of 256. Perhaps the calculation needs to be FD = 128256 + 281600 - 256 = 409600

So now I'll use the metric formula, but first I'll display the metrics needed for this calculation. Note: the min bandwidth is the 100Mbps (10000Kbps) on fa0/0 as shown below and the delay on fa0/0, the delay on loopback 3 at R3 is 5000 microseconds, therefore the cumulative delay is 6000 microseconds as shown directly below.

R1#sh ip eigrp topology 10.1.3.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.3.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 409600

Routing Descriptor Blocks:

10.1.100.3 (FastEthernet0/0), from 10.1.100.3, Send flag is 0x0

Composite metric is (409600/128256), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 6000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 1

Outputs below provided as added information to prove the outpu above is true.

R1#sh ip eigrp topology 10.1.100.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.100.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 281600

Routing Descriptor Blocks:

0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0

Composite metric is (281600/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 0

R3#sh ip eigrp topology 10.1.3.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.3.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256

Routing Descriptor Blocks:

0.0.0.0 (Loopback3), from Connected, Send flag is 0x0

Composite metric is (128256/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000000 Kbit

Total delay is 5000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1514

Hop count is 0

Now to calculate using the metric formula; metric = ((1E7/minbw)+(sum of delays/10ths of microseconds))*256

metric = ((10000000/10000)+(600))*256 = 409600

So the formula matches the FD shown in the topology output.

R1#sh ip eigrp topology 10.1.3.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.3.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 409600

Routing Descriptor Blocks:

10.1.100.3 (FastEthernet0/0), from 10.1.100.3, Send flag is 0x0

Composite metric is (409600/128256), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 6000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 1

I'm interested in anyone's view on the reasoning behind the said quote above because it does'nt appear to be right if taken literally, it might be true if you subtract 256. Can anyone confirm possible reasons for the difference of 256?

Perhaps the AD is not used in the FD calculation for a given route and is only used as the selector for Feasible Successors etc.. not sure.

The EIGRP formula matches the FD as expected.

thanks DM