EIGRP Metric Calculations with K values

[merveakis1]

Hello everyone,

I am studying for the cisco exam and while I was checking the EIGRP metric calculation part I got confused about several things. First of all according to the formula, 

Metric= 256*((K1*Scaled Bw) + (K2*Scaled Bw)/(256 – Load) + (K3*Scaled-Delay)*(K5/(Reliability + K4)))

if we use the default K values, then metric will be 0 (K5=0). Then some other part of the forum I saw people say that when K5 is 0 then (K5/Reliability+K4) wont be taken into account and then the simplified EIGRP formula can be achieved

256*(Scaled Bw + Scaled Delay)

Also, I saw examples where instead of multiplication the people use addition such as:

Metric= 256*((K1*Scaled Bw) + (K2*Scaled Bw)/(256 – Load) + (K3*Scaled-Delay)+(K5/(Reliability + K4)))

So lets say it is true that when K5 is 0, that part is not taken into account and everything works well, however I also saw in CCNP- Routing and Switching book the following comment:

"Cisco recommends against using K-values K2, K4, and K5, because a nonzero value for these parameters causes the metric calculation to include interface load and reliability. The load and reliability change over time, which causes EIGRP to reflood topology data, and might cause routers to repeatedly choose different routes (route flapping)"

Which I understand using a nonzero value for K5 also not recommendable by cisco , and when K5 is 0 then it wont taken into account so whats the point of using it then. Its just I saw several explanations for it and didnt make sense to me and I feed confused. If someone please clarify the k-values in eigrp metric calculation that would be really helpful.

Thank you.

[Diana Karolina Rojas]

Hello! Good morning,

I think it depends of your services and topology, read this information from Cisco:

"Reliability: Usage and Definition of K₄ and K₅

K₄ and K₅ are used to allow for packet loss-based path selection for networks where link quality and packet loss is critical. Packet loss caused by network problems result in highly noticeable performance issues or jitter with streaming technologies, voice over IP, online gaming and videoconferencing, and will affect all other network applications to one degree or another.

Critical services should pass with less than 1% packet loss. Lower priority packet types might pass with less than 5% and then 10% for the lowest of priority of services. The final metric can be weighted based on the reported link quality according to the adjustment:

Setting K₅ values up to 255 scales the impact of reliability quotient on the final metric."

EIGRP is not a protocol used frequently in large corporations or with services with high demand, is a limitated protocol mainly used in small or medium campus networks where services are limitated and non-critical. So is not common or is not a best practice to include this value into account. I hope this can be help full to you.

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Regards,

[paul driver]

Hello

It happens to me all the time, I just remember that by default Eigrp uses BW/delay to calculate its metric and their k value are K1 and K3 respectively.

You can set the eigrp  to just use either BW or Delay which would change the compositic metric of it routes and to do that you need to amend the eigrp weights within the routing stanza, And not just for that rtr either but to every router in theat eigrp domain, other wise you wont get adjacency's

Default metrics
router eigrp xx
metric weights 0 1 0 1 0 0 <---- note the first k1 value is reserved for tos which im not to sure when we would u utilise this - maybe others on this forum would have the answer?

sh ip protocols | sec Metric
Metric weight K1=1, K2=0, K3=1, K4=0, K5=0

BW only
router eigrp xx
eigrp weights 0 1 0 0 0 0

sh ip protocols | sec Metric

Metric weight K1=1, K2=0, K3=0, K4=0, K5=0

Delay only
router eigrp xx
eigrp weights 0 0 0 1 0 0 

sh ip protocols | sec Metric

Metric weight K1=0, K2=0, K3=1, K4=0, K5=0

res
Paul

[Richard Burts]

Paul makes a good point that if you change K values on one router you must also change them of every other router with which you want to form neighbor relationship because neighbors check the K value and will not become neighbor with a router whose K values are different.

I would also like to address one part of the original post which discusses the impact of using K values to use load and reliability. The statement was that this would cause EIGRP to repeatedly flood updates as the values change. But that is not the way that EIGRP actually handles the metric. EIGRP will calculate using load and reliability when it first calculates the metric and will use them if something forces a recalculation (change in the path, change in neighbor state, etc) but will not initiate a new metric calculation just because the load or reliability changes.

HTH

Rick