EIGRP K Values Formula


(Rene Molenaar) #13

EIGRP uses a 32 bit metric so the value used for infinity is 4294967295, the highest value you can create.


(Rene Molenaar) #14

Hi Hussein,

The MTU doesn’t influence the metric, it’s not used in the metric calculation.

Rene


(Rene Molenaar) #15

Apparently when you have two equal cost paths and EIGRP has to select one of them, it will use the path with the highest MTU. The low MTU path will not be used.


(Rene Molenaar) #16

Hi Hussein,

Good question, I tried to look it up but I couldn’t find anything why they picked 10^7…can’t find any explanation.

Rene


(Rene Molenaar) #17

Hi Hussein,

The advertised distance that a router advertises to a neighbor is the feasible distance that it has stored for a route, it’s the same calculation.

Rene


(Diego M) #18

Hi Rene,

Here you see the result of changing the bandwidth on the interface. Something to remember is that EIGRP will use the highest bandwidth in the path from A to B (since this is the bottleneck).
Is this right?. A think that EIGRP use the lowest bandwitch.

Thanks you


(Rene Molenaar) #19

Hi Diego,

That is correct, I just fixed it. Thanks for reporting!

Rene


(Efrain V) #20

I’m about 2 years too late but I’d like to take a crack at this for future reference. The reason 10^7 is used is because 10^7=10,000,000 kbps which is the same things as 10Gbps. When this formula was developed into what it is today, 10gbps was insanely, insanely fast and was decided to be used as the number utilized to calculate the weight of the minimum bandwidth number in the EIGRP formula.

The reason that we are adding weight to each number in EIGRP is easy to understand. EIGRP is a complex formula with different sets of numbers that have varying levels of relevance. We decide how much weight, or relevance, the minimum bandwidth has by taking 10^7 (dividend) and dividing it by the minimum bandwidth (divisor), the resulting number (quotient) multiplied by 256 [Read: 2^8 ‘binary octet max value’] is your weighted bandwidth.

“The multiplication of 256 is done so EIGRP is compatible with IGRP (the predecessor of EIGRP).”


(Rustem S) #21

Hi Rene!
K5 value is part of reliability and even though MTU is included K value configuration, Cisco router does not use it for composite metric calculation!


(Justin A) #22

doesn’t the speed command actually change the rate at which the interface transmits data? I know that the bandwidth command doesn’t.


(Quinton M) #23

Hi Rene

I am confused about the Feasible Distance and the Overall metric of a route. Are they the same thing?

Thanks


(Lazaros Agapides) #24

Hello Justin

Yes, the speed command will actually change the data rate of the interface in question. However, the speed command will only configure an interface to conform to a specific Ethernet technology that is supported by that interface. That is, 10Mbps, 100Mbps, 1000 Mbps, 10Gbps. Only these choices are available and only if that interface supports it.

Dynamic routing protocol metrics will also change according to the speed that is configured on specific interfaces.

I hope this has been helpful!

Laz


(Lazaros Agapides) #25

Hello Quinton

The feasible distance is the overall metric of the best route to a specific destination from the point of view of a particular router.

Take a look at this Cisco documentation that explains feasible distance, reported distance and feasible successor quite well. You may find it useful.

I hope this has been helpful!

Laz


(Muhammad Rasoul A) #26

Hi Lazaros,
Although the question was asked before but still it is not clear for me. the difference between feasible distance and metric.
And what is usec in Delay, is it millisecond or something else. please explain it in simple way.
Thanks in advance.


(KUNAL N) #27

IS MTU parameter considered in EIGRP metric as we are not seeing Metric consideration in networklesson notes but seen in other documents
please clarify


(Lazaros Agapides) #28

Hello Kunal

There is a general confusion when it comes to the K5 value for EIGRP and the metric involved with it. The confusion has to do with the fact that it is part of the metric calculation formula.

Now the MTU metric component essentially is used to advertise the minimum MTU along the route to the particular destination. Even though this value is carried in the EIGRP message, MTU is not actually used in the best path selection process. It is not factored in to the composite metric nor is it used as any kind of tiebreaker. However, it is still part of the equation.

As a reminder, the equation is the following:

image

Remember!!! The K values are only flags, and have values of either 1 or 0!

Notice that K5 is the only value that has no associated coefficient with which it is multiplied directly. This means that the actual value of the maximum MTU in the path in question is not taken into account anywhere in the metric calculation. Also note that if K5 = 0, K4, which is reliability, even if it is 1 is not taken into account. In other words, in order for reliability to be taken into account, bot K4 and K5 must be set to 1. So in actual fact, K4 and K5 both deal with reliability (as far as the metric calculation goes) and not with MTU values.

I hope this has been helpful!

Laz


(Lazaros Agapides) #29

Hello Muhammad

The metric is the term used to refer to the cost of a particular route. EIGRP uses a specific formula to calculate this metric. So the result of the formula calculation is called the metric. There may be many metrics to reach a particular destination, because there are multiple routes to get there. Each one has a metric of its own.

Now the feasible distance is the BEST metric to the destination, and it is the metric of the route that is ultimately installed in the routing table.

As for usec, this is actually microseconds. The “u” at the beginning is used to indicate the Greek letter mu or μ. It looks kind of the same, but not quite. In any case, u is used to substitute for μ so instead of getting μs you get usec.

I hope this has been helpful!

Laz


(Muhammad Rasoul A) #30

Nice explanation Lazaros thank you very much.


(Krishnendu B) #31

Hi, I have pasted the output below. I couldn’t compare the FD value and how its calculated. Please clarify.

R1# show int e0/0
Ethernet0/0 is up, line protocol is up
  Hardware is AmdP2, address is aabb.cc00.0100 (bia aabb.cc00.0100)
  Internet address is 12.0.0.1/24
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
 reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:03, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
 2826 packets input, 280114 bytes, 0 no buffer
 Received 2811 broadcasts (0 IP multicasts)
 0 runts, 0 giants, 0 throttles
 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
 0 input packets with dribble condition detected
 4179 packets output, 360367 bytes, 0 underruns
 0 output errors, 0 collisions, 1 interface resets
 7 unknown protocol drops
 0 babbles, 0 late collision, 0 deferred
 0 lost carrier, 0 no carrier
 0 output buffer failures, 0 output buffers swapped out
R1#show ip eigrp topo
R1#show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(13.0.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
   r - reply Status, s - sia Status

P 34.0.0.0/24, 1 successors, FD is 307200
    via 13.0.0.3 (307200/281600), Ethernet0/1
P 13.0.0.0/24, 1 successors, FD is 281600
    via Connected, Ethernet0/1
P 192.168.0.0/24, 1 successors, FD is 307200
    via 12.0.0.2 (307200/281600), Ethernet0/0
P 12.0.0.0/24, 1 successors, FD is 281600
    via Connected, Ethernet0/0

R1#

(Lazaros Agapides) #32

Hello Krishnendu

Assuming the default values are used, then only K1 and K3 are taken into account. This means we can use the following formula:

Metric = bandwidth (slowest link) + delay (sum of delays)

However, the information you provided in the above output is not enough to calculate the metric. This is because the bandwidth used is the slowest link in the path from R1 to the destination. We need information about the rest of the topology. Which is the slowest link? It may not be Ethernet0/0. Also, in order to determine the delay, we need a sum of all the delays involved for each hop between R1 and the destination. Only then can we calculate the metric and see if it is the same as that shown in the topology table you shared.

The routing table indicates that all routes that go via the Ethernet0/0 interface go to destinations that are not directly connected. This means that there are intervening routers for which the delay and the bandwidth must be known to calculate the metric.

I hope this has been helpful!

Laz