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EIGRP Wide Metric Calculation

This topic very accurately describes how the wide metric calculation works (and boy, is there a lot of bullshit on the web about this calculation with wide metrics).  Below is a simple topology to demonstrate how the EIGRP wide metrics work.

EIGRP-WIDE-METRICS

Below is the current configuration of each router. Simply put, a very basic EIGRP name mode config has been put on all three devices.  This lab will be using default K values.

R9#sh run | s eigrp
router eigrp A
 !
 address-family ipv4 unicast autonomous-system 1
  !
  topology base
  exit-af-topology
  network 155.1.79.0 0.0.0.255
 exit-address-family

R7#sh ru | s eigrp
router eigrp A
 !
 address-family ipv4 unicast autonomous-system 1
  !
  topology base
  exit-af-topology
  network 155.1.67.0 0.0.0.255
  network 155.1.79.0 0.0.0.255
 exit-address-family

R6#sh run | s eigrp
router eigrp A
 !
 address-family ipv4 unicast autonomous-system 1
  !
  topology base
  exit-af-topology
  network 155.1.67.0 0.0.0.255
 exit-address-family

 

In this example, we will look at R9 for the route 155.1.67.0/24, and work out how the metric was calculated.

R9#sh ip eigrp topology 155.1.67.0/24
EIGRP-IPv4 VR(A) Topology Entry for AS(1)/ID(150.1.9.9) for 155.1.67.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 1966080, RIB is 15360
  Descriptor Blocks:
  155.1.79.7 (GigabitEthernet1.79), from 155.1.79.7, Send flag is 0x0
      Composite metric is (1966080/1310720), route is Internal
      Vector metric:
         Minimum bandwidth is 1000000 Kbit
         Total delay is 20000000 picoseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 150.1.7.7

 

Explination of the forumla

It’s essentially Throughout + Latency. Where previously in classic EIGRP it was BW + Delay. They are effectively the same thing in the calculation, but just renamed.   The forumla breaks down into the following

 

Throughput = 65536 * 10^7 / minimum bandwidth across all links in the path

Latency =        65536 * total delay (in picoseconds) / 10^6

Composite metric = Throughput + Latency

 

Important Notes:

  • 65536 is a constant value that never changes, known as the “Wide-Scale-Constant”.  Note: this is not the RIB-Scale.
  • 10^7 is a constant value, i.e. it is always 10^7.  Cisco names this “EIGRP_BANDWIDTH” in the documentation.
  • 10^6 is a constant value, i.e. it is always 10^6.  Cisco names this “EIGRP_DELAY_PICO” in the documentation.

 

EIGRP wide metric calculation of the prefix 155.1.67.0/24

Using what we know from breaking down the formula (above) the calculation for my prefix of 155.1.67.0/24 is as follows:

 

Throughout = 65536*10^7/1000000 = 655360

Latency = 65536*20000000/10^6 = 1310720

Composite metric = 655360 + 1310720 = 1966080

 

As you can see it’s a perfect match.

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