1

I have a question regarding the EIGRP metric system.

so if K1 is the Bandwidth and formula is :

(K1 * BW + ((K2 * BW) / (256 - LOAD)) + K3 * DELAY ) * (K5 / (RELIABILITY + K4)

Whats is BW and RELIABILITY?

Greetings

2

Hi and welcome to Network Engineering at StackExchange.

BW in the formula is the MIN value of all bandwith values found on the set of links along path towards the destination.

Likewise (I am aware that you didn't specifically ask about this), Delay in the formula is the sum of all DLY delay values found for the set of links along the path to the destination.

The bandwidth and DLY value(s) are actually the "bandwidth" and "delay" property of the given link (interface), as can be seen in the output of show interface on a given interface:

TenGigabitEthernet1/6 is up, line protocol is up (connected)
  ...
  Internet address is ip.ip.ip.ip/30
  MTU 9216 bytes, BW 10000000 Kbit, DLY 10 usec,       <---- there is the BW and DLY for the given link!
     reliability 255/255, txload 1/255, rxload 1/255   <---- and here's the reliablity and load parameters
  Encapsulation ARPA, loopback not set
 ...

reliability and load are further properties of the given link/interface, but are almost never actually used in metric calculation.

https://www.practicalnetworking.net/stand-alone/eigrp-metric/ (and probably any of Cisco's EIGRPs white papers too) nicely explains why:

There are two reasons Load and Reliability are not included in the default EIGRP metric:

First, EIGRP does not do periodic updates — only triggered updates. As a result, the values for Load and Reliability are calculated once when a route is first learned, but are not updated dynamically as an interface becomes more or less saturated. A change in Load/Reliability does not trigger a new EIGRP update.

Second, the Load and Reliability values are not a reflection of the full path‘s load and reliability, but instead only of the directly connected link.

As such, Cisco elected to only consider Bandwidth and Delay, and to weigh them equally, in their default EIGRP metric calculation. The default K-Values are K1 and K3 set to one, and K2, K4, and K5 set to zero.

  • thank you! very much appriciated! – Alexandre Amaral Bednell Mar 22 at 12:00
  • Please note: by default, the BW value is derived from the actual link speed (like 10/100/1G/10G on Ethernet interfaces). But you can manipulate the BW value in interface configuration (bandwidth 10000) and this will have an impact on EIGRP's metric calculation and path selection. There's a few things to consider before manipulating the bandwidth parameter of an interface. Also see (for example) networkengineering.stackexchange.com/questions/57618 – Marc 'netztier' Luethi Mar 22 at 13:48

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