I'm confused about how eigrp calculates metric.

Greenmet29

I tried to read the second part of odems book last night and I don't know if it was from lack of sleep or what but I had a helluva time understanding FD/RD/SR/FS etc, so i'm probably going to be asking a few questions on here tonight and tomorrow... thanks in advance!

My first question:

if the bandwidth/delay of a serial link is 1544/20000, if I change the bandwidth on a router does it automatically calculate the delay? If not, how would I be able to calculate it?

soskar

The bandwidth and delay you mention is significant only to the local router. Changing one does not affect the other locally. It does affect the calculation of the metric for a route. Generally, lowering bandwidth and increasing delay will tell EIGRP to stay clear of that route, and vice versa. Here is how the router calculates the metric:

metric = ((10,000,000/minimum bandwidth) + total delay) x 256

Minimum bandwidth (kilobits) is the link on the way to the destination with the lowest bandwidth.

Total delay is the sum of all the delay values for each link. Remember that delay is measured in microseconds, so take the value from show int and divide it by 10.

For example, lets say we have the following:

R1

---

BW=56K/D=1000

---

R2

---

BW=128K/D=500

---

R3

Minimum bandwidth = 56
Total delay = 1500

((10,000,000/56 + 1500) x 256 (Round down when doing first step)
(178571 + 1500) x 256
180071 x 256
metric = 46098176

foxhenry

hi, if you're preparing for the CCNA..don't bother about this calculation but ofcourse it's good to know..but normally those such calculations are on the CCNP level..

just be concerned on how the routing table chooses the best path to place on its route table..there's a cisco rule on this but i can't remember the rule..but i'll try to dig my notes and let you know..