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EIGRP Metric Lab

I spent the last week reading my notes, answering questions and digging in topics that I didn’t feel complete about. I start seeing the big picture and get closer to the exam. This post will cover EIGRP metric and the next one will cover the SPF algorithm, two subjects I found both interesting and major exam topics.

EIGRP uses a relatively complicated metric system as it has five parameters:
Bandwidth, Load, Delay, Reliability and MTU.

By default only Bandwidth and Load are being used (and we’ll see why later) but in more complex scenarios the composite metric is calculated using the K values of each variable.
Bandwidth – calculated as 10^7/BW (using Kbit), the lowest along the path is chosen
Delay – cumulative along the path, the sum of all the delays in tens of microseconds (ms).
This is tricky and important for exam configurations – for 1000 ms we type 100.

The formula to calculate the metric is

Metric=[K1*bandwidth+(K2*bandwidth)/(256-load)+K3* delay]* [K5 /(reliability+K4)]

Values K1 through K5 can be changed with the metric weights command.
The default values are:
K1 & K3 = 1
K2, K4 & K5 = 0
The zero value of K2, K4 & K5 explain why we only use Bandwidth and Delay by default as the formula zero shorten the formula:

Metric = [256 * (10^7/BW) + 256 * (delay)]

In order to see the metric output I’ve built a small lab with 3 full-mesh routers and some Loopback interfaces on one of them.
One way to see the metric value is the command show interfaces s0/0:

Serial0/0 is up, line protocol is up
Hardware is M4T
Internet address is 10.2.2.2/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255

Another way is the show ip eigrp topology 10.0.0.0/8 command:

IP-EIGRP (AS 100): Topology entry for 10.0.0.0/8
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2169856
Routing Descriptor Blocks:
0.0.0.0 (Null0), from 0.0.0.0, Send flag is 0x0
Composite metric is (2169856/0), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 20000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
172.16.20.2 (Serial0/1), from 172.16.20.2, Send flag is 0x0
Composite metric is (2681856/2169856), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 40000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1

I broke the lab with the shutdown command on one of the Interfaces.
Using the command show ip eigrp topology I was able to catch the Active state

P 10.0.0.0/8, 1 successors, FD is 2169856
via Summary (2169856/0), Null0
A 10.0.1.0/30, 0 successors, FD is 2681856, Q
1 replies, active 00:00:00, query-origin: Local origin
Remaining replies:
via 172.16.20.2, r, Serial0/1

Another lab breaking step was changing the metric values on one side only. Using the command metric weights 0 3 2 4 1 2 resulted in:

*Mar  1 00:35:16.935: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.2.2.1 (Ser
ial0/0) is down: K-value mismatch
*Mar  1 00:35:21.399: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.2.2.1 (Ser
ial0/0) is down: Interface Goodbye received

Normal show ip eigrp topology show another mysterious buzz word – serno

P 172.22.71.208/29, 2 successors, FD is 46163456
via 172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via 172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266

This is Cisco’s explanation: serno stand for serial number

Some other definitions that this post will not be complete without:
Feasible Distance (FD) – the best metric along a path to a destination network, including the metric to the neighbor advertising that path
Reported Distance (RD) – the total metric along a path to a destination network as advertised by an upstream neighbor
Feasible Successor – a path whose RD <  FD (current best path)

Finally, I want to recommend reading a great post on EIGRP with non-default K values. It is well written and have good examples.

Find more labs here

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