Switch Duplex and Speed

By certskills January 10, 2012 09:39

For this next lab post, instead of just asking you what the IEEE auto-negotiation rules look like, the lab asks you to configure some switches in ways that affect auto negotiation. But the lab gives you the end goal: the resulting speed and duplex. The logic is not complex, but it if you have only read the basics in a table, and never applied it yourself, these basics are easy to miss. This exercise gives you a chance to apply the ideas.


This lab begins by giving you some details about the lab environment, and then giving you two sets of requirements. So first, here are some of the details about the lab environment, detailed in Figure 1:

  • The router has been configured already, and is working.
  • The router is connected to other links, not shown; those links are completely unimportant to the lab.
  • This lab uses only ICND1/CCENT concepts, so the link between the two switches is not a VLAN trunk, and only the default VLAN (VLAN 1) is in use.


Figure 1 – Lab Topology

This lab begins with the router configured correctly, but the lab ignores the router configuration for the most part. The two switches start with the configurations as shown. These initial configurations are here for reference, but the initial config has no real impact on how you configure the switches for the upcoming new requirements.

Example 1: SW1 Initial Config

Example 2: SW2 Initial Config



Problem 1: Configure to Match Desired Auto Negotiation Results

This exercise does not match what you might do in real life, but it does allow you to exercise the different command options. For this lab, configure the switch ports so that the results of the IEEE auto-negotiation process ends with the result for speed as shown in an upcoming table. That’s it! There are a few key assumptions and rules to keep in mind, though:

  1. The PC NICs use IEEE auto-negotiation
  2. The PC NICs have default settings, which means the PC NICs will attempt to negotiate speed an duplex
  3. All PC NICs can sense the speed ONLY through IEEE auto-negotiation (in real life, some can just look at the physical layer encoding and detect the speed)
  4. All interfaces have the correct cabling already installed
  5. All switch ports are 10/100 ports
  6. Show all possible configurations that result in the final state listed in the table

Table 1: Resulting Speed and Duplex; Configure to Cause These Settings





















Problem 2: Configure and Predict Results

For this last part of today’s exercise, for two ports, configure based on the written rules listed here. Then predict the speed and duplex that should result on that link, on each end of the link. (Note that with IEEE auto-negotiation, the two ends may not end up agreeing on the duplex setting; the result is bad.):

  1. On SW1 F0/3, configure the switch to automatically detect duplex, but force the speed to 100 Mbps.
  2. On SW2 F0/6, configure the switch to automatically detect speed, but force duplex to half.

That’s it! Configure away!

Answers: FR DLCI Drill #1
FR DLCI Drill #2
By certskills January 10, 2012 09:39
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  1. Sunny June 17, 10:37

    Dear sir,
    I’m facing some problem when I’m trying to get status of a configured switch using show interface command. It creates messages ‘Invalid input detected at’ >’ marker

    Reply to this comment
    • CCENTSkills June 19, 07:21

      Any chance you were in config mode at the time?

      In EXEC mode (user or enable mode), “show interfaces” should be accepted.

      In config mode, show commands are rejected, unless you preface them with the “do” command, which tells IOS that the rest of what you typed should be issued as an EXEC command. EG “do show interfaces” in config mode executes the command, but “show interfaces” in config mode fails.

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