Answers: Switch Duplex and Speed

certskills
By certskills January 17, 2012 10:52

This latest lab gave you some tasks related to how to configure switch ports to achieve a particular speed and duplex setting on some Ethernet NICs. Basically the lab gives you lots of exercises with choosing when to use the speed and duplex commands. Details below the fold. Enjoy!

Quick Review of Auto-Negotiation on Cisco Switches, and Lab Rules

This lab exercise revolves around both configuring the switches and relying on the switch to use auto-negotiation. Cisco switch ports (like 10/100 and 10/100/1000 UTP ports) that support multiple speeds can be configured for speed, duplex, and both. However, when both are configured, the switch port also disables IEEE auto-negotiation. In the lab setup, I mentioned to assume that the PC NIC would not sense the speed correctly if auto-negotiation were disabled on the switch.

The other reminder is more obvious: when both ends use auto-negotiation, they use the best speed that both support, and the best duplex (full is better than half). When the other end is simply not doing auto-n, and the local end cannot sense the speed by other means, it uses the slowest speed and the worst duplex (10/half in this case).

And as usual, as a reminder, here’s the LAN topology:

Figure 1 – Lab Topology

Switch Configurations – Problem 1

The configurations for Problem 1 are listed below. The answers show one way to configure for each of the four requirements in the original lab, Problem 1. However, the original lab asked for all. Which ways did you configure as an alternative to these? Post the answers here, and I can review if you’re unsure.

Example 1: SW1 New Config, Problem 1

Example 2: SW2 New Config, Problem 1

Switch Configurations – Problem 2

Problem 2 required very little configuration – the problem statement essentially tells you the configuration in English wording. The real question is to predict the speed settings on both end, and in particular, to pay attention to any possible duplex mismatches.

Examples 3 and 4 show the configuration added in each case.  In these two cases, note that the switch ports do not have both the speed and duplex configured, so the switch still uses auto-n. As a result, no matter the duplex settings, the PC NIC and the switch port will be able to agree to which duplex to use.

  • For SW1’s Fa0/3: Both ends use 100 Mbps, Full
  • For SW2’s Fa0/6: Both ends use 100 Mbps, Half

Example 3: SW1 New Config, Problem 2

Example 4: SW2 New Config, Problem 2

FR DLCI Drill #2
Answers: FR DLCI Drill #2
certskills
By certskills January 17, 2012 10:52
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19 Comments

  1. Jon February 15, 15:22

    This is definitely something to pay attention to for real life. I don’t know whether it’s on the exams, but learn it anyway, and learn it well because it should be. If you have two switch or router interfaces, one coded to 100/Full and the other defaulting to autonegotiate, the auto interface will pick 100/Half every single time, and the results are not pretty: Frame and CRC errors on the Full-duplex side, and Late Collisions on the Half-duplex side. The same would be true with hard-coding to 10/Full; the auto side would pick 10/Half and you would see the same errors. Only Ethernet links running at gigabit and faster speeds will default to full duplex. NOT capable of; only interfaces operating at those speeds will choose full duplex by default.
    I saw this all the time at work, until our network vendors got their configuration documentation squared away. If you see lots of Frame errors on an interface, or any Late Collisions at all, check the duplex settings for a mismatch.

    Reply to this comment
  2. Sharad March 5, 08:23

    Prb.1)

    SW1#config

    interface fa 0/1
    speed 100 —– alternative 1

    duplex full —– alt 2
    speed 100

    duplex half —– alt 3
    speed 100

    interface fa 0/2
    speed 10 —– alternative 1

    duplex full —– alt 2
    speed 10

    SW2#config

    interface fa 0/4
    speed 10 —– alternative 1
    duplex full

    interface fa 0/5

    speed 100 —– alternative 1

    duplex full —– alt 2
    speed 100

    duplex full —– alt 3

    Please review. If there is any missing or wrong alternative , please hint it I will like to try it again.

    Reply to this comment
    • Wendell Odom of Certskills March 5, 12:39

      Hi Sharad,
      Thanks for playing!

      So, I don’t think it’s all correct, but in the spirit of your request, here are hints only.

      When you configure both speed and duplex on an interface, a Cisco switch no longer participates in IEEE auto-negotiation. The PC on the other end of the link will then be forced to react to a case in which it tries auto-n, but gets no response. Check your cases versus the goals, and see where you’re still happy, and not happy, with your configs.

      Hint 2: If you set speed or duplex on the switch, that’s the only speed or duplex it can use. EG, speed 100 means that the switch cannot auto-negotiate to any other speed; it will run at 100.

      Enjoy!
      Wendell

      Reply to this comment
      • Sharad March 6, 12:20

        Thanks Wendell,

        I have again reviewed the answers and am changing few. Also mentioning my thought process for the answers.
        Please review & correct it.

        SW1#config

        interface fa 0/1
        speed 100 —– alternative 1 ( wrong, this allows auto-n for duplex, thus making it FD)

        ############
        duplex full —– alt 2 ( wrong, it fixes setting to 100 & fd; initially i thought that speed 100 with auto-n disabled on the switch, on basis of speed
        PC will used hd, but it is pointed in ques pc don’t sense
        speed)
        ##########

        duplex half —– alt 3 (that should be correct)
        speed 100

        ############

        interface fa 0/2
        speed 10 —– alt 1 (wrong, autoneg for duplex makes it fd)

        ##########

        duplex full —– alt 2 (wrong, as disabled autoneg makes switch use configured setting)
        speed 10

        Can’t think of no other alternative than one mentioned by you for 10-h setting.

        ############

        SW2#config

        interface fa 0/4
        speed 10 —– alt 1 (correct, switch fixes to this but it will lead to duplex mismatch)
        duplex full

        Can’t think of any other alternative for 10-f setting. Is there any? Please indicate.

        interface fa 0/5
        speed 100 —– alt 1 (correct)

        duplex full —– alt 2 (correct)
        speed 100

        duplex full —– alt 3 (correct)

        Reply to this comment
  3. Chris September 27, 13:22

    Ok, here’s my question. The requirements specify that on SW2 F0/4 should have a speed of 10 and duplex of Full. In your configuration for this switchport, you are manually setting the speed to 10 and leaving auto-negotiate on for duplex, which you’re saying would give us full duplex.

    IEEE rules state that if your speed is 10 or 100 Mbps, then use half-duplex. Wouldn’t this configuration then not meet the requirements of the lab?

    Reply to this comment
    • CCENTSkills November 17, 10:48

      Chris,
      Sorry, I imagine you don’t care about a reply 2 months later. I just missed your comment. Just in case…
      The short answer is No. The reason: In this lab, the problem statement says that the PCs use default settings and that they do use auto-negotiation. SW2’s F0/4, with speed set to 10, would use a speed of 10, but would allow either full or half duplex. When the PC off that port participates in auto negotiation, it states it would use multiple speeds, but use half or full duplex. The result: they both use speed 10 (because that is the only speed offered by the switch), and they both use full duplex (because both offer to use half or full, and full is better).
      Hope this helps…
      Wendell

      Reply to this comment
    • David Alon May 1, 15:43

      Hi Chris,

      Autonegotiation can be disabled for speed only or duplex only. In this case, the auto-n parts will still choose the best possible settings. So 10 can allow full duplex if both duplex are auto-n.

      To predict the results on the link: always analyze from the switch perspective (which ever end if negoctiating). If auto-n on switch. look at PC speed to define duplex results (if auto-n is disabled for duplex), hence possible mismatch. But if any static value is defined on the switch, then the result on link will be this switch static value.

      Reply to this comment
      • David Alon May 3, 17:08

        Examples:

        Case 1: switch has auto-n

        PC has 10 Mpbs and Full-duplex. Result: 10 Mbps with Half-duplex (duplex mismatch)
        PC has 100 Mpbs and Full-duplex. Result: 100 Mbps with Half-duplex (duplex mismatch).

        Case 2: PC has auto-n

        SW has 10 Mpbs and Full-duplex. Result: 10 Mbps with Full-duplex.
        SW has 100 Mpbs and Full-duplex. Result: 100 Mbps with Full-duplex.

        Reply to this comment
        • Michael Prejean May 18, 23:39

          Hi David, was wondering if you could clarify this scenario a little. I’m confused as to why the result differs in each case. For example, does case 1 imply that the PC has auto-n turned off, and vice versa for case 2? I understand the duplex mismatches and why the results in case 1 happened, but I’m confused as to why when the PC has auto-n the results don’t create a mismatch. Are there different rules for auto-n on a PC NIC than on a Cisco switch? Sorry if the answer is obvious, just trying to get the learning down.

          Reply to this comment
  4. HectorJ November 16, 19:28

    This lab was placed with chapter 8 questions. But I think that this belongs to the 9 one: What’s wrong?

    Reply to this comment
  5. Nghi Thai May 6, 15:54

    Hello,

    I just want to confirm my thought process on this.

    For switch#2 interface fa0/6, you set duplex half and leave speed auto.

    This means the interface is still in auto-n mode. So if a PC uses auto-n, then both Switch & PC can still negotiate to use either Half or Full. (Even though duplex half is set).

    The only time it cannot negotiate is when the interface set both duplex and speed.

    Is this thought process correct?

    Reply to this comment
  6. Nick Schultz June 10, 19:49

    Hi there,

    If speed and duplex are statically configured on a switch port – which successfully disables auto-n, can an end node such as a PC ever determine the speed like a Cisco switch can? Or does the PC automatically default to using its slowest supported speed?

    Thank you

    Reply to this comment
    • Chris June 18, 12:30

      There are many different makers of NIC’s. I do not know how they function. I would guess they have the ability to sense speed as well.

      Reply to this comment
    • certskills Author June 19, 08:15

      Hey Nick,
      Yep, NICs can sense the speed as well. The encoding is different for each speed – that’s the detail of how the standard varies to the electrical characteristics to encode bits – and from an electronics perspective, it’s a pretty simple task to distinguish between the encoding based on the incoming signals. So, sensed speed is pretty common.

      Reply to this comment
  7. Edin August 31, 17:40

    Hi. I have a confusion regarding Example 3. Wouldn’t there be a speed mismatch since the PC doesn’t hear autonegotiation from SW1 (which has configured a speed of 100), so it uses its slowest speed of 10?

    Reply to this comment
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