Answers: Trunking Puzzle 1

By certskills April 8, 2016 09:05

Lots of switch trunks, lots of different requirements, resulting in a variety of configuration combinations. Can you remember all the trunking config options without having to look up commands or using CLI help? Check out the lab first as always, and then come check out the answers here.



Figure 1: Four Switches with Trunks

Example 5: SW1 Config

Example 6: SW2 Config

Example 7: SW3 Config

Example 8: SW4 Config



When configuring trunking between switches there are a few question that need to be answered, including: Should dynamic trunking be used? And if so, which device should initiate the trunk? If not, and manual trunking is enabled, should you also disable DTP?

For this lab, you are tasked with configuring a number of trunks between devices. Obviously in a real environment, the trunking encapsulation and mode will be more consistent across the network; however, for this little network we require you to understand how to configure the various types of trunking modes.

First, the lab requirements tell you to make SW1 actively attempt to form a trunk with its neighboring switches. This mode is called dynamic desirable on Cisco switches and is configured with the switchport mode dynamic desirable command.

Second, the lab requirements tell you to configure the interface between itself (Gi0/2) and SW3 (Gi0/2) to use manual trunking and to disable DTP. To do so, configure the Gi0/2 interface on each switch to manually trunk using the switchport mode trunk command. Then, to disable DTP you use the switchport nonegotiate command.

Finally, the lab also asks you to configure SW4’s interfaces to use manual trunking. To do so, you should configure these ports on SW4 like SW2 and SW3 in this lab, except that you do not disable DTP.

Trunking Puzzle 1
EIGRP for IPv6 1
By certskills April 8, 2016 09:05
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  1. Chris October 16, 15:41

    Hi Wendell,

    I got tripped up by a couple things on this lab.

    The first was disabling DTP on the g0/2 interfaces between SW2 and SW3. In the VLAN Trunking Configuration section on p. 258, I did not see a mention of the switchport nonegotiate command and so thought that a manual configuration to dot1q along with a manual configuration to trunk would turn off DTP. I don’t think the switch type in the lab was specified, but since you usually use 2960s in your examples maybe I should’ve assumed these were 2960s and there for the switchport trunk ecapsulation command is not even supported, therefore the nonegotiation would be the only way to finish disabling DTP. My question is, if this was a switch that supported the trunk encapsulation command, would manually configuring dot1q or isl and with the mode to trunk also disable DTP?

    The second thing I think I made a mistake was configuring the trunk ports where you didn’t give a specific configuration (SW2: g0/1 & 3, SW3: g0/1 & g0/3). Since no configuration was specified for them but the diagram clearly shows them as trunks, I configured them as dynamic auto which I believe would allow them to trunk given the configurations of the configuration of the port they terminate at.

    And finally, I am really enjoying learning this material using your CCENT/CCNA ICND1 book. After I pass ICND1 I will be purchasing the companion volume to prepare for ICDN 2 to complete my CCNA



    Reply to this comment
    • CCENTSkills October 19, 07:57

      Hi Chris,
      Thanks for the post! And glad you’re finding these blog items useful.

      On your first thing, on the models that support both ISL and 802.1Q (older switches), the switch supports the switchport trunk encapsulation dot1q|isl command. However, in answer to your question, even on those switches, you’d need to also use switchport nonegotiate to disable DTP even with trunking statically configured.

      On your number 2, that’s really a set up to help you avoid the mistake of configuring switchport mode dynamic auto on both switches (which results in an access link, and not a trunk). I think from your phrasing that’s what you did, and that’d be wrong. I may be misunderstanding what you wrote, though.
      Hope this helps!

      Reply to this comment
  2. Mansoor May 22, 12:15

    Hi Wendall,

    Regarding second requirement – Configure the trunk between SW2 and SW3 using manual trunking and disable DTP, we know trunk can be accomplished with a command on either SW2 or SW3. So why do it on both switches ?. The question states between SW2 & SW3 so logically we do it on SW2. But here you have done on both switches ?. I’m confused here.

    What if on the exam we are to do this same task by configuring only on SW2 and not on SW3 will a candidate loose points ?. Nobody wants to lose points by overlooking this although the task can be accomplished using SW2 only.

    I bet this is one of the tricky questions that implies you should do it. 🙂

    Please guide me. Appreciate your work.

    Reply to this comment
    • CCENTSkills May 24, 08:01

      Hi Mansoor,
      Honestly, I probably did it on both sides because a responsible network engineer would do so on both sides to avoid ambiguity. I probably could not turn off the “configure it right” bit when writing the lab. 🙂

      That said, Cisco goes out of their way to write questions that, if you truly understand the topic, would leave you with one answer. This is a lab, with much more concept in the scenario than any single question on the real CCNA exam, so there’s more here to wonder about. Each CCNA exam question will be a much smaller world. I think if you understand the tech, learn the breadth, go in rested and relaxed, you won’t find “tricky” questions at all, because you’ll think and see the one way to answer. Just my $.02.

      Reply to this comment
      • Mansoor May 24, 12:47

        Quick question…

        Considering the exam. I understand the implication in the real world. From exam point of view should this be done the same way as did here ?. Just not to loose points. 🙂

        Reply to this comment
  3. Kevin February 22, 23:52

    Hi Wendell,

    I was wondering if it is ok to use interface range on those ports that we are configuring in the example, or should they be done individually per the answers?


    Reply to this comment
    • CCENTSkills February 28, 15:21

      Hey Kevin,
      Sure, go for it. When you do, then do a “show run”. You’ll see that it splits out the config to repeat the commands on individual interfaces – ie, it’ll look like the above examples. 🙂 I probably used “interface range” myself when testing, and then did a show run to grab the above.

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