Which settings for trunking allow trunking to work?

[de0]

I am currently working through the CCNA official cert guide and I got this question wrong:

Imagine that you are told that switch 1 is configured with the dynamic auto parameter for trunking on its Fa0/5 interface, which is connected to switch 2. You have to configure switch 2. Which of the following settings for trunking could allow trunking to work? (Choose two answers.)
a. on
b. dynamic auto
c. dynamic desirable
d. access
e. None of the other answers are correct.

I answered C and E.

However, A and C were the correct answers.

In which way does the command "on" configure trunking and how is this command accessed and executed? There was no mention of it in the chapter of the cert guide.

[M02@rt37 · Accepted solution]

Hello @de0,

The "on" command is used to manually enable trunking on a switch interface. When a switch port is configured with the "on" mode, it forces the port to become a trunk port regardless of the negotiation with the neighboring device.

In the given scenario, switch 1 is configured with the "dynamic auto" mode on its Fa0/5 interface. This means that switch 1 will passively wait for the neighboring switch (switch 2) to initiate trunking negotiation. Switch 2, in order to establish trunking with switch 1, needs to be configured in a mode that can actively initiate the negotiation.

The correct answers to the question are A and C:

A. "on" mode: This mode manually enables trunking on the interface, allowing it to actively form a trunk without waiting for negotiation. It would be an appropriate configuration on switch 2 to establish trunking with switch 1.

C. "dynamic desirable" mode: This mode actively attempts to form a trunk with the neighboring switch if the neighboring switch is in a compatible mode (such as "dynamic auto" or "on"). It would also be a suitable configuration on switch 2 to establish trunking with switch 1.

For "mode on", you would use the following command on switch 2's interface:

interface <interface-id>
switchport mode trunk

[Joseph W. Doherty]

Just to add to M02@rt37 information . . .

For A (and C?) to be correct, DTP needs to be active on both switches' trunk ports.

Some Cisco network devices used to (still?) support manual only trunking mode (small [older] routers come to mind and perhaps very, very [very] old Cisco switches).

For devices that cannot run DTP, including Brand X switches, you would need the trunk manual ("on") mode.

BTW, in many years of networking, don't recall ever seeing Cisco's dynamic trunking being intentionally used.  I.e. both sides were always configured "switchport mode trunk".  This done, I believe, firstly, to make clear the trunk is INTENTIONAL, and secondly, I believe the non-DTP (manual) trunk connection comes up into operation just a tiny bit faster.

[de0]

Thank you for clarifying. In which situations would the "dynamic auto" and "dynamic desirable" modes be used intentionally?

[Joseph W. Doherty]

None, readily, come to mind.

Keep in mind when studying for a vendor specific cert, like a CCNA, vendor will expect you to understand their proprietary features.

For example, also related to Cisco trunks, Cisco tested for knowledge using ISL, even after they began to also support .Q (which many started to use rather than ISL).

[de0]

Thank you for clarifying. In which situations would the "dynamic auto" and "dynamic desirable" modes be used intentionally?