To understand why DTP (Dynamic Trunking Protocol) did not negotiate a trunk between the two Cisco 3560 switches, it's essential to know the operational modes of the switch ports regarding DTP.
For DTP to negotiate a trunk link, one of the ports must be set to "desirable" mode or "trunk" mode, and the other port must be set to either "desirable" or "auto" mode. If both ports are set to "access" or "on" mode, no trunk negotiation will occur.
Let's analyze the given options:
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The ports on each side of the link are set to desirable.
- If both ports are set to "desirable," they would negotiate a trunk. So this option is not correct.
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The ports on each side of the link are set to auto trunking.
- If both ports are set to "auto," they will not initiate trunking since they wait for the other side to request it. Thus, if both are "auto", no trunk will be formed. This is a valid reason.
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The ports on each side of the link are set to dynamic.
- "Dynamic" is not a valid mode in the context of DTP. The modes relevant are "access", "trunk", "desirable", and "auto".
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The ports on each side of the link are set to on.
- If both ports are set to "on", they are forced to trunking mode but wouldn’t negotiate anything, and they would create a trunk link. So this option is not correct.
Based on this analysis, the reason DTP did not work is likely:
The ports on each side of the link are set to auto trunking.
Both ports set to "auto" would prevent them from initiating a trunk negotiation, leading to them remaining as access ports.
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