Neighbor switch's port doesn't change to access mode

Question

When Switch A's port is configured to access mode from trunk mode, Switch B's port stays in trunk mode.

Switch A is DTP disabled (#switchport nonegotiate). Switch B has default configuration which is dynamic auto and DTP enabled.

Is this normal?

Answer 1

Depending on when the commands were typed in, this sounds like the expected behaviour. If Switch A were plugged into Switch B, and Switch A moved to the trunk state, then DTP on Switch B would have negotiated trunk mode.

This is how DTP normally negotiates the interface mode:

Table 3-8 DTP Negotiated Interface Modes

                Dynamic       Dynamic     Trunk        Access
                Auto          Desirable

Dynamic Auto    Access        Trunk       Trunk        Access <-- Should be here


Dynamic         Trunk         Trunk       Trunk        Access
Desirable

Trunk           Trunk         Trunk       Trunk        Limited 
                                                   connectivity

Access          Access        Access      Limited      Access
                                      connectivity

^{Cisco Press, Cisco Networking Academy's Introduction to VLANs, Table 3-8}

However, if you disabled DTP on Switch A, then Switch B would have no further knowledge of any changes in interface changes on Switch A. So moving Switch A to access will not be conveyed to Switch B, as it would have normally expected.

If you reset the connection between them, Switch B would have never changed it's mode to trunk and remain in access mode. Then you'd be having the total opposite problem (Switch B not turning into a trunk).

Answer 2

DTP by default has 300 seconds of Dynamic Trunk timeout. So, Switch B will change its port operational mode to "static access" if it will have not received any DTP advertisement to trunk in the next 300 seconds [after it changes the port operational mode to "static access" or since the receive of the DTP advertisement to trunk].

I tested and the Switch B's port operational mode was changed from "trunk" to "static access" after 300 seconds.

Note: The statement in [ ] is not confirmed. It is supposed to be either one.