switchport modes

TWX

Figured I'd clarify through experimentation. It has been debated what happens for switchport mode dynamic auto and switchport mode dynamic desirable when connected to another switch that is set switchport mode trunk, so I did it.

All equipment is 2960-S, mix of IOS 12 and 15. These happen to be 24-port non-PoE with Lan Base on them.

Both dynamic modes (auto and desirable) came up as trunk when the switch they were connected-to was set switchport mode trunk and DTP negotiation was not changed from default. Ie, switchport nonegotiate was not set.

Port      Name               Status       Vlan       Duplex  Speed Type
Gi1/0/24                     connected    trunk      a-full a-1000 10/100/1000BaseTX

When I set switchport nonegotiate on the set-as-trunk switch, I got the following error with both dynamic modes:

*Mar  1 00:12:18.381: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/24, changed state to up
*Mar  1 00:12:20.383: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/24, changed state to up
*Mar  1 00:12:20.750: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet1/0/24 VLAN1.
*Mar  1 00:12:20.750: %SPANTREE-7-BLOCK_PORT_TYPE: Blocking GigabitEthernet1/0/24 on VLAN0001. Inconsistent port type.

The interface shows as access in the status:

Port      Name               Status       Vlan       Duplex  Speed Type
Gi1/0/24                     connected    1          a-full a-1000 10/100/1000BaseTX

So, to clarify, if trunk is set on one end and any dynamic is set on the other end, as long as DTP is enabled, the trunk will form.

Chev Chellios

Nice one TWX thanks for the heads up, I was wondering that when reading about dynamic modes but sadly lack the equipment to experiment with it myself currently!

Simrid

Nice clear overview. Good job.