Good thread in here
check Peter Paluch reply, hope its useful
note, its not only about ISL or dot1q
it can also negotiate how to port works as access or trunk
you can test this by setting one switch port to trunk and leave the second switch as default settings (no configs)
you will notice the port is working correctly
Hi, but i refer specifically to the encapsulation method - whats it negotiating with? As i said, 802.1q is the only option on most switches so desnt even have the negotiation option. If any other switches support ISL and 802.1q then ISL is preferred.
Deciding whether to trunk as i understand is the result of the mode - trunk, access, dyn desirable, dyn auto. Having 'negotiate' as on option for trunk encapsulation is a different thing right?
There are links about DTP that you could see:
This is a bit complicated. Lets start this topic with type of switch: L2 or L3. L2 -access layer- switches will have default settings set to be access ports. L3, distribution switches will have trunks ports. There are some exceptions especially for older devices.
Most of switches will have both ISL and 802.1q protocols operating. I think ISL is still preferred over 8021q. Long time ago, old L2 switch did not have ISL, only 8021q
Now, check switch IOS for default operation mode: Access or Dynamic desirable modes. those settings not always follow type of sw mentioned above but it should in mine opinion.
L2 or not, access or not, DTP is on and will try to negotiate link. this is evident from my labbing using virtual IOS.
Answers to your questions are: DTP will negotiate either access or trunk and if it it trunk, it will decide on dot1q or ISL, ISL is preferred.
If only 802.1q is supported, DTP negotiates on 802.1q trunk or access depending on other switch.
True, at least on c355/60, c3750, ISL is preferred.
True, Deciding whether to trunk is the result of the default port mode - trunk, access, dyn desirable, dyn auto.
"negotiate" option is sort of there by default as long as DTP runs on port. This brings us to turning DTP off.
switchport nonegotiate is a command you can use to stop DTP frames and preventing negotiation of a link. Using this command means manual configuration on both switches.