trunk negociation

[boboshlap]

Hi,

Which configuration option will cause the link between 2 switches to become a functional trunk?

1. switchport access vlan 31
switchport mode dynamic desirable

2. switchport mode trunk
switchport nonegociate

3. Leave the port with default trunk settings

4.  switchport mode auto
switchport mode auto

[glen.grant]

1 and 2  .

[boboshlap]

Hi Glen,

Can you tell me why you have chosen number 1 and 2?

[Nathan Spitzer]

"Switchport mode dynamic desirable" Makes  the interface actively attempt to convert the link to a trunking link.  The interface becomes a trunk interface if the neighboring interface is  set to trunk, desirable, or auto mode.

"switchport mode nonegotiate" makes it a trunk regardless of the state of the other end. This can lead to loops or other bad things.

[Nathan Spitzer]

Note that option 2 is NOT recommended except in the case where you are connecting a Cisco switch to another vendors device that does not support DTP.

[boboshlap]

Both switches are CISCO.

I am curious why do you consider number 1 to be the right choise. Dynamic desirable actively tries to convert the link to trunk, but the opposite is configured with switchport access command.

[Nathan Spitzer · Accepted solution]

Keep in mind the "switchport access vlan"  command applies ONLY if the ports operational mode is access, if the ports operational mode is trunk it has no effect. In the case of option 1 the other end will determine whether the ports operational mode mode is trunk.

[boboshlap]

I got it, I think here's the trick. So option 1 is for sure.

Thanks a lot Nathan.