VLAN Trunking question

veritas_libertas

If you have two switches running 802.1Q and you want them to communicate over a trunk, do both of the interfaces between switches need to be in trunk mode? The Boson practice test I took said only one is needed, but I can't find another source to verify.

drkat

Depends if you wanna use DTP or not

If one side of a switch is either in trunk or dynamic desirable and the other side is left blank, DTP will negotiate a trunk on the link, or should I say try.

you can do

show int fa0/x switchport to confirm the presence of DTP

SW1#show interface fast 0/2 switchport
Name: Fa0/2
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: down
Administrative Trunking Encapsulation:
dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1
(default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN:
none
Administrative private-vlan host-association: none
Administrative
private-vlan mapping: none
Administrative private-vlan trunk native VLAN:
none
Administrative private-vlan trunk encapsulation: dot1q
Administrative
private-vlan trunk normal VLANs: none
Administrative private-vlan trunk
private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled:
ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs
Allowed: ALL
Protected: false
Appliance trust: none

NetworkVeteran

veritas_libertas wrote: »

If you have two switches running 802.1Q and you want them to communicate over a trunk, do both of the interfaces between switches need to be in trunk mode? The Boson practice test I took said only one is needed, but I can't find another source to verify.

If one side is set to "switchport mode trunk", the other side can be any of these modes and there will be a trunk link will successfully be setup: trunk, dynamic auto, dynamic desireable. So yes, setting one Catalyst to trunking is enough to ensure both are set to trunking given the other side's DTP mode and negotiation settings are left at their defaults.

Boson tests usually link to the source of their answers.

YFZblu

Sounds like Boson just poorly worded the process of Desirable trunking. Sure, you might only have to configure one switchport as a trunk port if the other side is in a status that allows for trunk negotiation. But make no mistake, both ports must be trunking for this to work.

veritas_libertas

NetworkVeteran wrote: »

If one side is set to "switchport mode trunk", the other side can be any of these modes and there will be a trunk link will successfully be setup: trunk, dynamic auto, dynamic desireable. So yes, setting one Catalyst to trunking is enough to ensure both are set to trunking given the other side's DTP mode and negotiation settings are left at their defaults.

DrKat wrote:

Depends if you wanna use DTP or not

Thanks for explaining this!

Boson tests usually link to the source of their answers.

Usually they do, but this question didn't. Thanks for taking the time to clarify this for me.

mapletune

If one switch is set to switchport nonegotiate or switchport mode access then there is nothing another switch can do to trunk with this one.

kmcintosh78

Industry practice, from my world, is to hard code ALL ports that connect to other network hardware and network devices.
AUTO, IMO, is good for end devices.