VLAN Trunking question

veritas_libertasveritas_libertas CISSP, GIAC x5, CompTIA x5Greenville, SC USAPosts: 5,735Member ■■■■■■■■■■
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.
Currently working on: Linux and Python

Comments

  • drkatdrkat Posts: 703Banned
    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
    Married to the game but she broke her vows. That's why my bars are full of broken bottles And my night stands are full of open bibles
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    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.
  • YFZbluYFZblu Posts: 1,462Member ■■■■■■■■□□
    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_libertasveritas_libertas CISSP, GIAC x5, CompTIA x5 Greenville, SC USAPosts: 5,735Member ■■■■■■■■■■
    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.
    Currently working on: Linux and Python
  • mapletunemapletune Posts: 316Member
    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.
    Studying: vmware, CompTIA Linux+, Storage+ or EMCISA
    Future: CCNP, CCIE
  • kmcintosh78kmcintosh78 Posts: 195Member
    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.
    What I am working on
    CCNP Route (Currently) 80% done
    CCNP Switch (Next Year)
    CCNP TShoot (Next Year)
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