DTP / Trunk Question

Hey guys, bit of a silly question here - I think I've gotten myself confused.

I haven't labbed this up (as I only have one switch right now :P)

In terms of trunking and interface states, I know that:

- If a port is set to switchport mode trunk it will be "unconditionally trunking."
- If a port is set to switchport mode access it will be "unconditionally not-trunking."
- If a port is set to switchport mode dynamic desirable it will attempt to negotiate a trunk.
- If a port is set to switchport mode dynamic auto it will only form a trunk if it receives DTP frames requesting a trunk (ie if the other end is set to trunk or desirable.

My question is: if you have two switches connected, one side set to trunk and one side set to access, will the switch set to trunk actually tag frames (assuming you're using 802.1Q and they aren't destined for the native VLAN), or will it cease to tag them due to the fact that a trunk isn't established (because the other end is set to access.)

Would I also be correct in saying that if one end is set to access and the other set to dynamic desirable, a trunk will not form and frames would not be tagged regardless?

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Comments

chmorin

chX wrote: »

Hey guys, bit of a silly question here - I think I've gotten myself confused.

I haven't labbed this up (as I only have one switch right now :P)

In terms of trunking and interface states, I know that:

If a port is set to switchport mode trunk it will be "unconditionally trunking."

If a port is set to switchport mode access it will be "unconditionally not-trunking."

If a port is set to switchport mode dynamic desirable it will attempt to negotiate a trunk.

If a port is set to switchport mode dynamic auto it will only form a trunk if it receives DTP frames requesting a trunk (ie if the other end is set to trunk or desirable.

My question is: if you have two switches connected, one side set to trunk and one side set to access, will the switch set to trunk actually tag frames (assuming you're using 802.1Q and they aren't destined for the native VLAN), or will it cease to tag them due to the fact that a trunk isn't established (because the other end is set to access.)

Would I also be correct in saying that if one end is set to access and the other set to dynamic desirable, a trunk will not form and frames would not be tagged regardless?

Well a full trunk connection will not form. Even if they were both working, the trunk side would attempt to forward tagged frames and the access port would strip them.

I also have not labbed this, but I'm pretty sure because of the mismatch an 'up and up' status would not be made. Line protocol would probably be down. I'll lab it in a moment and update the post.

chmorin

Well it looks like the interface does come up/up on both ends, however traffic will only cross if the native vlan on the trunk side and the access vlan on the access side match, which makes sense. They send STP and DTP packets between each other, but a trunk will not form.

chX

Thanks for doing that.

It still leads me to wonder though - does the "trunk" side actually tag and forward frames not destined for the native VLAN (and the "access" end would probably discard the frame) or does it not even bother purely because a trunk hasn't been negotiated?

networker050184

chX wrote: »

Thanks for doing that.

It still leads me to wonder though - does the "trunk" side actually tag and forward frames not destined for the native VLAN (and the "access" end would probably discard the frame) or does it not even bother purely because a trunk hasn't been negotiated?

If you hard code the trunk then yes it will. There is no negotiation of a trunk at that point.

chX

networker050184 wrote: »

If you hard code the trunk then yes it will. There is no negotiation of a trunk at that point.

Thanks for the response. I need to get a few more switches so I can answer things for myself. :P

So if you were to hard code one end as a trunk port and the other as an access (and considering it will still tag frames, as you mentioned), what will the switch configured as an access port do with the tagged frames?

networker050184

If the a tagged frame arrives on an access port the switch will drop the frame unless the VLAN id matches the access VLAN on that port. If the VLAN id matches the access VLAN the switch will strip the 802.1q header and forward the frame. This is where the premise of VLAN hopping comes in to play.