Your guess is pretty close. Conceptually right. I saw that if I had multiple links between two switches,
dtp uses a different mac -address for each link. this mac address is only used by the dtp protocol for
dtp traffic pertaining to a specific link. they show up in the mac address table.
the numbers used by dtp are something like d,e etc., in the last field, it may have some logic to it.
If you loo at the mac address table and see closely resembling mac addresses that differs only in their last letter or number, they are most likely redundant links. for trunk links they show up in every vlan in the mac -address table. Rightly so. wonder if the switch then has to send dtp traffic duplicated for each vlan. this might drive up the bandwidth usage. a non-negotiate therefore helps a lot on trunk links. wonder if there are any cons to setting a trunk to non-negotiate.
my understanding is:
If looking at the mac address table you see closely resembling mac addresses that differs only in their last letter or number in every vlan on trunks, I believe they are not due to DTP traffic. As DTP negotiation should stop when the ports moves to the trunk status, shouldn't it?
So I believe these MAC addresses could be also used by STP BPDUs?
If I'm right you might see them on one trunk side only, not on the other trunk side?
The cons to setting a trunk to non-negotiate are following:
The port will become a trunk no matter of the other side port status.
So it the other side port is configured as an access port (even with negotiaition enabled) or not supporting trunk negotiation (non-Cisco, e.g.), it will remain an access port. So you will finish in a discrepancy: a trunk port on one switch connected to an access port on the other switch.
All Layer-2 and Layer-3 protocols have hello timers.
DTP hello messages are sent every 30 seconds.
In case you change the VTP domain name after the trunk is established, the DTP will send that ' text string ' in its packet across the trunk to the neighbor switch. the neighbor switch will notice the changed VTP domain name and take down the link.
Even in the Layer-2 trunk link down state the DTP hello packets go through and when you revert the VTP domain name to match on both sides, DTP will bring the trunk back up.
Whenever a change is configured on the trunk or in stp or in vtp, it triggers a dtp exchange even after the trunk is established.
the only time dtp is never sent is when the dtp non negotiate command is issued.
We are pleased to announce availability of Beta software for 16.6.3.
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