I did some looking around the forums and found what I think might explain the behavior we ran across today, but I wanted to ask the experts to see if we are on the right track. Here is what we saw:
We had a 3550 switch and a 3745 router. Both of the units were set to VTP mode "server". Their domain setting was identical and everything was working fine. A co-worker accidentally changed the mode of the 3550 to "transparent" (thought he was connected to a different switch) and on the 3745 we lost VLAN 800 as soon as the change was made. On the 3550 we lost VLAN 900. Both VLANs were present on both units prior to the accidental change. Ironically, when I ran the "show vtp status" command on the 3745 (after the change of the 3550 to transparent mode) it clearly indicated that the last update/change came from the 3550 at the exact time it was changed from "server" mode to "transparent" mode. So, the change actually resulted in an update of some sort.
My question is the following: If both units were in "server" mode, by changing one of them to transparent (in our case the 3550) why/how could the VLANs just disappear from the switch/router? The only post I could find in the forums indicated that if the VLAN that was lost (let's say VLAN 900 on the 3550) that it would have been pushed/learned when added to the 3745. So VLAN 900 was added on the 3745, pushed/learned by the 3550 via VTP and then lost when the mode was changed to "transparent". Is this the case/behavior that we would expect?
We are trying to understand the behavior as we have run across a number of other switches that are set to "server" mode and we want to make sure we don't run into these issues again. We were able to manually add the missing VLANs to both units and were back up and running before long, but it did cause quite a scare. Okay, thank you so much for your help!!!