Hello,
No magic here :) Let me explain it.
The VTP Pruning is a mechanism added to the VTP protocol to dynamically allow or deny transporting individual VLANs on a trunk. Imagine that two switches are connected with a trunk, on both switches (call them S1 and S2) the VLANs 10 and 20 are created, however, on S2, there is currently no access port in the VLAN 20. It doesn't make sense to send VLAN 20 frames to S2 because they would be dropped. The VTP Pruning allows the S2 to tell S1 that the VLAN 20 is currently unused on it and that it is not necessary to send frames in VLAN 20 to it. The S1 switch will therefore prune the VLAN 20 from the trunk - it is as if you wrote "switchport trunk allowed vlan except 20" on the S1's trunk interface. Of course, when the VLAN 20 becomes used on S2, it will again tell the S1 to unblock its trunk for the VLAN 20.
The VTP Pruning is internally provided by switches exchanging periodic VTP Join messages that contain a list of all VLANs and their used/unused status (as a bit field). If no VTP Join messages are received then no information about the usage of VLANs can be provided. In this case, the switch assumes that all VLANs are unused and therefore, all VLANs will pruned on a trunk.
When you separated your network into two VTP domains, you gave each VTP domain a unique name (perhaps even a different password). A switch in one VTP domain drops VTP packets coming from other VTP domain. This is the cause of what happened: the VTP Join messages were sent by both switches but they were dropped upon receiving. As a result, no usable information about VLAN usage on the trunk was available to either switch, so the VTP Pruning pruned all VLANs from that trunk.
By entering the command I suggested in my previous post, you have told the switch to not to prune any VLAN from the trunk using the VTP Pruning, no matter what. We have simply opened the trunk for all VLANs despite the VTP Pruning would tell otherwise.
I am glad you got it running.
Best regards,
Peter
