Spanning-tree operation requires that each switch have a unique BID (Bridge ID). In the original 802.1D standard, the BID was composed of the bridge priority and the MAC address of the switch, and all VLANs were represented by a CST, Common Spanning Tree. Because Cisco started to use unique instance in PVST+ PVRST+ for each VLAN STP Process, there came need to provide Unique BID for each separate instance of STP per VLAN. So what Cisco Did! divided the Bridge priority field of 16 bits into two parts, 4 bits for priority and 12 bits for VLAN ID and named it as Extended VLAN ID. Now because only left most four bits are reserved for Bridge priority, you can only make the combinations of discrete values in increments of 4096 with those bits.
I was getting this error while removing Operating System ISO image mounted on the Virtual Machine. What worked for me, is 1. Uncheck the "Connected and Connect at power on" from Device Status. 2. Then Change the Device type from " Datastore ISO File to Client Device " Radio Button 3. and press OK to save the changes. Note:- I was able to remove the mounted ISO only by directly logging to the ESXi at https://esxi-ip-address/ui where it asks "The guest operating system has locked the CD-ROM door and is probably using the CD-ROM, which can prevent the guest from recognizing media changes. If possible, eject the CD-ROM from inside the guest before disconnecting. Disconnect anyway and override the lock?" You need to select yes to eject the CD-ROM and then remove the ISO file successfully.