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.
If you planning to upgrade your graylog and want to see what your graylog current version is, here is how you can check it. Go to the Graylog Web Interface Click on System/Nodes Under System/Nodes --- Click on the N odes Scroll down to the ' System ' Here you can see the current version of your graylog server