I'm not sure you posted this in the right place, you might have gotten a response (or more responses) sooner if it was posted in, say, CCNA R&S or something else that was related to routing and switching. Anyhow, I will try to answer your question as best I can.
The extended system ID is utilized by spanning-tree to include the vlan ID in the STP bridge ID, it also uses the same MAC address in each bridge ID. So, normally the bridge-priority would be a value from 0 to 65535, with a default of 32768. With the extended ID, the value will be the vlan ID plus the bridge priority value, with a range of 0 to 61440 for the bridge priority value, in increments of 4096. Note, the bridge priority value will never be higher than 61440 as the last increment of 4096 in the use of the extended system ID, because with a bridge priority value of 61440 plus the vlan value of 1024 it still remains under 65535, which is the highpoint of the priority value. If you were able to use the last value 65535, any vlan ID added to that value would exceed the limits set for the STP bridge priority value (the 16-bits allocated for that portion of the 64-bit bridge ID).
Bitwise, a normal STP bridge ID contains a 16-bit bridge priority (0 to 65535) , followed by a 48-bit unique MAC address for the VLAN. With the STP extended ID you have a 4-bit priority multiplier (0 to 61440) followed by a 12-bit vlan ID, followed by a non-unique (common) MAC address. The use of a non-unique MAC address is also called MAC address reduction. In the non-extended system ID for STP, the switch would assign a unique MAC address (the switch has a certain number reserved) to each instance of STP (one per vlan). With the extended system ID, this is not the case, as the same MAC is used for all instances of STP.
Also, on switches that do not support 1024 unique MAC addresses for its own use, like the 2950, the extended system ID is enabled by default.
Del provide you a very nice explanation. Good job Del. Check out this link
to a calculator I created in MSExcel that shows you how the bridge ID and VLAN ID make up the bridge priority value. Please let me know if you have any questions. Sheet 1 is the "extended system ID" and Sheet 2 is the "traditional system ID".
The STP and RSTP standard specifications mandate that each switch running STP/RSTP must have a unique Bridge ID (BID). Because Cisco runs STP or RSTP in each VLAN separately (called PVST and RPVST or PVRST), in each VLAN, the switch behaves like a standalone (albeit virtual) switch and thus, each STP/RSTP instance is required to have a unique BID to comply with the standard. Simply, having X VLANs means having X separate STP/RSTP instances and X unique BIDs.
The question now is how to make sure the BIDs of STP/RSTP instances run on the same switch in different VLANs are truly unique. Older switches actually had a large reserve of MAC addresses. As new VLANs were created, these switches allocated a new MAC address for each new STP/RSTP instance in a new VLAN (recall that the BID originally consisted of the priority and the MAC address), making the BIDs unique.
However, the consumption of MAC addresses this way was simply too large and ineffective. At the same time, having 65536 different values for priority in the BID was largely useless. So IEEE came with the idea of Extended System ID in which they reused a part of the priority field for a unique instance identifier. In Cisco's implementation, this field is populated with the VLAN number the STP/RSTP instance runs in. This easily and effectively makes the BID unique - even with the same priority for all VLANs on a single switch, and a single switch MAC address, multiple STP/RSTP instances running on this same switch with the same priority have different BIDs thanks to different VLAN numbers embedded into the BID.
Some switch platforms actually allowed you to deactivate the Extended System ID and revert to the older style of assigning unique MAC addresses to individual STP/RSTP instance BIDs. That is why the command spanning-tree extend system-id exists in the first place. However, removing this command is only possible on those switching platforms which are equipped with 1024 MAC addresses for their disposal. Most new switching platforms have only 64 MAC addresses for their internal use, and while the spanning-tree extend system-id command is present in their configuration, you can not remove it. It is simply there to inform you that the Extended System ID is being used but you can not really deactivate it.
Read more here:
Peter (Not me copied from other discussion group)