Final answer:
The 12-bit extended system ID in a BPDU contains VLAN ID information which is used in conjunction with the 2-byte Bridge Priority to allow for multiple instances of STP on the same physical network, thus helping to prevent loops at the VLAN level in network engineering.
Step-by-step explanation:
The 12-bit extended system ID in a Bridge Protocol Data Unit (BPDU) contains additional information used for the Spanning Tree Protocol (STP) in network engineering. Specifically, this extended system ID includes information about the VLAN (Virtual Local Area Network) to which the BPDU pertains, enhancing the basic 2-byte Bridge Priority with VLAN ID information. This extension was part of the enhancements introduced in IEEE 802.1t and IEEE 802.1w (Rapid Spanning Tree Protocol) to support multiple instances of STP on the same physical network.
Traditionally, the Bridge ID (BID) consisted of a 2-byte Bridge Priority and a 6-byte MAC address. When VLANs were incorporated into the protocol, it was necessary to include the VLAN ID in the BPDU to distinguish between different STP instances. Therefore, the 4 bits of Bridge Priority were reduced to accommodate the additional 12 bits for VLAN ID, making it possible for STP to operate within individual VLANs, helping to prevent loops at the VLAN level.