Final answer:
The 12-bit extended system ID in a BPDU provides the VLAN ID to prevent Bridge ID collisions in a network, allowing for multiple spanning trees for different VLANs.
Step-by-step explanation:
The 12-bit extended system ID in a Bridge Protocol Data Unit (BPDU) contains additional information to prevent Bridge ID (BID) collisions when using the spanning tree protocol (STP) in a network. This additional information consists of the VLAN ID (VLAN Identifier) that the BPDU is associated with. In a traditional BPDU, the Bridge ID is composed of a 2-byte priority field and a 6-byte MAC address. However, with the introduction of the extended system ID, the 2-byte priority field now includes both a 4-bit priority value and the additional 12-bit extended system ID, which allows for the identification of 4096 different VLANs. This enhancement allows multiple spanning trees to coexist within the same network for different VLANs, improving network efficiency and loop prevention.