The new 2-event classification mechanism and power up procedure ensures that PoE+ devices will be backwards compatible with legacy PoE devices. This means that PoE+ PSEs can source power to both Type 1 and Type 2 PDs. The IEEE 802.3at standard also states that a PD must operate under 12.95W until it recognizes a Type 2 PSE. Initializing a PD in the 12.95W mode eliminates interoperability issues between Type 1 and Type 2 devices. The PSE will limit the power to 12.95W until it confirms via the 2-event classification that it is attached to a PoE+ compliant PD. If the PSE only performs a single event classification, then the PD may negotiate higher power via the LLDP protocol. A PoE+ compliant PD must be able to respond to both Layer 1 and Layer 2 classification mechanisms while the Layer 2 Ethernet protocol and 2-event classification is optional for the PSE. Once the PD recognizes a Type 2 PSE, it is required to wait an additional 80ms in 12.95W operation before switching to 25.5W.