When the pre-standard PoE is used, why does the 7970G IP phone negotiate 15.4W with the 802.3af protocol?
A. A Cisco IEEE+CDP powered device, such as a Cisco IP phone 7970G, comes up in low power mode (6.3W) and transmits a Cisco Discovery Protocol (CDP) message with an inline power (ILP) type length value (TLV) that informs the Power Source Equipment (PSE) of the actual power required by the device. If the power is less than the default 15.4W, the PSE acknowledges the request with its available power and modifies the PSEs power budget. If the requesting powered device exceeds the power budget for the line card or switch, the port is either powered down, or the port remains in low power mode (7W). This management scheme is implemented in order to provide backward compatibility and investment protection to the installed base of Cisco Catalyst pre-standard Power over Ethernet capable line cards and switches. Cisco IP phones are power efficient and require 6.3W maximum power as reflected within the pre-standard Power over Ethernet implementation. However, the development of new high-power pow
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