CCNA Network Infrastructure and Connectivity Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  PoE (IEEE 802.3af) can deliver up to 15.4 watts of power per port.

  Why this is correct

  Under the IEEE 802.3af standard, a power-sourcing equipment (PSE) such as a switch can supply a maximum of 15.4 watts per port at the source. Because of cable resistance and heat losses over the 100-meter link, the powered device (PD) is guaranteed to receive at least 12.95 watts. This is the baseline PoE capability commonly seen on older enterprise switches.

• ✓

  PoE+ (IEEE 802.3at) can deliver up to 30 watts of power per port.

  Why this is correct

  IEEE 802.3at, commonly called PoE+, raises the per-port power budget to 30 watts at the PSE, with 25.5 watts guaranteed at the powered device. It also introduced additional power classifications and a significantly higher power level for devices like pan-tilt-zoom cameras and thin clients. The standard remains backward-compatible with 802.3af devices, which are powered normally when connected to a PoE+ port.

• ✗

  Both PoE and PoE+ use the same four pairs of a twisted-pair cable for power delivery.

  Why it's wrong here

  PoE (IEEE 802.3af) and PoE+ (IEEE 802.3at) standards deliver power over two of the four twisted pairs in an Ethernet cable, not all four. These standards can use either the data pairs (Mode A) or the spare pairs (Mode B) for power. This option is tempting because the newer 4PPoE (IEEE 802.3bt) standard, also known as PoE++, does utilise all four pairs to provide significantly higher power, making the statement correct for that specific, later standard.

• ✓

  Powered devices (PDs) can negotiate power requirements using Link Layer Discovery Protocol (LLDP) with PoE extensions.

  Why this is correct

  Powered devices can use LLDP with the IEEE 802.3 Power via Media Dependent Interface extensions, often implemented through LLDP-MED, to communicate their actual power requirements to the switch. Rather than relying only on the coarse hardware classification defined by resistor signaling, LLDP enables dynamic negotiation and adjustment of power allocation during operation. This allows the PSE to allocate just enough power and to respond to changing demands from the PD, provided the switch and PD both support the LLDP power extension TLVs.

• ✗

  PoE+ requires Category 3 cabling or better.

  Why it's wrong here

  PoE+ does not require Category 3 cabling; in fact, the standard explicitly recommends Category 5 or better (e.g., Cat5e) twisted-pair cabling to maintain reliable power delivery and signal integrity. Category 3 cabling is only rated for 10BASE-T and lacks the pair-to-pair crosstalk and attenuation characteristics needed to support 1000BASE-T at 100 meters, let alone the higher current levels of PoE+. Therefore, claiming Cat3 is acceptable for PoE+ is incorrect; Cat5/5e is the minimum practical baseline.

• ✗

  PoE automatically disables power if a non-PoE device is detected.

  Why it's wrong here

  A PSE does not 'automatically disable' power after detecting a non-PoE device; instead, the 802.3af/at process begins with a detection phase that looks for a 25 k-ohm signature resistor as evidence of a valid powered device. If this signature is not found, the port simply never applies power, because applying power to a legacy device could damage it. The statement implies an active disabling action after connection, whereas in reality the absence of the valid signature prevents power from ever being applied in the first place.

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