Which PoE Mode Guarantees Power After Device Detection?

200-301 By islever Editorial Last updated

Explain the role and function of network components Describe physical infrastructure connections of WLAN components (AP, WLC, access/trunk ports, and LAG)

Short answer

B Static mode enables powered-device detection and pre-allocates power, guaranteeing it will be provided when a device is detected.

Question

Which PoE mode enables powered-devices detection and guarantees power when the device detected?
  1. auto
  2. static Correct Answer
  3. dynamic
  4. active

Key Takeaways

The question tests whether you can distinguish Cisco's auto and static PoE port modes; the trap is seeing 'powered-device detection' and picking auto, while the keyword 'guarantees' points to static's pre-allocated power reservation.

Cisco Catalyst switches offer PoE port modes (auto and static) that both enable powered-device detection, but only one pre-allocates power so it is guaranteed on detection. This page establishes that static mode is the correct answer because it reserves wattage in advance, while auto mode grants power first-come, first-served.

Common mistake: Choosing A (auto) is the most common error: auto does enable detection, but it only allocates power if budget remains at that moment, so a detected device is not guaranteed power when other ports consumed the budget first.

Expert Analysis

Why the Answer Is Correct

Cisco's Catalyst configuration guides define static mode as enabling powered-device detection while pre-allocating (reserving) power for the port before discovery, and the switch "reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection." Because wattage is already committed, any powered device drawing less than or equal to the configured maximum wattage is guaranteed to boot when connected. The question's decisive phrase is "guarantees power," which maps exactly to this static pre-allocation behavior. Community contributors DL86, RougePotatoe, khaledsh00, and VanessaR05 all converged on B for precisely this reason, with RougePotatoe quoting the Cisco wording verbatim. Thus static (B) is the only mode that both detects powered devices and commits power ahead of time.

Why the Other Options Are Wrong

Option A (auto) is the tempting distractor and the mode most learners select: it does enable powered-device detection, but Cisco documents it as granting power on a first-come, first-served basis only "if enough power is available," so no guarantee exists once the power budget is exhausted. Options C (dynamic) and D (active) are not valid Cisco PoE port-mode names at all; dynamic PoE terminology belongs to allocation schemes like IEEE 802.3at dynamic power negotiation, not to the switch port mode setting, and "active" is a fabricated distractor here. As TrueLearner and cybercerberes argued for A, their quoted text itself contains the giveaway phrase "first-come, first-served," which contradicts a guarantee. Since the stem explicitly asks for detection plus guaranteed power, auto, dynamic, and active all fail the second half of the requirement.

Community Comment Notes

The comments split almost evenly between A and B, but the B side anchored itself in vendor documentation rather than intuition. DL86 captured the logic concisely: "B, keyword is guarantees power. Auto does not guarantees, static does." RougePotatoe reinforced this by noting that "static reserves power and guarantees its availability" and that "Auto is first come first serve," supported by the official Catalyst 9200 PoE guide. Nadamos added the nuance that auto "does not guarantee power in the same way that static mode does," which is the exact distinction the stem tests. On the A side, cybercerberes asserted "See in google search, the cisco site explain....Correct answer is A (Auto)," but the Cisco text quoted by TrueLearner actually describes auto as granting power only when the switch "has enough power," confirming rather than refuting the static verdict.

Official Reference

https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/switches/lan/catalyst9200/software/release/16-12/configuration_guide/int_hw/b_1612_int_and_hw_9200_cg/configuring_poe.html.xml

This link opens the official vendor documentation in a new tab.

Exam Strategy

On CCNA PoE questions, isolate the exact guarantee keyword: 'guarantee,' 'reserve,' or 'pre-allocate' always points to static mode, while 'first-come, first-served' or 'if enough power is available' points to auto. Remember that both modes perform powered-device detection, so detection alone never distinguishes them.

Frequently Asked Questions

Why is auto mode wrong if it also performs powered-device detection?

Auto grants power only if budget remains, on a first-come, first-served basis. Because nothing is reserved, a detected device can be denied power, so auto fails the 'guarantees' requirement.

What does static PoE mode reserve power for?

Static mode pre-allocates the configured wattage for the port even with no device attached, so any powered device drawing at or below that maximum is guaranteed to power up on connection.

More 200-301 FAQ →

Discussion 71 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 30 of 71

  1. BATSIE 👍 26
    auto - Enables powered-device detection; if enough power is available, automatically allocates power to the PoE port after device detection (default setting). max max-wattage - limits the power allowed on the port; if no value is specified, the maximum is allowed. max max-wattage - limits the power allowed on the port; range is 4000 to 30000 mW; if no value is specified, the maximum is allowed. never - disables device detection, and disable power to the port. Note: If a port has a Cisco powered device connected to it, do not use the power inline never command to configure the port. A false link-up can occur, placing the port into the error-disabled state. static - Enables powered-device detection; pre-allocate (reserve) power for a port before the switch discovers the powered device; the switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection. The switch allocates power to a port configured in static mode before it allocates power to a port configured in auto mode.
  2. splashy 👍 6 Answered B
    https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/switches/lan/catalyst9200/software/release/16-12/configuration_guide/int_hw/b_1612_int_and_hw_9200_cg/configuring_poe.html.xml#id_114542
  3. RougePotatoe 👍 5 Answered B
    Static reserves power and guarantees its availability when it detects a device that requires PoE is connected. Auto does NOT guarantee the availability of power. Auto is first come first serve static is pre-allocated. "Because power is pre-allocated, any powered device that uses less than or equal to the maximum wattage is guaranteed to be powered when it is connected to the static port...The device powers the port only if it discovers a powered device. Use the static setting on a high-priority interface." https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/switches/lan/catalyst9200/software/release/16-12/configuration_guide/int_hw/b_1612_int_and_hw_9200_cg/configuring_poe.html.xml#id_114542
  4. splashy 👍 5
    keyword is "guarantees" auto : Enables powered-device detection. If enough power is available, automatically allocates power to the PoE port after device detection. This is the default setting. static : Enables powered-device detection. Pre-allocate (reserve) power for a port before the device discovers the powered device. The device reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection. Static does not provide/allocate power, it pre-allocates/reserves it. The power is reserved (will not be spent on anything else) and only applied to port after device detection.
  5. DL86 👍 5 Answered B
    B, keyword is guarantees power. Auto does not guarantees, static does.
Show 25 more comments
  1. khaledsh00 👍 4 Answered B
    static static—Enables powered-device detection. Pre-allocate (reserve) power for a port before the switch discovers the powered device. The switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection.
  2. nadamos 👍 4 Answered B
    It's important to note that while the auto mode also enables powered-device detection, it does not guarantee power in the same way that static mode does
  3. dropspablo 👍 3 Answered A
    Answer A, because there are three configuration modes in PoE, Static, Auto and Dynamic. The three modes CAN guarantee the power. But only Auto and Dynamic modes DETECT the device (along with the required power). And the Dynamic mode, in addition to guaranteeing energy, detecting the device, it also NEGOTIATES energy, because if the switch has less watts to supply, it can partially feed the device. As it was only asked in the question to GUARANTEE the energy and DETECT the device, the Auto mode already answers (Answer A). If it asked to also "negotiate", it would have to be Dynamic mode. Static mode is wrong as it cannot detect the device, it just guarantees power. And Active mode does not exist in the configuration.
  4. AbdullahMohammad251 👍 3 Answered B
    The switch allocates power to a port configured in static mode before it allocates power to a port configured in auto mode. This means if we have limited power and 2 devices we connect to the switch at the same time with one port set to auto mode and the other set to static, The static port will guarantee power since it's served first but the auto port wouldn't if we ran out of power. So, I think it's static. https://www.thinlabs.com/faq/configure-cisco-switch-for-powering-poe-client#:~:text=static%20%2D%20Enables%20powered%2Ddevice%20detection,be%20provided%20upon%20device%20detection.
  5. SAAVYTECH 👍 3
    A. Auto In Power over Ethernet (PoE) terminology, "auto" mode, also known as "auto-detect" or "auto-sensing" mode, enables powered device detection and guarantees power when a compatible device is detected. When a device is connected to a PoE switch or injector configured in auto mode, it will check whether the connected device is PoE-compatible and supply power accordingly. If a PoE-compatible device is detected, the switch will provide power to it. If the connected device is not PoE-compatible, power will not be supplied, ensuring that non-PoE devices are not damaged.
  6. enehana_777 👍 3
    auto mode —The switch automatically detects if the connected device requires power. If the switch discovers a powered device connected to the port and if the switch has enough power, it grants power, updates the power budget, turns on power to the port on a first-come, first-served basis, and updates the LEDs. For LED information, see the hardware installation guide. If the switch has enough power for all the powered devices, they all come up. If enough power is available for all powered devices connected to the switch, power is turned on to all devices.
  7. VanessaR05 👍 3 Answered B
    https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html#:~:text=static%E2%80%94Enables%20powered%2Ddevice%20detection,be%20provided%20upon%20device%20detection. static—Enables powered-device detection. Pre-allocate (reserve) power for a port before the switch discovers the powered device. The switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection.
  8. lofidawg 👍 3 Answered B
    "static—Enables powered-device detection. Pre-allocate (reserve) power for a port before the switch discovers the powered device. The switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection." https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html
  9. vktsupport 👍 3 Answered B
    The question said: guarantees power when the device detected? It is answer B - Static Go to index - Power Management Modes. Then, at the end of the Static description, it said: Use the static setting on a high-priority interface. Source: https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html#con_1874624
  10. TR3Y 👍 3 Answered B
    From this cisco site itself. Please correct me if I am wrong. The definition has the same wording as the question. "Guarantees power will be available upon detection." https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html
  11. TrueLearner 👍 3 Answered A
    auto—The switch automatically detects if the connected device requires power. If the switch discovers a powered device connected to the port and if the switch has enough power, it grants power, updates the power budget, turns on power to the port on a first-come, first-served basis, and updates the LEDs
  12. Yinxs 👍 3 Answered B
    The key word is "guarantee". A. auto mode can't guarantee, if enough power is available, the connected device can get the power. B. static mode can pre-allocate the power, so it can guarantee the power to the device.
  13. mhayek 👍 3 Answered A
    A is the correct answer. Keyword is "when". Static provides power even when device is NOT detected.
  14. cybercerberes 👍 3 Answered A
    See in google search, the cisco site explain....Correct answer is A (Auto)
  15. VanessaR05 👍 3 Answered B
    It is static, key word is "guarantees"
  16. tonye1OO 👍 3
    sorry i am wrong, correct answer is A
  17. Da_Costa 👍 3 Answered B
    Static is the correct answer
  18. Doman01 👍 2
    According to this: https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html Answer is AUTO Configures the PoE mode on the port. The keywords have these meanings: auto—Enables powered-device detection. If enough power is available, automatically allocates power to the PoE port after device detection. This is the default setting. never —Disables device detection, and disable power to the port. Note If a port has a Cisco powered device connected to it, do not use the power inline never command to configure the port. A false link-up can occur, placing the port into the error-disabled state. static—Enables powered-device detection. Pre-allocate (reserve) power for a port before the switch discovers the powered device. The switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection.
  19. xbololi 👍 2 Answered B
    Configures the PoE mode on the port. Keywords and meanings: auto - Enables powered-device detection; ""if enough power is available"", automatically allocates power to the PoE port after device detection (default setting). static - Enables powered-device detection; pre-allocate (reserve) power for a port before the switch discovers the powered device; the switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection. "if enough power is available" means its not a guarenteed method as the question... So definetly not Auto... " the switch reserves power for this port even when no device is connected and guarantees that power" this is guarenteed method. https://www.thinlabs.com/faq/configure-cisco-switch-for-powering-poe-client#:~:text=static%20%2D%20Enables%20powered%2Ddevice%20detection,be%20provided%20upon%20device%20detection.
  20. RougePotatoe 👍 2
    Static reserves power and guarantees its availability when a device that requires PoE is connected. Auto does NOT guarantee the availability of power. Auto is first come first serve static is pre-allocated. "Because power is pre-allocated, any powered device that uses less than or equal to the maximum wattage is guaranteed to be powered when it is connected to the static port." https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/switches/lan/catalyst9200/software/release/16-12/configuration_guide/int_hw/b_1612_int_and_hw_9200_cg/configuring_poe.html.xml#id_114542
  21. Aymen91 👍 2 Answered A
    auto - Enables powered-device detection; if enough power is available, automatically allocates power to the PoE port after device detection (default setting). static - Enables powered-device detection; pre-allocate (reserve) power for a port before the switch discovers the powered device; the switch reserves power for this port even when no device is connected and guarantees that power will be provided upon device detection in the question, is saying when device is detected and not before even detecting device so Auto is correct
  22. rodrigocalcarao 👍 2 Answered B
    https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html#:~:text=static%E2%80%94Enables%20powered%2Ddevice%20detection.%20Pre%2Dallocate%20(reserve)%20power%20for%20a%20port%20before%20the%20switch%20discovers%20the%20powered%20device.%20The%20switch%20reserves%20power%20for%20this%20port%20even%20when%20no%20device%20is%20connected%20and%20guarantees%20that%20power%20will%20be%20provided%20upon%20device%20detection.
  23. Isuzu 👍 2 Answered A
    The PoE mode that enables powered-devices detection and guarantees power when the device detected is "auto" mode. In "auto" mode, the Power Sourcing Equipment (PSE) detects the powered device (PD) before providing power, and then delivers the appropriate power level to the PD. If the PD is not detected, or if it is not compatible with the PSE, no power is supplied. This ensures that only compatible devices receive power, and that the appropriate power level is delivered to them.
  24. RahmNn 👍 2 Answered A
    A. auto Explanation: Auto Mode: In Power over Ethernet (PoE), auto mode allows for the detection of powered devices (PDs) connected to the PoE switch or injector. When a device is detected, the PoE switch negotiates and guarantees the appropriate amount of power for that device. This mode ensures that power is supplied only to devices that require it and are capable of using it, helping to prevent power wastage and potential overloads.
  25. akareem999 👍 2 Answered B
    Answer is B static—Enables powered-device detection. Pre-allocate (reserve) power for a port before the switch discovers the powered device. https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_011010.html#:~:text=static%E2%80%94Enables%20powered%2Ddevice%20detection,switch%20discovers%20the%20powered%20device.

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