What Prevents a Workstation from Receiving a DHCP Address?

200-301 By islever Editorial Last updated

Interpret basic operations of Rapid PVST+ Spanning Tree Protocol Configure and verify DHCP client and relay

Short answer

A STP prevents a workstation from receiving a DHCP address if PortFast is not enabled, causing the DHCP request to time out during the listening and learning states.

Question

What prevents a workstation from receiving a DHCP address?
  1. STP Correct Answer
  2. VTP
  3. 802.1Q
  4. DTP

Key Takeaways

This question tests the interaction between STP port states and DHCP timeout; the common trap is assuming trunking protocols like 802.1Q or DTP are the direct cause instead of the STP forwarding delay.

Spanning Tree Protocol (STP) can prevent a workstation from receiving a DHCP address if PortFast is not enabled on the access port. This page establishes that the STP listening and learning delay causes the DHCP request to time out before the port reaches the forwarding state.

Common mistake: Choosing 802.1Q or DTP, thinking trunk negotiation or tagging blocks DHCP, when the actual blocking mechanism is the 50-second STP delay dropping the DHCP Discover packet.

Expert Analysis

Why the Answer Is Correct

Spanning Tree Protocol (STP) calculates a loop-free topology by placing ports into blocking, listening, learning, and forwarding states. Without the PortFast feature enabled on an access port, a workstation's connection will delay in the listening and learning states for approximately 30 seconds combined. During this time, the switch drops incoming frames, including the workstation's DHCP Discover broadcast. Because DHCP clients typically time out before STP transitions the port to the forwarding state, the workstation fails to receive an IP address.

Why the Other Options Are Wrong

VTP (VLAN Trunking Protocol) synchronizes VLAN databases across switches and does not affect individual port forwarding states or block DHCP broadcasts. 802.1Q is simply the framing standard for VLAN tagging on trunks; while an incorrect trunk configuration can cause communication failures, it is not the protocol mechanism that inherently delays or blocks initial DHCP discovery like STP does. DTP (Dynamic Trunking Protocol) negotiates trunk links dynamically; although an unexpected trunk can place a port in a different VLAN, STP is the specific protocol known to time out DHCP requests due to port state delays.

Community Comment Notes

Several users correctly identified STP as the answer by referencing Cisco's DHCP troubleshooting documentation, which explicitly states to "verify that the port has STP portfast enabled" as noted by Ghugs. rijstraket pointed out that "The time it takes to get to the Forwarding state might be too long for a client's DHCP process". Others incorrectly argued for 802.1Q, claiming trunking prevents DHCP, but they miss the specific STP port-state delay mechanism that is the classic textbook cause for this issue.

Exam Strategy

When a question asks about a workstation failing to receive a DHCP address upon connecting, immediately think of STP PortFast. If PortFast is disabled, the STP convergence delay will drop the initial DHCP packets before the port can forward them.

Community Votes

A
60%
B
40%

60% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 32 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 32

  1. Ghugs 👍 9
    I think its STP, specifically portfast. I found this one the cisco white pages, under the DHCP troubleshooting section. "...verify that the port has STP portfast enabled and trunking/channeling disabled. The default configuration is STP portfast disabled and trunking/channeling auto, if applicable. For the 2900XL/3500XL/2950/3550 switches, STP portfast is the only required configuration. These configuration changes resolve the most common DHCP client issues that occur with an initial installation of a Catalyst switch." from https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/27470-100.html#anc72
  2. fransCISCO 👍 7
    HEY SPLASHY, YOURE MY IDOL,YOU HAVE MANY COMMENTS IN SOME QUESTIONS. I HAVE SOMETHING FOR YOU. WHATS YOUR EMAIL SO THAT I CAN CONTACT YOU
  3. rijstraket 👍 6 Answered A
    The time it takes to get to the Forwarding state might be too long for a client's DHCP process (which starts after the interface on the client becomes 'up'). Using Spanning-Tree PortFast can mitigate this exact issue. So yes, STP can prevent workstations from getting an IP-adress using DHCP.
  4. rogi2023 👍 5 Answered A
    in such questions the best practise is to exclude the wrong asnwers first. VTP - says how to spread out VLANs across the network between sw - this one is out. DTP - says how to create a working trunk. btw on trunk int you won't get an dhcp - ip, so this one out as well. 802.1q - says how to tag VLAN-id in trunk - so also this one is out. so just with exclusions the last option is STP, which after reading the explanatory comments make sense. So A is definitely correct.
  5. splashy 👍 5
    Still undecided on this one really... STP ok but you will eventually get a DHCP address. I can however find a lot of issues with vlans not getting a dhcp address (because of various reasons from wrong tagging, not having a dhcp server on the vlan, not having a helper address when the dhcp server is on a different vlan, adding vlans without adding a dhcp pool for the vlan, ...)
Show 25 more comments
  1. rogi2023 👍 4
    this is a very stupid question, I hope not to see such garbage on the exam especially for ccna level.
  2. akamong 👍 3
    The correct answer is C. 802.1Q. Explanation: A. STP (Spanning Tree Protocol): STP is a protocol that prevents loops in a network by blocking redundant paths. It does not prevent a workstation from receiving a DHCP address. B. VTP (VLAN Trunking Protocol): VTP is used for managing VLAN configurations in a network. It does not prevent a workstation from receiving a DHCP address. C. 802.1Q: This is a standard for frame tagging in VLANs. If a switch port is configured as an 802.1Q trunk port and the workstation is not configured to handle VLAN tagging, the workstation may not be able to receive a DHCP address because it is not on the correct VLAN. D. DTP (Dynamic Trunking Protocol): DTP is a Cisco proprietary protocol used to negotiate trunking between switches. It does not prevent a workstation from receiving a DHCP address. In summary, 802.1Q, which deals with VLAN tagging, is the most relevant option when considering issues related to a workstation not receiving a DHCP address in a VLAN environment.
  3. kenkari 👍 3 Answered C
    In this context, the correct option is C. 802.1Q. The reason is that 802.1Q is a VLAN tagging standard used to transport traffic for multiple VLANs over a trunk link. If a port is configured as a trunk (802.1Q), and the workstation is not configured to tag its traffic with the correct VLAN, the workstation might face difficulties in receiving an IP address via DHCP.
  4. splashy 👍 3 Answered C
    STP can't prevent you from getting an DHCP address, it prevents infinite loops of traffic by blocking ports, not by blocking traffic from going to every client on the subnet. If you cannot authenticate yourself on the network however, you will not be able to send a DHCP request and get an offer.
  5. 4ffc2ad 👍 2
    So guys, under no circumstances does STP prevent a host from receiving DHCP. Let's imagine the following scenario: We have 1 L3 switch, and in this switch we have a DHCP server configured in VLAN 10, if the host port is in the default VLAN, this equipment will not receive an address from that DHCP, on the other hand, if its interface is in VLAN 10, it will receive the address, therefore the correct answer is C.
  6. dropspablo 👍 2
    That is, by using PortFast it is possible to mitigate the problem caused by the STP transition time and ensure that workstations obtain IP addresses via DHCP without significant delays.
  7. BigBluElephant 👍 2
    Correct Answer: STP STP can prevent DHCP clients from getting an IP address because the port doesn't start forwarding traffic until STP goes into the forwarding state.
  8. RODCCN 👍 2 Answered C
    Ok, how about C? If the port is in access mode, on vlan X and DHCP on vlan Y, and there's no dhcp relay configured, it'll prevent the computer to get IP.
  9. jeebz 👍 2
    the same question was answered "STP" in question 108 a public discussion thread on this question
  10. Da_Costa 👍 2 Answered A
    STP will prevent a workstation from receiving a dhcp message
  11. Murphy2022 👍 2
    Dot1q ist VLAN tagging not authentication
  12. mohdhafizudddinesa 👍 2
    You will not have IP from trunk port
  13. xplinty666 👍 1 Answered A
    A. STP (Spanning Tree Protocol) Spanning Tree Protocol (STP) can potentially prevent a workstation from receiving a DHCP address if it blocks or disables the port through which DHCP requests are transmitted. STP operates at Layer 2 of the OSI model and is responsible for preventing loops in network topologies by blocking redundant links. If a port is in a blocking state due to STP convergence or a topology change, DHCP requests sent from workstations connected to that port may not reach the DHCP server, resulting in the inability to obtain an IP address dynamically.
  14. eb63e5a 👍 1
    The correct answer is D. DTP (Dynamic Trunking Protocol). DTP (Dynamic Trunking Protocol) is a Cisco proprietary protocol used to negotiate trunking between switches. It operates on trunk ports and helps switches dynamically determine whether to form a trunk link with another switch. The other options (STP, VTP, and 802.1Q) are not directly related to DHCP address assignment:
  15. Starlord2535 👍 1
    The DHCP (Dynamic Host Configuration Protocol) address assignment process is not directly affected by options A (STP - Spanning Tree Protocol), B (VTP - VLAN Trunking Protocol), C (802.1Q - VLAN tagging), or D (DTP - Dynamic Trunking Protocol). DHCP issues are typically related to network connectivity, DHCP server availability, or configuration errors.
  16. Starlord2535 👍 1
    The options provided (STP, VTP, 802.1Q, DTP) are not directly related to preventing a workstation from receiving a DHCP address. Therefore, none of the given options are the correct answer in the context of DHCP issues. It's important to troubleshoot DHCP problems by checking network connectivity, DHCP server availability, and proper configuration.
  17. cedc60e 👍 1
    STP can prevent DHCP clients from getting an address because the port doesn't start forwarding traffic until STP goes into the forwarding state. That can be up to 30 seconds. By then the client has timed out. Cisco has a command called "portfast" to fix this.
  18. chelvamc 👍 1 Answered A
    A. STP Prevents Layer-2 loops; forwarding delay can affect DHCP ✅ B. VTP Distributes VLAN information ❌ C. 802.1Q VLAN trunk tagging ❌ D. DTP Negotiates trunking ❌
  19. matass_md 👍 1
    STP because the original 802.1d protocol had a 15+15+20 50 second convergence time , this made hosts fail in getting a IP address via DHCP.
  20. Da_Costa 👍 1
    STP is the best option because when the port is blocked the client will not receive a dhcp address hence apipa will be used
  21. Fakecon 👍 1 Answered C
    I used ip-helper on VLANs for WiFi and cable connections, never was STP issue. So for me its C out from experience
  22. Secsoft 👍 1
    sorry but I removed STP too thinking it as the stuffs related to port convergence
  23. ShadyAbdekmalek 👍 1 Answered A
    Answer is A : STP At is also same as Question 108 which has the correct answer
  24. splashy 👍 1
    Correct i got it mixed up with 802.1X... doh
  25. splashy 👍 1
    Thx for sharing i guess it's A then

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