Examkin

Question

Difficulty: MediumSTP Port States and PortFast Benefits

Which primary operational benefit is achieved when Cisco STP PortFast is enabled on a switch access interface connected to an end-user host?

  1. The port transitions immediately from Blocking to Forwarding state upon link UP and avoids generating Topology Change Notifications.Answer
  2. B
    The interface suppresses incoming BPDUs and converts into an 802.1Q trunk link to carry multiple VLANs to the endpoint.
  3. C
    The switch port converts into a Layer 3 routed interface, preventing broadcast frames from traversing adjacent switch connections.
  4. D
    The switch dynamically reduces the port cost by increments of 4096 to ensure the interface is selected as the root port.

Answer

Enabling PortFast causes the access interface to immediately transition into the STP Forwarding state upon link activation while preventing Topology Change Notifications (TCNs) from being sent to the root bridge.
Enabling PortFast on an access interface allows edge devices (such as PCs and servers) to begin transmitting data immediately upon link activation by bypassing the Listening (15s) and Learning (15s) states. Furthermore, because edge ports joining or leaving the network do not reflect core topology changes, PortFast prevents the switch from generating Topology Change Notifications (TCNs) to the Root Bridge.

Step-by-Step Solution

1
Analyze standard 802.1D STP timer delays on link initiation.
Standard 802.1D STP requires ports to spend 15 seconds in Listening and 15 seconds in Learning before reaching Forwarding.
This 30-second delay ensures no temporary switching loops exist before user data is forwarded.
2
Evaluate the effect of PortFast on edge device connectivity.
Edge ports connected directly to workstations or servers cannot form switching loops, making the 30-second delay redundant.
PortFast bypasses Listening and Learning states directly to Forwarding and suppresses TCN generation to maintain network stability.

Key Concept

STP PortFast Benefits and State Bypass Behavior
Rate this question

Topics

STP Port States and PortFast Benefits

More questions from this topic