Examkin

Question

Difficulty: MediumSTP Port States and PortFast Benefits

To reduce network join latency for desktop computers during operating system startup, a Cisco switch interface connected to an end-user host is configured with PortFast. Which specific operational behavior does PortFast introduce on this interface?

  1. The interface bypasses the Listening and Learning states to transition directly from Blocking to Forwarding upon link initialization.Answer
  2. B
    The interface stops sending and processing Bridge Protocol Data Units (BPDUs), enabling it to connect to other switches without triggering STP topology changes.
  3. C
    The interface modifies the bridge priority calculation by adding an incremental offset of 4096 to force the switch to lose the Root Bridge election on that port.
  4. D
    The interface restricts Layer 2 broadcast traffic, converting the single switch port into an isolated broadcast domain to speed up IP address assignment.

Answer

The interface bypasses the Listening and Learning states to transition directly from Blocking to Forwarding upon link initialization.
Enabling PortFast on an access port allows the switch interface to transition immediately into the Forwarding state upon detecting link status up, bypassing the normal Listening (15 seconds) and Learning (15 seconds) STP states. This eliminates initialization delays for end hosts such as workstations and servers.

Step-by-Step Solution

1
Identify standard 802.1D STP port state transition timers for access ports.
Standard 802.1D STP transitions through Blocking -> Listening (15s) -> Learning (15s) -> Forwarding, totaling 30 seconds of convergence delay.
During Listening and Learning states, user data frames are dropped while the switch listens for BPDUs and populates its MAC address table.
2
Analyze the functional impact of enabling Cisco STP PortFast on an access interface.
PortFast causes the interface to skip the 15-second Listening and 15-second Learning delay, moving immediately into the Forwarding state when the physical link comes up.
End-user workstations do not create Layer 2 loops, so delaying data forwarding for 30 seconds is unnecessary and causes application initialization timeouts.

Key Concept

STP PortFast operational mechanics and state bypass
Rate this question

Topics

STP Port States and PortFast Benefits

More questions from this topic