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Difficulty: EasySTP Port States and PortFast Benefits

An administrator is configuring edge interfaces on a Cisco Catalyst switch connected directly to end-user host computers. Which two operational characteristics or benefits occur when Cisco PortFast is enabled on these access ports?

  1. The interface transitions immediately to the Forwarding state by bypassing the Listening and Learning states.Answer
  2. The switch prevents Topology Change Notifications (TCNs) from being generated when the link state on the port toggles.Answer
  3. C
    Spanning Tree Protocol (STP) is completely disabled on the interface, preventing it from listening for incoming BPDUs.
  4. D
    The switch automatically decrements its bridge priority value to ensure it retains root bridge status when hosts connect.

Answer

PortFast allows access interfaces connected to host devices to bypass Listening and Learning states to transition immediately into Forwarding, and suppresses Topology Change Notifications (TCNs) when host links change state.
Enabling PortFast on an access port allows host devices to bypass the standard 802.1D Listening and Learning states (saving 30 seconds) and enter the Forwarding state immediately upon link initialization. Furthermore, because end hosts frequently turn on and off, PortFast suppresses Topology Change Notifications (TCNs) for those ports, preventing unnecessary MAC address table aging across the switch network.

Step-by-Step Solution

1
Identify the primary operational function of Cisco STP PortFast on access ports.
PortFast allows an edge interface to skip the traditional 802.1D Listening (15s) and Learning (15s) states, placing the port immediately into the Forwarding state upon link up.
End-user workstations do not create Layer 2 loops, so waiting for standard convergence timers is unnecessary and can cause application timeouts (such as DHCP).
2
Identify the network-wide benefit regarding Spanning Tree topology notifications.
The switch does not generate Topology Change Notifications (TCNs) when a PortFast-enabled port goes up or down.
Preventing TCN generation for simple workstation reboots avoids constantly flushing MAC address tables on all switches across the Layer 2 domain.

Key Concept

Cisco STP PortFast operation and benefits on edge access ports
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Topics

STP Port States and PortFast Benefits

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