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Zorluk: OrtaRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator is analyzing the operational characteristics of a Cisco Catalyst switch block running Rapid PVST+. Which two statements accurately describe Rapid PVST+ port roles and bridge priority structures?

  1. An Alternate port remains in the Discarding state while receiving BPDUs from neighboring switches to provide an alternate path to the Root Bridge.Cevap
  2. A Backup port receives BPDUs from the same switch on a shared medium segment to offer redundant connectivity for the designated segment.Cevap
  3. C
    Configuring bridge priorities for VLANs in Rapid PVST+ must be specified in increments of 2048 to allow space for the 12-bit System ID Extension.
  4. D
    Enabling PortFast on switch-to-switch trunk ports causes the interface to transition immediately into the Learning state to accelerate STP convergence.

Cevap

The correct statements are that an Alternate port remains in the Discarding state while receiving BPDUs to provide an alternate root path, and a Backup port receives BPDUs from its own switch on a shared segment to provide redundant segment access.
In Rapid PVST+, an Alternate port receives BPDUs from neighboring switches to serve as an instant backup to the Root Port while maintaining a Discarding state. A Backup port receives BPDUs from its own switch across a shared collision domain (such as a hub) to serve as a backup to a Designated port.

Adım Adım Çözüm

1
Evaluate the operational characteristics of Rapid PVST+ Alternate and Backup port roles.
Alternate ports receive BPDUs from other switches to back up the Root Port (in Discarding state). Backup ports receive BPDUs from the local switch on shared media to back up a Designated port.
RSTP (802.1w) defines distinct roles for discarded links based on whether the incoming BPDUs originate from an external switch (Alternate) or the local switch itself (Backup).
2
Evaluate the calculation rules for System ID Extension and Bridge Priority in Cisco switches.
Bridge priority uses the 4 most significant bits of the 16-bit priority field, requiring configured values to be multiples of 4096 (2122^{12}).
Values like 2048 are invalid because the lower 12 bits are strictly reserved for carrying the VLAN ID.
3
Analyze PortFast state transitions and placement rules.
PortFast transitions ports directly to Forwarding (bypassing Discarding and Learning) and is intended for end-user edge devices.
Applying PortFast to switch-to-switch trunks risks immediate Layer 2 loops during topology changes.

Anahtar Kavram

Rapid PVST+ Port Roles and System ID Extension Rules
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Rapid PVST+ Spanning Tree Protocol Operations and Roles

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