Examkin

Question

Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator is configuring Rapid PVST+ on a Cisco Catalyst switch to optimize Spanning Tree Protocol (STP) operations for VLAN 30. The administrator wants to manually set the bridge priority so that this switch becomes a predictable root bridge for VLAN 30. Which command will be successfully accepted by Cisco IOS CLI without returning a configuration error?

  1. spanning-tree vlan 30 priority 20480Answer
  2. B
    spanning-tree vlan 30 priority 18000
  3. C
    spanning-tree vlan 30 priority 20510
  4. D
    spanning-tree vlan 30 priority portfast

Answer

The command 'spanning-tree vlan 30 priority 20480' is valid because priority values must be specified in increments of 4096.
The correct answer specifies a base priority value (2048020480) that is a valid multiple of 4096 (5×40965 \times 4096). In Cisco switches running Rapid PVST+, the Bridge ID uses a 4-bit priority field and a 12-bit System ID Extension containing the VLAN number. Consequently, configured priority values must be multiples of 4096.

Step-by-Step Solution

1
Analyze the IEEE 802.1t / Rapid PVST+ Bridge Identifier (BID) structure.
The 8-byte BID consists of a 2-byte Bridge Priority (4-bit Priority + 12-bit System ID Extension) and a 6-byte MAC Address.
Because the 4 highest bits represent the configurable priority, valid CLI priority values can only change in steps of 212=40962^{12} = 4096.
2
Evaluate the mathematical validity of the priority values.
20480/4096=5.020480 / 4096 = 5.0, which is a whole integer. 18000/4096=4.394518000 / 4096 = 4.3945, which is invalid.
Cisco IOS enforces priority inputs to be strictly in multiples of 4096 (0,4096,8192,12288,16384,20480,24576,28672,327680, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, etc.).
3
Verify System ID Extension behavior.
The administrator specifies the base priority (2048020480), and the switch automatically adds the VLAN ID (3030) to form a priority of 2051020510 in BPDU frames.
Entering 2051020510 manually in the CLI is rejected because the CLI expects only the un-extended base priority value.

Key Concept

Rapid PVST+ System ID Extension and Bridge Priority Calculation
Rate this question

Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

More questions from this topic