352-001 Question #775: Real Exam Question with Answer & Explanation

The correct answer is B: STP is increased by a factor of 10 convergence time.. Scaling from 90 to 990 VLANs with PVST+ multiplies STP convergence workload by roughly 10x and significantly raises CPU utilization because each VLAN runs an independent STP instance.

Question

You want to add 900 VLANs to the existing 90 VLANs on a data center. Which two spanning-tree concerns must you consider? (Choose two)

Options

ATo add 990 VLANs to the switching hardware reserved VLANs requires you to use extended
BSTP is increased by a factor of 10 convergence time.
CBPDU does not support 990 VLANs.
DThe diameter of the STP topology is increased.
EThe PVST+ increases CPU utilization

Explanation

Scaling from 90 to 990 VLANs with PVST+ multiplies STP convergence workload by roughly 10x and significantly raises CPU utilization because each VLAN runs an independent STP instance.

Common mistakes.

A. 990 total VLANs falls within the standard VLAN range (1-1005), so extended VLANs (1006-4094) are not required; this choice is based on a false premise.
C. BPDU frames fully support any number of VLANs within the 802.1Q and PVST+ specifications - there is no hard VLAN count limit in the BPDU format.
D. STP topology diameter refers to the maximum number of switches in the longest path between any two endpoints, which is a physical design constraint independent of how many VLANs are configured.

Concept tested. PVST+ scalability impact of large VLAN counts

Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24092-147.html

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