400-007 · Question #373
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 )
The correct answer is C. The diameter of the STP topology is increased. D. The PVST+ increases CPU utilization. Scaling from 90 to 990 VLANs raises two key STP concerns: increased CPU load from PVST+ running a separate instance per VLAN, and potential growth in STP topology diameter affecting convergence.
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
- ASTP is increased by a factor of 10 convergence time
- BTo add 990 VLANs to the switching hardware reserved VLANs requires you to use extended
- CThe diameter of the STP topology is increased.
- DThe PVST+ increases CPU utilization
- EBPDU does not support 990 VLANs
How the community answered
(32 responses)- A6% (2)
- B3% (1)
- C81% (26)
- E9% (3)
Why each option
Scaling from 90 to 990 VLANs raises two key STP concerns: increased CPU load from PVST+ running a separate instance per VLAN, and potential growth in STP topology diameter affecting convergence.
STP convergence time is governed by topology diameter and port timer values, not by the count of VLANs; there is no defined factor-of-10 relationship between VLAN count and convergence time.
VLANs 1 through 1001 (excluding reserved 1002-1005) accommodate 990 VLANs within the standard range, so extended-range VLANs (1006-4094) are not required for this specific scale.
As 990 VLANs span across more switches the logical STP diameter can grow, directly extending convergence timers (forward delay, max age) and increasing the time required for the topology to stabilize after a link failure.
PVST+ and Rapid-PVST+ maintain an independent spanning-tree instance for each VLAN, so increasing from 90 to 990 VLANs multiplies the per-instance BPDU processing and topology calculation load on each switch by approximately a factor of eleven, significantly raising CPU utilization.
PVST+ sends per-VLAN BPDUs and fully supports hundreds of VLANs; there is no protocol-level BPDU limitation that prevents operation at 990 VLANs.
Concept tested: PVST+ CPU utilization and STP diameter at large VLAN scale
Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html
Topics
Community Discussion
No community discussion yet for this question.