You are currently using VLAN IDs 2 through 300 within your Layer 2 domain and you need to configure VSTP to prevent loops. You must ensure that all VLANs are loop free.
In this scenario, which statement is correct?
A. You must ensure that the VLANs are balanced between two different root bridges.
B. You must enable RSTP to account for all VLANs.
C. You must ensure that the bridge priority is set to the lowest value on all switches in the Layer 2 domain.
D. You must enable all VLANs, 2 through 300, under the VSTP configuration.
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VLAN Spanning Tree Protocol (VSTP) enables Juniper Networks switches to run one or more Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP) instances for each VLAN on which VSTP is enabled. For networks with multiple VLANs, VSTP improves intelligent tree spanning by defining best paths within the VLANs instead of within the entire network.
You can configure VSTP for a maximum of 509 VLANs.
VSTP and RSTP can be configured at the same time. If you configure VSTP and RSTP at the same time and the switch has more than 253 VLANs, VSTP is configured only for the first 253 VLANs. For the remaining VLANs, only RSTP is configured. RSTP and VSTP are the only spanning-tree protocols that can be configured at the same time on a switch.
correct Answer is D
ref:
https://www.juniper.net/documentation/en_US/junos/topics/concept/stp-vstp-qfx-series-understanding.html
I would say B, as VSTP is able to support 253, when it is reached then RSTP has to be enabled.