200-301 · Question #325
When using Rapid PVST+, which command guarantees the switch is always the root bridge for VLAN 200?
The correct answer is C. spanning -tree vlan 200 priority 0. To guarantee a switch is always the root bridge for VLAN 200 in Rapid PVST+, set its spanning-tree priority for that VLAN to 0.
Question
Options
- Aspanning -tree vlan 200 priority 614440
- Bspanning -tree vlan 200 priority 38572422
- Cspanning -tree vlan 200 priority 0
- Dspanning -tree vlan 200 root primary
How the community answered
(47 responses)- A4% (2)
- B13% (6)
- C74% (35)
- D9% (4)
Why each option
To guarantee a switch is always the root bridge for VLAN 200 in Rapid PVST+, set its spanning-tree priority for that VLAN to `0`.
`614440` is not a valid STP priority value; priorities must be multiples of 4096 and range from 0 to 61440.
`38572422` is not a valid STP priority value; priorities must be multiples of 4096 and range from 0 to 61440.
In Spanning Tree Protocol, the root bridge is elected based on the lowest bridge ID. Setting the `spanning-tree vlan 200 priority` to `0` assigns the lowest possible bridge priority, ensuring the switch will be chosen as the root bridge for VLAN 200.
The `spanning-tree vlan 200 root primary` command sets the priority to `24576` (or `28672` in certain conditions), which makes the switch likely to be the root, but it does not guarantee it if another switch has a lower priority (e.g., 0).
Concept tested: STP root bridge priority configuration
Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12_2_55_se/configuration/guide/scg/sw_stp.html#wp1077673
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