Which statement about spanning-tree root-bridge election is true?

Question:
Which statement about spanning-tree root-bridge election is true?

• It is always performed automatically
• Each VLAN must have its own root bridge
• Each VLAN must use the same root bridge
• Each root bridge must reside on the same root switch

Explanation: Answer A is not correct as we can choose which switch to become root bridge by configuring bridge priority. The switch with lowest bridge priority (value) would become the root bridge.

For answer B, this paragraph from Cisco confirms it is the correct answer:

“When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. Each VLAN must have its own root bridge because each VLAN is a separate broadcast domain. The roots for the different VLANs can all reside in a single switch or in various switches.”

Reference: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

The meaning of answer C is not clear but maybe it means “every VLAN must use the same root bridge” which is not correct as Sw1 can be the root bridge for VLANs 1, 3, 5 but Sw2 can be the root bridge for VLAN 2, 4, 6…

From the quote above we can say answer D is not correct.

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