Which statement about spanning-tree root-bridge election is true?
A. It is always performed automatically
B. Each VLAN must have its own root bridge
C. Each VLAN must use the same root bridge
D. Each root bridge must reside on the same root switch
A is correct because It’s talking about STP, not PVST.
Would it not say each vlan has its own route bridge, not must have its own root bridge as this is not a choice
The election is performed automatically. You can manually set a different route bridge using priority
I think the key to this question is that it just says “spanning-tree.” It does not specify which version of STP. Original STP and RSTP only provide one instance of STP regardless of the number of VLANs configured on the switch. MSTP groups several VLANS together into one STP instance. Only PVST+ and Rapid PVST+ provide an instance of STP on a per-VLAN basis.
A is incorrect at least from my perspective, root bridge is not always elected automatically since you can use the command spanning-tree vlan 1 root primary, I would choose B
The correct ansewr is B ,cisco reference
B
With STP, the key is for all the switches in the network to elect a root bridge that becomes the focal point in the network. All other decisions in the network, such as which port to block and which port to put in forwarding mode, are made from the perspective of this root bridge. A switched environment, which is different from a bridge environment, most likely deals with multiple VLANs. 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.
Note: The selection of the root switch for a particular VLAN is very important. You can choose the root switch, or you can let the switches decide, which is risky. If you do not control the root selection process, there can be suboptimal paths in your network.
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.
How to set root-bridge manually
spanning-tree [vlan vlan-number] root primary
Global configuration command that changes this switch to the root switch. The switch’s priority is changed to the lower of either 24,576 or 4096 less than the priority of the current root bridge when the command was issued.
So, A is incorrect (B is correct)
A as indicated above is the correct answer .Other websites indicate B. This is ideal but not the rule . other options are not valid. which leaves out A .Hence correct.
Agree
A is correct, because it’s always at first automatically, only then if we want can manipulate with priority