Can network professionals answer this question?!

[MikeNY85]

Which configuration ensures that the switch is always the root for VLAN 200?
A. Switch(config)#spanning-tree vlan 200 priority 32003655
B. Switch(config)#spanning-tree vlan 200 root primary
C. Switch(config)#spanning-tree vlan 200 priority 604240
D. Switch(config)#spanning-tree vlan 200 priority 0

A, C are absolute wrong. What's the deal with B and D?

-Any help from network professionals is much appreciated!

[Enes Simnica · Accepted solution]

option D. is the correct one.

This sets the switch’s priority to 0, which is the lowest possible value in Spanning Tree. That means it will win the root bridge election, unless another switch also has priority 0 and a lower MAC address. So, as long as no one else sets the same priority, this switch will stay the root bridge.

[MikeNY85]

Here is my confusion, "root primary" keeps the switch as a root bridge no matter what, even when another switch is inserted with lower MAC address or lower priority. Priority of 0 will break if another switch gets in and has lower MAC address. Please confirm if I am right or wrong. Otherwise, I appreciate you helping and responding!

[Joseph W. Doherty · Accepted solution]

Here is my confusion, "root primary" keeps the switch as a root bridge no matter what . . .

Oh?  I recall it's just a symbolic way of setting a low enough priority value (24k?) that such a switch, if other switches are using defaults (32k?) or secondary (28k?) symbolic, it will be the candidate root.  Ignoring the question of setting a value of zero, what precludes two, or more, switches being configured as root?  If not precluded, what should happen?

Hmm, long, long ago, I recall (???) root selection didn't have symbolics, i.e. you had to select your own numeric value.

I also recall (???) the symbolic primary root value can be over come by setting a lower (0, 4k, 8k, .. 20k)  manual value, which if true, may have been intentional so that, if needed, you could move root to another switch just changing the one switch.  Possibly useful if you had a need to temporarily move root.

Rack1SW1(config)#spanning-tree vlan 1 priority 1

% Bridge Priority must be in increments of 4096.

% Allowed values are:

0 4096 8192 12288 16384 20480 24576 28672 32768 36864 40960 45056 49152 53248 57344 61440

[Jens Albrecht · Accepted solution]

Hello @MikeNY85,

"root primary" will not keep the switch as root. This is a common misunderstanding of this command.

When you execute the "root primary" command, the switch will check the current root priority and will try to go below this value. Why do I say try? Because this command cannot set the priority to '0' and it will never lower the MAC address!
This means if another switch has a priority of 4096 the "root primary" command will do nothing!

In addition, this command is a one-timer that gets executed when you enter the command and it will not monitor what happens later. So if later on someone lowers the priority on another switch, then this other switch will become root.

Therefore, answer 'D' is the only correct one. Of course, if you add another switch with priority '0' and a lower MAC address, then this switch will still become root.

So you could say that answer 'D' is not perfect but it is the best one.

HTH!

[Joseph W. Doherty · Accepted solution]

So you could say that answer 'D' is not perfect but it is the best one.

@Jens Albrecht is correct.  None of the answers are fully correct for the OP question:

Which configuration ensures that the switch is always the root for VLAN 200?

@Enes Simnica also explicitly (and I implicitly) touched upon this.  I.e. priority, alone, doesn't guarantee root selection.  However, a zero priority with a LAA (if possible) might.