Command question of spanning-tree vlan 1 root primary & secondary

[he2413297250214]

Hi, I'm practicing spanning tree protocol with packet tracer and was confused about spanning-tree vlan 1 root primary/secondary this two commands how it usually works.

I've set up two switches(2960-24TT) and link them together through G0/1-2 on each side.

In Normal condition, both of the switches' priority is 32769(=32768+1),

Switch_1(config)# spanning-tree vlan 1 root primary

[Switch_1 output priority] >> 24577 (=32768-8192+1)

Switch_2(config)# spanning-tree vlan 1 root secondary

[Switch_2 output priority] >> 28673 (=32768-4096+1)

[My Question]

As the priority mentioned above, now I've given the lower priority to Switch_1 using command:

Switch_1(config)#spanning-tree vlan 1 priority 12288

[Switch_1 output priority] >> 12289 (=12288+1)

Then the command on Switch_2:

Switch_2(config)# spanning-tree vlan 1 root primary

[Switch_2 output priority] >> 8193

and the command on Switch_1:

Switch_1(config)# spanning-tree vlan 1 root secondary

[Switch_1 output priority] >> 28673

I expected the output priority would be 4097(=12288-8192+1) on Switch_2 and 8193(=12288-4096+1) on Switch_1,

but it seems quite different from the result?

--

Best Regards,

Ray.

[paul driver · Accepted solution]

Hello
Prior to any previous stp priority changes
ROOT primary defaults to 24,576 +vlan X
ROOT secondary ALWAYS defaults to 28,672 +vlan X

Any prevously set stp root
ROOT primary defaults to minus 4096 to whatever the current stp root is.

So as the stp root already has its root priorty set, Then the stp root primary marco will decrease the stp root by 4096 as you see on your switch 2

Switch_2(config)# spanning-tree vlan 1 root primary

[Switch_2 output priority] >> 8193

However the secondary root marco ALWAYS sets it value to 28,672, as you see here

Switch_1(config)# spanning-tree vlan 1 root secondary

[Switch_1 output priority] >> 28673

[he2413297250214]

Hi Pual,

I got it! Really appreciate for your help!!