Spanning tree primary

Alceo

If we use command: spannig tree vlan X primary root
it check for other switches priority end it use one that is lower among all.

But what happens if another swith lower it's priority below the root one?
Does the command above dynamically lower again it's priority or it is only a one time shot?

I've done some testing and my switch does not lower it's priority again, can you confirm?

james43026

I believe whichever switch last had the spanning tree vlan x root primary command used on it would take over as root bridge. This command is a one off command, and after a reload of the switch it would continue to use it's default or manually configure priority. It's best to manually set your bridge priority on switches that you want to be your root.

TWX

spanning-tree vlan # root primary tells the switch to, at this moment, determine the priority needed to take over as the root bridge on the specified VLAN. The actual command that the switch saves to its configuration is the command that defines a specific priority.

I ran spanning-tree vlan 5 root primary on one switch that was not the primary. This switch's original BID originally was 32773, the root was 24581. After running the command this switch BID became 20485, and spanning-tree vlan 5 priority 20480 (note 20485 sans 5 for vlan 5) was added to my running-configuration.

I then went to the switch that I originally had as root and had just demoted, its running config had spanning-tree vlan 1,5,10 priority 24576 still part of the running-configuration, it did not automagically recalculate to become root after the other switch's BID changed for VLAN 5.

I finally made my now-demoted switch the root switch again for VLAN 5, with spanning-tree vlan 5 root primary run in global config. The re-promoted original root before I started any of this now had this in the running-configuration:

spanning-tree vlan 1,10 priority 24576
spanning-tree vlan 5 priority 20480

And the switch that I had temporarily promoted still had spanning-tree vlan 5 priority 20480 set. The original-then-demoted-then-promoted switch became root again.

Now what I am curious about is why the priority didn't decrement by 4096, and was set to the same 20480. The original switch has a lower MAC address so it became root when the BIDs matched, but I expected, based on what I read, that it would become 16384 as that is 4096 less than the other switch as root-bridge was at the time I ran the command. It's an ancient 2950 running 12.1 so perhaps Cisco implemented the function to look at MAC addresses, or perhaps the function is buggy in this IOS version, or perhaps I didn't give the switches enough time to achieve convergence before I continued making changes, but either way it did not behave as the Odom book indicated it should.

volfkhat

Great Feedback Guys!!

I am studying for the 200-101 as we speak (well... on again, off again).

But it's these practical, real-life questions that help me really "get" the concepts being taught.

:]