Question regarding STP - Change of Root Switch - Help needed, plz.

Blanco

Hello everyone,:)



There's something I don't understand about Spanning-Tree Protocol (STP) and I would like your help, if you know the answer pls.


¤¤¤¤¤¤R1
¤¤¤¤¤¤/
S3--S1
¤¤\ /
¤¤S2


Let's say, there's 3 switchers and 1 router. Trunks been made, vlan, vtp. Everything's configured, but there's still STP missing, I think.


Switch 3 is the Root (via command show spanning-tree -> Root ID -> This bridge is the root. <- all vlans on s3 is root) I have created several vlans which are: They are all root in switch 3.


VLAN0001 = default = (This bridge is the root.)
VLAN0010 = sales = (This bridge is the root.)
VLAN0020 = guests = (This bridge is the root.)
VLAN0077 = Management (Native Vlan) = (This bridge is the root.)


And now I want to change switch 1 to root, because it's the closest to router.


If I want to do that, do I have to make all vlans including native vlan to become root in switch1 via the command:
spanning-tree vlan x primary root or I need only to make VLAN0001 (vlan1) as the root? Because if its only vlan1, then I dont understand, because vlan should'nt be used, but rather be shutdown?


Correct me, if there's anything wrong.


Thanks in advance.

veritas_libertas

Yes, since each VLAN has it's own Spanning Tree. This should help you further delve into the topic: Multiple Spanning Tree (MST) - Packet Life

It's an interesting topic and makes Layer-2 more complex than you would initially think it is.

edit: I think I'm understanding you correctly. You want to make Switch1 the root for all the STP instances, correct?

ChickenNuggetz

spanning-tree vlan vlan-id root {primary|secondary}

The above command is a global command. When used on a switch it will reset the priority number for the specified vlan making that switch the primary root bridge or secondary (whichever you selected). Use it on the switch you want to be the new root bridge.

You can also manually set the priority number of a swtich using the spanning-tree vlan vlan-id priority priority-value global command. Although I find the first command easier to use.

Do a show spanning-tree or a show spanning-tree vlan vlan-id command before and after using the above commands to see how each one exactly changes the priority numbers and root bridges for each vlan. Hope that helps!

theodoxa

veritas_libertas wrote: »

Yes, since each VLAN has it's own Spanning Tree. This should help you further delve into the topic: Multiple Spanning Tree (MST) - Packet Life

It's an interesting topic and makes Layer-2 more complex than you would initially think it is.

edit: I think I'm understanding you correctly. You want to make Switch1 the root for all the STP instances, correct?

By limiting which VLANs are allowed across which links you can make it so that no ports are in a blocking state and that each VLAN has a different root bridge.