Why per vlan spanning tree

Root is elected by the lowest mac address if the priory is left at the default , or by the lowest priority. Spanning tree uses different port modes to form a layer two switching topology to ensure no layer two loops exist in the network.

You need to be familiar with the different port modes in PVST as given below;. Step 1. Verify your configuration on SW2. Remember the lower the priority number the higher higher chance the switch will be the root bridge during an election.

If the switch has the lowest priority of all switches then it will automatically become the root per that vlan. Bridge priorities can be a number and must use increments to abide by the IEEE standard using the sum of the bridge priority and sys-id-ext which is the VLAN Number. So if you set a priority on vlan 1 to , the sum of the bridge priority and the sys-id-ext will be and that will be the bridge priority on that switch for that vlan.

Shown below is an example root bridge configuration using the spanning-tree vlan root primary command;. To verify your configuration you can use the show spanning-tree vlan command or the show spanning-tree vlan root command as shown below;.

If you have a root cost and root port then that displays the cost to get to the root and which port is the root port per VLAN basis as shown above. As shown in the verification below you can see that the root bridge has a priority of and the MAC address of c. The best path to the root bridge is out the root port which is Po12; interface Port-Channel12 is directly connected to SW2.

Well first and foremost, the switch, in this example Switch A, looks at the current priority for the root bridge and determines what it is. Is it currently the default value? If it is, we will lower our priority to 24, and some change. I believe it is 24, by default. If it's not the default priority, then what we will do is we will lower our priority to 4, less than the priority that is currently being used by the root bridge, making us the root bridge.

So let's look at the top output here, first of Switch A. We typed in spanning-tree vlan 1 root primary on Switch A forcing us to make ourselves the root bridge. If you look down below at the output after that for show spanning-tree vlan 1, we can see we are now the root bridge. It explicitly says that. This bridge is the root and the Priority is My math is right here, that is 4, less than the original value of that root bridge.

We do want to be mindful of the fact that we don't want Switch C to be the root of the topology even if Switch A goes down. Let's trade-off that role and responsibility. So root secondary says I want to back myself off one notch to be next in line for root bridge functionality should the primary fail. Now be mindful about this command. When you type it in, it does not say I am always the root bridge, you can never take it away from me.

It's just trying to find the specific priority value that would make it the root bridge. If we go to another switch and we type in spanning-tree vlan 1 root primary, it will go through the same process and become the root bridge. So this is not a permanent command that states I'm always the root bridge, somebody could still take over the root bridge process if that command is typed in somewhere else. How do we analyze our spanning-tree topology?

That is a really critical skill that we want you to pick up here. Well you have to have a good map. You have to have a good topological map. If you're trying to do this based on show output without a topological map, good luck to you.

Spanning tree is going to show us when we do show spanning-tree vlan 1 or vlan 2 or maybe we will just do show spanning-tree, which lists spanning tree for all VLANs. It's going to show us our root bridge. Is it us? Is it something else? It's going to show us timers, but more importantly, it's going to show us root ports and designated ports and that is what we taught you how to analyze.

Here is a short reference of the most used commands when we are analyzing the STP topology:. So there is the possibility that we look at all this stuff and it doesn't make sense to us, because maybe we have a rogue root bridge that came in below Switch C, someone trying to take over our root. Maybe we injected a new switch out of the box that had a better bridge ID because we never manipulated our distribution layer. It is possible that a low-end switch that was built 15 years ago is going to have the best default bridge ID.

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