Since you are reading this I assume you understand how “classic” spanning-tree works. If you don’t, it’s best to read my Introduction to spanning-tree first before you continue.
Having said that, let’s start with a nice picture:
- VLAN 10 is configured on SW1 and SW2.
- VLAN 20 is configured on SW1, SW2 and SW3.
Question for you: do we have a loop in VLAN 10? What about VLAN 20?
There’s a big difference between our physical and logical topology. We don’t have a loop in VLAN 10 because it only runs on the link between SW1 and SW2. We DO have a loop within VLAN 20 however.
How does spanning-tree deal with this? Simple…we’ll just calculate a different spanning-tree for each VLAN! The oldest version of spanning-tree is called CST (Common Spanning-Tree) and is defined in the 802.1D standard. It only calculates a single spanning-tree for all VLANs.
Another version of spanning-tree is able to calculate a topology for each VLAN. This version is called PVST (Per VLAN Spanning-Tree) and it’s the default on Cisco switches.
Let’s look at an example where we have three switches and two VLANs. Both VLANs are available on all switches:
Above you can see that we have two root bridges. If we use PVST we can create a different root bridge for each VLAN if we want. SW1 could be the root bridge for VLAN 10 and SW2 could be the root bridge for VLAN 20. Why would you want to do this? Here’s an example:
If I would make one switch root bridge for both VLANs then one interface will be blocked for both VLANs. In my example above SW1 is the root bridge for VLAN 10 and 20 and as a result the fa0/16 interface on SW3 is blocked for both VLANs. No traffic will be forwarded on the fa0/16 interface at all. Imagine these were 10 Gigabit interfaces. It would be a shame if one of those expensive interfaces wasn’t doing anything right?
If I choose another switch as the root bridge for VLAN 20 we will see different results:
lagapides
Thank you so much. I got it now.
Hi Zeko,
You change it with the following command:
SW1(config)#spanning-tree vlan 1 priority 4096Which sets the priority to 4096 for VLAN 1. This way, you can change it for each individual VLAN. You can also do this for a range:
SW1(config)#spanning-tree vlan 1-200 priority 4096There’s also this command:
SW1(config)#spanning-tree vlan 1 root primaryYou can use “primary” or “secondary”. Basically it’s a macro that checks for the current root bridge and then sets the priority of the switch to a value just above or below the current root bridge.
By default, the
... Continue reading in our forumThanks Andrew…
Can you please confirm by default which version runs on switch PVST or PVST+ ? and which command do we use to enable PVST+
Hello Aniket
PVST+ is the default configuration on most modern Cisco switches. If you have changed this default and you want to revert back to PVST+ you can do this by issuing the following command:
spanning-tree mode pvst.Now the fact that the ‘+’ is not included can be a bit confusing. Keep in mind that plain PVST is no longer configurable on a switch, it’s always PVST+ so even if you see ‘pvst’ alone in the command, it is always PVST+ that is being referred to.
The advantage of PVST+ is that it sends 802.1D compatible BPDUs, as well as Cisco proprietary,
... Continue reading in our forumThanks a lot as always Lazaros …