Multiple Spanning Tree (MST)

Lessons Discussion
102

Hello Tshepiso

Glad that makes you happy! :grinning: That makes me happy too! :grin: Keep networking!

Laz

103

Hello,
I hope you’re doing well. I have a question regarding the spanning-tree configuration, specifically related to MST (Multiple Spanning Tree) and VLAN 14.
I have the following configuration:

spanning-tree mst 14 priority 24576
no spanning-tree vlan 14
spanning-tree mst configuration
  name region1
  revision 14
  instance 14 vlan 14

My question is:

  • Will MST instance 14 send BPDUs with this configuration?
  • Will VLAN 14 disabled in the spanning tree protocol (STP) for due to the no spanning-tree vlan 14 command?

I would appreciate your help and clarification on these points.

104

Hello Houssem

I went into the lab for this, using CML with a Cisco L2 IOSv device. I changed the spanning tree mode to MST, and put in the configuration you shared. When I issue the show spanning-tree vlan 14 command, I get this:

SW1#show spanning-tree vlan 14

Spanning tree instance(s) for vlan 14 does not exist.

SW1#

The MST instance is active for the other VLANs on the switch. So it would seem that the no spanning-tree vlan 14 command overrides the spanning-tree mst configuration subcommands that include VLAN 14 in the MST instance.

So no BPDUs will be sent for instance 14, and VLAN 14 will have STP disabled.

Just to make sure it is clear, this is an excellent exercise to understand how these commands interact. However, in a production network, such a configuration should be avoided to ensure clarity and predictable behavior.

I hope this has been helpful!

Laz

1 Like
105

Thank you @lagapidis

1 Like
106

Can you please explain what does “MST will only advertise BPDUs from the IST to the outside world” mean?

SW4 has the same root bridge for all vlans and it is running PVST.
From SW4 perspective, it is connected to SW2 and SW3 and it should see it is connected to One single switch with 2 links? So, does it learn SW1 via SW2 and SW3 and chooses Gi0/0 as the root port?

107

Hello Adjhithya

It is true that SW4 sees the whole MST region as one big switch, and interacts with it in that manner. However, this does not mean that SW4 doesn’t see the root ID of individual switches within the MST region (i.e. the root bridge SW1) nor does it mean that it does not take into account the internal costs of links to reach that root bridge in choosing its own root port.

What it does mean is that the BPDUs that are sent to the “outside” switch are those of the IST only, and it is as if SW4 sees only one big switch. But the root ID of that switch is that of SW1, and the costs advertised by each port of that switch include the internal costs to reach the root bridge. This is enough information for SW4 to learn about the root bridge ID, evaluate the cost to the root bridge from each of its ports, and thus determine its own root port. Does that make sense?

I hope this has been helpful!

Laz