MSTP ISSUE

[tzachi.slav]

Dear Experts,

We have just bought 3 new 3550 24 fx smi switches and one 3550 12 Gbic Switch. All the 24 fx smi switches are connected to the 12 GBIC switch.

Also, Two of the 24 fx Switches are connected to each other on the fast Ethernet ports 22/23.

We are using MSTP protocol. The 12G Switch is the root bridge.

For some strange reason, the MSTP protocol decides that one of the Gbic ports that connect the 12G switch is in blocking state. Meaning, best root path for one of the interconnected switches is through the 24 fx connected to it.

I tried lowering the priority and the cost of the ports 22/23 but the mystery remains.

I will be grateful if you could help me in this manner.

For your connivance, I have attached detailed connection diagram to this conversation.

Tzachi Slav

[Georg Pauwen]

Hello Tzachi,

from what I can tell from your diagram, the port is blocked because there are redundant paths, which would be the default spanning tree behaviour. Are you using one MST instance for all your VLANs ?

Regards,

GP

[tzachi.slav]

Dear GP,

First, thank you very much for the fast replay.

I am using MST instance for all of our vlans (mst instance 0. vlan range 1-1000).

I even checked it on each switch and they are all using MST00.

I don't think it’s the issue. I tried to change the redundant paths into one Gibic Trunk but the port state stays the same (I have lowered priority and the cost of the port, did not work).

How do I configure port state manually?

Just for the record, from my experiments and reading, MST has a better convergence then PVST. Thus, it is better working with MSTP, is it right?

[Georg Pauwen]

Hello,

the problem I see with your setup is that with one MST instance, there is actually only one spanning tree in your network, hence there is no way you will ever get both ports to become active, since there are redundant links. The only way to have both ports active is to use PVST and distribute the VLANs evenly across both the ports, using the 'spanning-tree vlan X port-priority' command...

Does that make sense ?

Regards,

GP

[tzachi.slav]

You misunderstood my intentions. I would like to disable the redundant ports. They need to act as backup. The active port should be the Gibic connected to the 12G Switch (the servers are connecting to it). All VLAN Traffic should be delivered through a single port.

[Francois Tallet]

Hi Tzachi,

Please, make sure that all your bridges are in the same mst region. The mst region is defined by the region name, region revision and vlan to instance mapping. All this has to match on all the switches. My guess is that the link is seen as external by the 3550 24 FX that is blocking (an MST switch always prefers an internal port to an external port when electing its root port). If you could btw give the output of "show span mst" on the switch that is blocking, that would help understanding the reason why the port blocks.

Note that you can still do load balancing using different instances should you need it later. But for this also, you need to make sure that all the switches are in the same region.

Regards,

Francois

[k.adepetu]

from your diagrams, the default spanning tree behaviour should be to make the gigabit port conecting to the root the root port. You shouldnt reduce the port priority of the ports fastethernet ports.

What you need to do is increase the port priority of the fast ethernet port on the 3550-24 swich ,connecting to the other 3550-24, and make it higher than the port priority of the giga bit port connecting to the 3550-12.

[Francois Tallet]

Actually, the priority of the different ports is irrelevant in this setup. STP compare in order: root ID, root path cost, sender bridge ID and at last port ID. The priority is part of the port ID and will only be used if all the highest ranking parameters are identical, which cannot be the case in this network.

Here, what is relevant is the cost of the links. If everything is left to its default, the topology should be indeed what Tzachi and you described with regular STP. MST is a little bit more tricky in the way that there are two different costs for the CIST. An external cost, used between MST region and an internal cost, used within the region. You start by comparing the external cost. If the two 24FX are in the same region, the external root path cost is null between them and one of the two gig interface will block. That's really what I expect here.

Regards,

Francois