MSTP Ring - Maximum Number Of Switches?

Question

We have inherited a ring topology with 12 switches configured with MSTP. Each switch is either 4 or 8 ports but in general most are 4 port with 2 fiber uplinks that form the ring.

We are getting a what appears to be a significant number of topology changes in the stats:

stats from switches show a high number of topology changes: 3185

I`ve looked at a few posts that suggest there is a maximum number of switches/bridges in a ring. Although the posts do relate to RSTP not MSTP.

RSTP recovery time with Ethernet ring

I`m trying to find concrete primary proof that this is the case and to identify the maximum switches in a ring. It may that the maximum is also influenced by MSTP settings of which I have limited knowledge at present.

Kind Regards

Aidan

Answer 1

stats from switches show a high number of topology changes: 3185

While large STP rings are not always a good design, I doubt that the high number of Topology Changes you observe has to do with ring size.

I suggest reviewing the ports where edge devices are connected. These should be configured as spanning-tree port type edge [trunk] or spanning-tree portfast [trunk] (depending on platform and vendor, the syntax may vary).

If STP edge ports are not configured as such, each time one of them transitions from LRN to FWD state, a STP Topology Change Notification (TCN) will be flooded across the given spanning-tree(s). That will happen if the given attached server or PC boots.

That might readily explain the high number of (apparent) TCs the switches report.

Answer 2

In addition to Marc's answer:

• The number of switches is irrelevant. The tree depth is. Even with a ring, MSTP forms a (spanning) tree.
• Make sure you've defined a root bridge (lowest priority number) and don't just leave that to chance. If some edge switch is randomly elected root, every change triggers at least one TCN.
• You should also define a failover root bridge (designated, second lowest priority number) to still guarantee a defined tree in case the root bridge is offline.
• With a switch off the ring elected as root, the total depth may exceed MSTP's design limit of seven bridges in depth (=radius from the root's POV), so the tree never really converges. Trees of that depth require tweaked STP parameters.
• Better yet, those tree depths and rings should be avoided in general - consider rewiring to a tree topology (=one or two core switches =STP root and designated) plus redundant links where required.

Answer 3

Overall answers seem to vary ranging from 7 to 21.

Future designs now use ERP.

Thanks