Maximum VLANs Supported in PVST+

[Rémi]

Hi Team,

Is there a maximum number of VLANs when using PVST+ on Catalyst 9407R and/or on Catalyst 9300/9200 series?

I couldn’t find any datasheet addressing this point. Does this mean there is no limitation?

If we assume that in PVST+, each VLAN creates one instance (I hope I’m not mistaken), we could also check how many instances are supported by each switch model. However, once again, I couldn’t find this information in the datasheet.

Thank you in advance

Rémi

[Rémi]

My bad ... I finally find "PVST instance maximum number" in datasheet

So my only remaining concern is to confirm whether the "PVST instance maximum number" is equal to the "maximum VLAN number in PVST+ mode."

Thank you

Rémi

[Joseph W. Doherty]

Ah, I see it.

Looks like a logical limit much lower then possible VLANs.  Likely, trying to have more of a practical limit, but even something like 128 instances, still may often be impractical.

[Joseph W. Doherty]

In theory, the logical limit might be the maximum number of VLANs supported on the switch.

In practice, limitation likely depends on many factors, mostly I suspect, on the actual topology (and its stability), PVST or rapid-PVST, and the actual hardware supporting the control plane.

The latter likely not documented on a datasheet, but possibly mentioned in actual case examples, especially if an example of "breaking" the network with too many PVST instances (which may be rare for need of PVST at all, vs. MST, if running lots and lots of VLANs).

If no one responds with such info, suggest searching the Internet and/or past Cisco Live presentations.

[Rémi · Accepted solution]

Hi Joseph,

Thank you for your reply.

What do you think about this information in the datasheet?

I believe this is the information I was looking for.

Rémi

[Joseph W. Doherty · Accepted solution]

As I saw another platform with a lower limit, suspect it confirms what I replied with earlier, that limits, both logical and practical, depends on multiple factors.

Whatever the logical limit is, it's likely much higher than you might be able to effectively use, but high enough you're unlikely to bump into it as an actual limitation.

In other words, it would seem unlikely anyone should truly need 100+ PVST instantances.  For such cases, possibly network design should be reconsidered.

BTW, what prompts this question?

[Rémi]

Working for a customer (manufacturer) who currently has around 110 instances on C9407R.

Thank you for your replies. I now have a better understanding of the "limitations" of instances.

[Joseph W. Doherty]

"I now have a better understanding of the "limitations" of instances."

Great!  However, maximum instance count is "easy", practical instance count is much more complex as it's dependent on multiple variables.