RSTP vs MST

[Jason Fraioli]

For a scenario with 50 vlans, each vlan residing on an individual switch, is it best to run MST or RSTP? We want to have the flexibility to add more than one vlan to a switch, but at the same time, 50 vlans would be 50 STP instances using RSTP correct? Is 50 vlan RSTP instances going to slam my core switchs processor? What is the maximum number of vlans you would run in RSTP, before migrating to MST?

[ebanks2006]

I can't recommend a specific number of VLANs where migration to MST would be indicated. I can say that we run well more than 50 VLANs in our core in PVST+ mode. We use "switchport trunk allowed vlan" statements on the core-to-dist layer trunk uplinks. This keeps STP calculations reasonable by limiting the number of ports the core switch has to consider. CPU impact on our core switches by the STP process is negligible.

I don't think you'd have any problem running RSTP with 50 VLANs, assuming you use the "switchport trunk allowed vlan".

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml#block_ports

[vsri]

Yes, one can configure more than 50 vlans in the switch. Mainly we need to consider SP CPU and Number of virtual ports on the switch

before adding vlans.

[glen.grant]

50 shouldn't be a problem , we have like 80 on a first gen Sup 2 6500 and its not a problem with cpu or anything else , we run just pvst+ not rapid but it still shouldn't matter , just manually prune off any vlans you don't actually need down the links to the individual switches.

[chatasos]

In large switches, it's the number of spanning instances that matters, not the number of vlans.

Use "sh spanning-tree summary totals" to find it.