why does cisco support 16 instance for mstp?

or 64 instances? reason?

e:

It's a power of 2?

Honestly I do not know. What does the standard say?

Ed

ote:

I don't find the standard mention it. In real word, how many instances is used usually? if connect to end station only one port assign to one vlan and one vlan is only in one instance. Is that reason we don't need more instances? Also, configuring more instances is very complex. right?

I have never heard of more than two STP instances being used in a real network. This was done to "load share" traffic across main+backup path trunks. Each trunk carrying several VLANs.

Even that was more complex than anyone really wanted.

In modern networks Layer 2 (and hence STP) is out and hardware based L3 routing is in.

Why would anyone care about >16?

Classic spanning tree originally meant 1 spanning tree per physical network design. The hassle came with VLANs, when the topology of the physical paths and particular VLANs might be different and it was easy to design a topology where some vlans were split apart by the spanning tree.

The fix was more spanning trees - most manufacturers cam up with a way to run more than 1.

The cisco PVST protocol (per vlan spanning tree) often meant several or dozens of spanning trees on a single switch, never mind across an entire network.

depending on the hardware you could run "lots" of PVSTs on a box - ARAIR Catalyst 6509 is 64 or 250 - the limit was software and Sup dependent.

MST gives a middle compromise - 1 MST instance can contain multiple VLANs, and you can have multiple MSTs, but you do not need 1 per VLAN.

True :)

agreed. the real fix for spanning tree problems is not to have network wide L2, so the spanning tree "sprawl" can be contained to relatively local portions of the network.

the limiting issue used to be things like management VLANs, which tend to be built as L2 across entire campus networks.

probably a tick box item in the tender boilerplate beloved of consultants......

Full circle I guess.

Still, at layer 3 the routing protocols are all striving to arrive at a loop-free set of forwarding tables, not all that different from STP. What layer 3 (eg IP) has that layer 2 doesn't is an ability to actually tolerate (deal with, to an extent anyway) loops when they happen - thanks to the TTL in the IP header.

rick jones