Max STP Active Ports?

[procopius1980]

Hello,

Can anyone point me in the right direction to determine the maximum number of STP active ports a WS-C3850-48P switch stack can handle.  Please see the output below.  The command is a little mess when pasted into this thread, but this switch has hit the maximum 128 STP instances and it currently has 1,437 active STP ports.  Thank you for your help!

MDF-3850-Stack-05#show spanning-tree summary totals | begin STP Active
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
128 vlans 3 0 0 1434 1437

[Reza Sharifi]

Hi,

What is the output of "show spanning-tree summary"?

Is the switch running rapid-pvst?

Also, how many vlans you have on this stack?

HTH

[procopius1980]

Hi!

The switch is running in rpvst+ mode.  Here is the output you requested:

MDF-3850-Stack-05#show spanning-tree summary
Switch is in rapid-pvst mode
Root bridge for: none
EtherChannel misconfig guard is enabled
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is enabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short

Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 0 0 0 8 8
VLAN0002 0 0 0 9 9
VLAN0003 0 0 0 45 45
VLAN0004 0 0 0 8 8
VLAN0007 0 0 0 8 8
VLAN0008 0 0 0 8 8
VLAN0009 0 0 0 8 8
VLAN0015 0 0 0 8 8
VLAN0020 0 0 0 8 8
VLAN0021 0 0 0 8 8
VLAN0022 0 0 0 8 8
VLAN0023 0 0 0 8 8
VLAN0024 0 0 0 8 8
VLAN0025 0 0 0 8 8
VLAN0026 0 0 0 8 8
VLAN0027 0 0 0 8 8
VLAN0028 0 0 0 8 8
VLAN0029 0 0 0 8 8
VLAN0030 0 0 0 8 8
VLAN0031 0 0 0 8 8
VLAN0040 0 0 0 8 8
VLAN0041 0 0 0 14 14
VLAN0042 0 0 0 8 8
VLAN0043 0 0 0 8 8
VLAN0044 0 0 0 8 8
VLAN0045 0 0 0 8 8
VLAN0046 0 0 0 8 8
VLAN0098 0 0 0 8 8
VLAN0099 0 0 0 8 8
VLAN0101 0 0 0 8 8
VLAN0104 0 0 0 8 8
VLAN0112 0 0 0 8 8
VLAN0113 0 0 0 8 8

Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0114 0 0 0 8 8
VLAN0115 0 0 0 8 8
VLAN0116 0 0 0 8 8
VLAN0117 0 0 0 8 8
VLAN0118 0 0 0 8 8
VLAN0124 0 0 0 8 8
VLAN0125 0 0 0 8 8
VLAN0126 0 0 0 8 8
VLAN0128 0 0 0 8 8
VLAN0129 0 0 0 8 8
VLAN0130 0 0 0 8 8
VLAN0131 0 0 0 8 8
VLAN0132 0 0 0 8 8
VLAN0133 0 0 0 8 8
VLAN0134 0 0 0 8 8
VLAN0135 0 0 0 8 8
VLAN0136 0 0 0 8 8
VLAN0139 0 0 0 8 8
VLAN0144 0 0 0 8 8
VLAN0145 0 0 0 8 8
VLAN0154 0 0 0 8 8
VLAN0155 0 0 0 8 8
VLAN0201 0 0 0 8 8
VLAN0210 0 0 0 30 30
VLAN0211 0 0 0 8 8
VLAN0212 0 0 0 8 8
VLAN0213 0 0 0 8 8
VLAN0214 0 0 0 8 8
VLAN0215 0 0 0 8 8
VLAN0216 0 0 0 8 8
VLAN0220 0 0 0 8 8
VLAN0221 0 0 0 8 8
VLAN0222 0 0 0 8 8
VLAN0223 0 0 0 8 8
VLAN0224 0 0 0 8 8
VLAN0230 0 0 0 8 8
VLAN0231 0 0 0 8 8
VLAN0232 0 0 0 8 8
VLAN0233 0 0 0 8 8
VLAN0240 0 0 0 8 8
VLAN0241 0 0 0 8 8
VLAN0242 0 0 0 8 8
VLAN0250 0 0 0 30 30
VLAN0251 0 0 0 8 8
VLAN0260 0 0 0 8 8
VLAN0262 0 0 0 30 30
VLAN0270 0 0 0 8 8

Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0271 0 0 0 8 8
VLAN0272 0 0 0 8 8
VLAN0275 0 0 0 8 8
VLAN0276 0 0 0 8 8
VLAN0277 0 0 0 8 8
VLAN0299 0 0 0 30 30
VLAN0301 0 0 0 8 8
VLAN0302 0 0 0 8 8
VLAN0303 0 0 0 8 8
VLAN0304 0 0 0 8 8
VLAN0305 0 0 0 47 47
VLAN0306 0 0 0 8 8
VLAN0307 0 0 0 8 8
VLAN0308 0 0 0 8 8
VLAN0310 0 0 0 8 8
VLAN0350 0 0 0 8 8
VLAN0400 0 0 0 8 8
VLAN0402 0 0 0 8 8
VLAN0404 0 0 0 8 8
VLAN0406 0 0 0 8 8
VLAN0408 0 0 0 8 8
VLAN0410 0 0 0 8 8
VLAN0412 0 0 0 8 8
VLAN0414 0 0 0 8 8
VLAN0416 0 0 0 30 30
VLAN0418 0 0 0 30 30
VLAN0420 0 0 0 8 8
VLAN0422 0 0 0 8 8
VLAN0424 0 0 0 8 8
VLAN0426 0 0 0 8 8
VLAN0428 0 0 0 8 8
VLAN0430 0 0 0 8 8
VLAN0432 0 0 0 8 8
VLAN0434 0 0 0 8 8
VLAN0436 0 0 0 8 8
VLAN0438 0 0 0 8 8
VLAN0480 0 0 0 30 30
VLAN0482 0 0 0 30 30
VLAN0490 0 0 0 8 8
VLAN0492 0 0 0 30 30
VLAN0494 0 0 0 8 8
VLAN0500 0 0 0 30 30
VLAN0532 0 0 0 30 30
VLAN0564 0 0 0 30 30
VLAN0566 0 0 0 30 30
VLAN0567 0 0 0 30 30
VLAN0568 0 0 0 30 30

Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0580 3 0 0 5 8
---------------------- -------- --------- -------- ---------- ----------
128 vlans 3 0 0 1434 1437

[Reza Sharifi]

128 vlans

So, do you have 128 vlans configured on this switch?

Does this number match with "sh vlan" or "sh vlan brief"?

HTH

[procopius1980]

Reza,

Thank you for taking the time to look into this with me.  I considered giving a little back story just for context, but decided against it.  I have recently stepped into a role where I am helping to manage this network.  The network team understand they are in need of a serious network rework.  I am just helping them to build a case so they can present it to their decision makers, who are not necessarily technical.  

To answer your question, the switch has a total of 133 Vlans being carried over the trunk, which is probably about 125-128 more Vlans than necessary  At this point, I am really just looking for a baseline for how many Active STP ports a 3850 stack can handle.  I am aware that this model of switch has a Vlan instance max of 128, which means they have exceeded that limit.  

Thank you again.

[Joseph W. Doherty]

I'm unsure there's a limit on number of "active" STP ports, but a limit of 128 STP instances sounds about right.

Generally, there's rarely need for more than a one or just a few STP instances, but as Cisco's PVSTP defaults to having one for every VLAN, with lots of VLANs, it's not that difficult to run out of STP instances. Further, there's the potential processing load maintaining all those STP instances, especially when truly not needed.

Your aware of MSTP?

[procopius1980]

Thanks for the input, Joseph.  MSTP is certainly an option.  Like I said earlier, this is a network I recently inherited, and I have to build a level of trust with the decision makers before I can make significant modifications.  I asked about the Active STP ports issue because I once had a very interfesting experience with a 4506.

It was August 2009, and I was an in-house network admin just cutting my teeth on how to really be a network engineer.  So I spun up about 20 new Vlans in preparation for a redesign.  A few days later, this MDF 4506 starts rebooting every few minutes.  To summarize, it had 200+ active ports configured as follows:
switchport mode trunk
switchport trunk native vlan 50
switchport voice vlan 51

So these were "Data" ports with a voice vlan, only they were trunk ports!  In short, we had about 10,000 active STP ports and that was driving the CPU to max (each vlan defined on the switch and every port was a trunk).  At that time, I actually found a Cisco troubleshooting document that said the supervisor engine on my 4506 could only handle 2,000 active STP ports.  I had almost all the ports to  sw mode access/sw access vlan 50/sw voice vlan 51 by about 9:00 p.m.  Man I wish I had more days like that!