How to find the PORT cause STP change

[Ethan and Mia]

Hello , I just start learning Networking

Let 's start

My system have more than 30 cisco 2960 switch

PVST was on  , The core switch is ROOT

Everyday I found some logging why STP often change  so It shouldn't

I read one topic on this community , try this CMD 

show spanning-tree detail | in ieee|from|occur|is exec

this is output from Core Switch

VLAN1016 is executing the rstp compatible Spanning Tree protocol
  Number of topology changes 284 last change occurred 00:28:27 ago
          from GigabitEthernet2/0/11
 VLAN1017 is executing the rstp compatible Spanning Tree protocol
  Number of topology changes 266 last change occurred 02:34:01 ago
          from GigabitEthernet2/0/11
 VLAN1050 is executing the rstp compatible Spanning Tree protocol
  Number of topology changes 195 last change occurred 00:59:12 ago
          from TenGigabitEthernet2/1/6

Its show ports  Gi2/0/11 and Ten2/1/6    I enter to these switch  then.... How I know which ports make STP change ?

and Can access port make STP change ?

[Martin L]

Yes, access port can make STP change going up/down; that is why Cisco came out with improvement of PortFast for all access ports.  PortFast must be enabled either for all ports (access ports) or per access port. it is not enabled by default.

In STP a topology change travels to Root switch then down to all switches. So, it is a bit hard to find it.  Going over each switch and look at each port for spanning tree details and number of BPDUs.

You should run RSTP on all switches anyway;  In RSTP, by default, RSTP recognize access port as edge port and those will not make topology change when going up/down.  In RSTP only non-edge (trunking) ports will make TCN.

Regards, ML
**Please Rate All Helpful Responses **

[Ethan and Mia]

Hello Martin L

"Yes, access port can make STP change going up/down; that is why Cisco came out with improvement of PortFast for all access ports.  PortFast must be enabled either for all ports (access ports) or per access port. it is not enabled by default."

Do you mean  if access port at ROOT BRDIDE oftens up/down can make STP change right ?

Should I enable " Spanning tree portfast" to all "access mode" interface on all switches in system ?

Our network using  PVSTP and RSTP  , Core Switch was ROOT and others were default.

I read from many article ,  Can "access port" except  " ROOT BRIDGE "  can makes STP change(when its up/down) ? 

Thanks for your reply .

[Martin L]

Enable PortFast on every single access port on every switch you have either PvSTP or Rapid PvSTP.  Even if you switch to RSTP mode, Port Fast is not enabled by default (but switch knows port is access port).

you should not have old PVSTP at all. there is no reason to have old STP mode, it is ok to have mix modes during transition (for short time).  You should have run RSTP all over, on all switches.  Yes, Core Switch should be always set up as the ROOT.

not sure what you read but read Cisco CCNA book.

Regards, ML
**Please Rate All Helpful Responses **

[balaji.bandi]

Topology changes occur when any change physically or STP changes.

[Ethan and Mia]

Hello BB

Do you mean on root bridge or others switches ?

[paul driver]

Hello

---

@Ethan and Mia wrote:

Can access port make STP change ?

Yes it can so make sure that ALL access-ports are set to spanning-tree portfast
So they don’t participate in any stp convergence and cause any stp notification changes as/when these ports transition up/down

switches
int x/x
description access ports
spanning-tree portfast