STP - high number of topology changes

[Jason Fraioli]

RD-CORE-1#sh spanning-tree detail

 VLAN0010 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 8192, address 00d0.02d5.740a
  Configured hello time 2, max age 20, forward delay 15, tranmsit hold-count 6
  We are the root of the spanning tree
  Topology change flag set, detected flag not set
  Number of topology changes 13589 last change occurred 00:00:33 ago
          from GigabitEthernet3/41
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 1, notification 0, aging 300

I would appreciate any comments on the topology change number in question here.  Is this something I should be concerned about?

[Ganesh Hariharan]

Hi Jason,

TCN comes into picture in switching world when ever there is link up/down has happen in the switching domain,so by default in all cisco swirtches run PVST so if any change or any port goes up/down it will result in TCN and STP convergence for taht particular vlan.

As per the logs in vlan 10 it can happen that any host is getting booted or getting some problem with the switch port which is connected with host getting fluctuated result in TCN and stp convergence for that VLAN 10.

To avoid the TCN frequently portfast is the only feature which can be used in switching before enabling portfast just check out the necessaity check before enabling.

check out below link in understanding of TCN in switching domain hope that helps out your query !!

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml#topic1

Regards

Ganesh.H

[lusandi]

Hello,

I would like to ask you if you can provide me this information.

show version of the switch, also if you can provide me the show run of the unit, and a description of the edge points of your topology in order to confirm what is creating the topology changes.

I will be waiting for your information.

Thanks,

[umeshpathrwal10]

Hi 

I am observing continuous stp flaps for a vlan on c3560.

How can I find the details from where the tcn is initiated ?

Stp is not flapping for other vlans on the switch 

[Georg Pauwen]

Hello,

as seen in the original post, the output of 'show spanning-tree detail' should tell you which port is the source of the the STP flap. Check what is connected to that port.

If you know (or have determined) which switch is/should be  the root for that Vlan, then you can set the priority:

spanning-tree vlan X priority 0

in order to make sure that switch is the root for the specified Vlan.

[paul driver]

Hello

---

@Georg Pauwen wrote:

If you know (or have determined) which switch is/should be  the root for that Vlan, then you can set the priority:

spanning-tree vlan X priority 0

---

in order to make sure that switch is the root for the specified Vlan.

VLAN0010 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 8192, address 00d0.02d5.740a
  Configured hello time 2, max age 20, forward delay 15, tranmsit hold-count 6
  We are the root of the spanning tree
  Topology change flag set, detected flag not set
  Number of topology changes 13589 last change occurred 00:00:33 ago

I believe the above isnt regarding to what switch is the stp root or what it should be its pertaining to many times a root sw has generated and sent tcn messages through the estate (or in this case with rstp any non root switch could initiate and send a tc) due to some stp event being initiated (flapping non edge-port interface, udld, stp loop etc..)

So to identify a switch to be the stp root shouldn’t reduce these tcns if there is an issue within the network, I would say the most important thing about this is that if tcns are being consistently generated over a very short period then all other switches could potentially flush their mac tables in response to receiving a tcn which isn’t a good thing for your entire switching estate to be dong consistently even if rstp is running.

[paul driver]

Hello
Make sure all access ports have spanning-tree fast enabled, otherwise every time they become active they will participate in stp election and tcns would be created which would result in high numbers of topology’s being logged

int x/x/
description access-port
spanning-tree portfast