[c-nsp] Reconstructing a spanning-tree break

[Sam Stickland]

Hi,

In the "sh span vlan X detail" command there's output similar to the 
following:

  Root port is 47 (GigabitEthernet1/47), cost of root path is 14
  Topology change flag not set, detected flag not set
  Number of topology changes 11 last change occurred 2d00h ago
          from GigabitEthernet1/47

What is the meaning of the number of "Number of topology changes". Is 
this only incremented when a BPDU with the TC bit set it received? Or is 
it set when a switch sends a TCN? Or perhaps even against a root port 
that has gone down or stopped receiving BPDUs?

We have had a strange spanning-tree occurance that we are trying to 
reconstruct.

Looking at the ports listed under topology changes, we have this occurance:

SW7 >------< SW8
 |            X
 |            |
/|\           |
SW3          SW4 (R)
 |           \|/
 |            |
/|\           |
SW1 -------< SW2

SW4 is the root switch.
X is a blocking port
Arrows represent the port that received a topology change (all at the 
same time).

So SW4 received a TC from SW2, which received a TC from SW3, which 
received a TC from SW7, which recevied a TC from SW8.

But SW8 claims to have recevied a TC from SW7. :| This doesn't seem to 
make sense unless SW8 is listing the port for some other reason?

"logging event link-status" (or "spanning-tree logging" was not 
configured on any switch so don't know if any of the ports went up or down.

SW3 and SW4 are L3 switches, running HSRP. Oridinarily SW4 is active and 
SW3 is standby, but for a period of time both went active.

Can anyone explain what happened here?

Sam