09-24-2009 12:32 AM - edited 03-06-2019 07:51 AM
After setting "logging event status" on my access switches, I see the following message every few hours for several of the ports simultaneously:
%SPANTREE-6-PORT_STATE: Port Gi0/X instance XYZ moving from forwarding to forwarding
Where XYZ is the STP instance (this switch is running RPVST).
I'm a little confused how exactly a port changes from forwarding to forwarding. Does this message indicate there's a Spanning Tree topology change and the tree is being reset?
09-24-2009 01:14 AM
You might have configure portfast on these ports and there might be a topology change happening if you have not configured these ports in host mode, when any system/access port comes up.
09-24-2009 09:19 AM
The ports going from forwarding to forwarding are configured with portfast.
I turned on STP debugging and indeed the access switch is seeing a loss of BPDUs from the distribution switch on certain VLANs. This appears to then cause the STP instance to basically reset.
switch#sh debug
Spanning Tree:
Spanning Tree event debugging is on
Spanning Tree root changes debugging is on
switch#
Sep 24 17:10:41: RSTP(123): Po1 rcvd info expired
Sep 24 17:10:41: RSTP(123): updt roles, information on root port Po1 expired
Sep 24 17:10:41: RSTP(123): Po2 is now root port
Sep 24 17:10:41: RSTP(123): Po1 blocked by re-root
Sep 24 17:10:41 UTC: %SPANTREE-6-PORT_STATE: Port Gi0/11 instance XYZ moving from forwarding to forwarding
Sep 24 17:10:41 UTC: %SPANTREE-6-PORT_STATE: Port Gi0/12 instance 123 moving from forwarding to forwarding
Sep 24 17:10:41: RSTP(123): Po1 is now designated
Sep 24 17:10:41: RSTP(456): Po2 rcvd info expired
Sep 24 17:10:41: RSTP(456): Po2 is now designated
Sep 24 17:10:42: RSTP(456): updt roles, superior bpdu on Po2 (synced=0)
Sep 24 17:10:42: RSTP(456): Po2 is now alternate
Sep 24 17:10:42: RSTP(123): updt roles, superior bpdu on Po1 (synced=0)
Sep 24 17:10:42: RSTP(123): Po1 is now root port
Sep 24 17:10:42: RSTP(123): Po2 blocked by re-root
Sep 24 17:10:42 UTC: %SPANTREE-6-PORT_STATE: Port Gi0/11 instance 123 moving from forwarding to forwarding
Sep 24 17:10:42 UTC: %SPANTREE-6-PORT_STATE: Port Gi0/12 instance 123 moving from forwarding to forwarding
Sep 24 17:10:42: RSTP(123): Po2 is now alternate
Sep 24 17:10:43: RSTP(123): Po1 received a tc ack
The question is why is the access switch no longer seeing BPDUs from the distribution. I don't see any errors on the ports.
09-29-2009 07:30 PM
OK, so when I converted the 6500s from PVST to Rapid-PVST, I got the following warning which shed some light on the subject:
%PM-SP-4-LIMITS: The number of vlan-port instances on module X
exceeded the recommended limit of 1800
The module in this case is a WS-X6548-GE-TX, so not too old. However, we have 75 VLANs configured on 35 ports, which works out to 2,600 VLAN port instances. So that would explain why the BPDUs were timing out.
I setup VLAN filtering on the trunk ports and that seems to have resolved the issue. Converting to MST would also fix it.
Cisco TAC also recommended increasing max-age timer on the STP root, which lessened the frequency but didn't make the issue totally go away.
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