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Cisco Bug: CSCuo36213 - N5K: STP port is not placed into BKN PVST_Inc causing loop

Last Modified

Mar 01, 2018

Products (10)

  • Cisco Nexus 5000 Series Switches
  • Cisco Nexus 5548P Switch
  • Cisco Nexus 5596UP Switch
  • Cisco Nexus 6004 Switch
  • Cisco Nexus 6001 Switch
  • Cisco Nexus 5020 Switch
  • Cisco Nexus 5672UP Switch
  • Cisco Nexus 5548UP Switch
  • Cisco Nexus 5010 Switch
  • Cisco Nexus 5596T Switch

Known Affected Releases

5.2(1)N1(7) 6.0(2)N2(4) 7.0(1)N1(1)

Description (partial)

Symptom:
Under specific conditions we can see that when there is a loop for particular VLAN between Nexus 5000 ports,
STP is not blocking this port. 

In N5K VPC domain when access switch connected not via VPC, but separate trunks, and these conditions are met:
- vpc configured
- vpc peer-switch enabled
- MSTP configured on N5K
- RSTP configured on access switch

we can see loop is happening once we disable spanning tree for particular VLAN on access switch and these messages would be generated:

Nexus5596UP-2 %$ VDC-1 %$ %FWM-2-STM_LOOP_DETECT: Loops detected in the network for mac 0018.ba9a.ac41 among ports Eth1/2 and Po1 vlan 100 - Disabling dynamic learn notifications for 180 seconds 

But Nexus will not blok this loop.

NOTE: Issue noticed not during normal production time but during migration when access layer still on RSTP and aggregation moved to MSTP

Conditions:
Issue is noticed when all of these are configured:
- N5K VPC domain
- peer-switch is enabled and configured correctly
- MSTP configured on N5k side
- RSTP configured on access swtich
- access switch connected via 2 links to both N5k via separate trunks (not VPC)
- spanning tree disabled for particular vlan on access switch
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