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Cisco Bug: CSCun16599 - N7K SVI may go down even when there are active ports in the VLAN

Last Modified

Nov 01, 2018

Products (1)

  • Cisco Nexus 7000 Series Switches

Known Affected Releases


Description (partial)

In a few scenarios the SVI (interface VLAN) on the Nexus 7000 can go down even when there are layer 2 ports carrying the VLAN with STP in forwarding state for those VLANs

This happens when changes are made that invloves removing and re-adding VLANs on a port or bouncing a port/port-channel carrying any VLANs.

To validate if the Nexus 7000 is susceptible to this failure, check the following output.

N7K# show spanning-tree summary

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0040                     0         0        0          1          1
VLAN0049                     0         0        0          1          1
VLAN0100                     0         0        0          3          3
VLAN0101                     0         0        0          3          3
VLAN0102                     0         0        0          3          3
VLAN0103                     0         0        0          3          3

N7K# show spanning-tree internal flc-info
VlanId    FwdPorts   LC Bitmap  FwdPorts per LC
VLAN0040         1  0x08000000  LC3:1
VLAN0049         1  0x08000000  LC3:1
VLAN0100         3  0x08000000  LC3:1
VLAN0101         3  0x08000000  LC3:1
VLAN0102         3  0x08000000  LC3:1
VLAN0103         3  0x08000000  LC3:1

If you notice Forwarding counter in "show spanning-tree summary" matches the FWDPorts counter in "show spanning-tree internal flc-info"

If you see a few VLANs missing or you see the FWDPorts counter less than what you see in "show spanning-tree summary" , the box is in a broken state and could end up bringing down those SVIs when changes are being made or link flaps.
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