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Cisco Bug: CSCvi81086 - In N+N redundancy mode, running different capacity pwr supply, "Full protected" state not guaranteed

Last Modified

Jun 19, 2018

Products (1)

  • Cisco Catalyst 9400 Series Switches

Known Affected Releases

16.8.1

Description (partial)

Symptom:
When system is running with the different capacity power supplies with redundancy mode as N+N and power state as "Full protected", System could fail to provide same backup power upon switchover in case all active power supply fails. This is due to hardware current limitation upon switchover, where new active tries to come up within 20ms time window to backup failed active and lower capacity power supply could end up in over current due to equal load sharing. This could also lead to a partial or full system crash and should be treated as a "Normal protected" mode.

There are some corner cases where system would be in "Full protected" mode with different capacity power supply,  however upon failure of all active power supply , new active psu would fail to provide same power.

Case 1 :
Slot 1        Slot 2        Slot 3        Slot 4
Active        Active        Stdby        Stdby
3200w       2100w       3200w       2100w

Redundancy state : N+N
Operating state     : Full protected ( as sum of active power == sum of standby power )

==========> Now if Slot 1 and Slot 2 power supply fails, Standby power supply would try to become Active but it will fail to provide same power, as during transition, both of the new Active will initially try to current share the load and hence 2100w power supply will end up drawing more current than its max capacity.( ~38A ). This could result in Over Current Protection (OCP) for 2100w power supply and system could crash partially or fully.

Note : This behavior is very similar to "Normal Protected" where sum of standby power is less than sum of active power.

Case 2 :

Slot 1        Slot 2        Slot 3        Slot 4
Active        Active        Stdby        Stdby
230v          115v          230v         115v
3200w       1570w       3200w       1570w

Lets say we have all 4 power supply of 3200w , but some connected to 230V input and some connected to 115V input as shown.  This will have same behavior as Case 1.

Conditions:
Same as Symptoms
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