Buggy power supply can brick buggy Nexus switches

Network admins in Cisco Nexus shops might want to check their inventories for a particular combination of switches and power supplies.

A reader alerted Vulture South to two field notices: FN63893 and FN64094.

On their own, they're just routine, but together on the same device they can brick the switch.

The first relates to PSUs (power supply units N55-PAC-1100W and N55-PAC-1100W-B). When running in a dual-PSU configuration, failure in one supply can force a switch restart. This affects Nexus 5596UP and Nexus 600x switches.

The system restart would be inconvenient, but it's not a particularly remarkable bug. However, the sysadmin reader pointed out that the more recent field notice, FN64094, provides the fatal combination.

N5K-C5596UP, N5K-C5596T, and UCS-FI-6296UP switches – careful readers will notice “5596” in the model numbers – need a microcode upgrade because they can fail to boot after a power cycle.

That field notice, issued in January 2016, explains: “There are two power controller devices in the system and each controller device controls four ASIC devices 0.9V power rails on board. It was found that the firmware on the two controllers have different soft-start compensation parameters.”

In other words, as the reader told us, if the PSU triggers a restart on the wrong switch, it'll be bricked. And yes, the combination can exist in the field, because that's why the reader noticed: “Turns out we have both that model switch with that model power supply”, he said in a message to Vulture South.

“You wouldn't want the first one happening if you've not patched for the second one”.

The good news: Cisco's TAC can walk users through running a script in the Nexus devices to upgrade the microcode. ®