I have been doing firmware upgrades in HP switches. Two different models have been upgraded:
- ProCurve Switch 5406zl Intelligent Edge (J8697A): upgraded from K.15.06.0008 to K.15.12.0012
- HP 2520-24G-PoE Switch (J9299A): upgraded from J.14.54 to J.15.09.0021
Checking each switch right after booting the new image, I have observed something:
- Switches loaded the new firmware image without errors and connectivity was recovered as soon as the switch booted up. At this point CPU usage was low (under 10%)
- Few seconds later CPU usage raised up to 100% and stood there for several minutes. I could not detect any issue at this point apart from CLI through SSH being a little bit sluggish: normal connectivity, no log messages...
- After five to ten minutes at 100%, CPU came back to normal without any change from my side.
Both models were having this behaviour. I rolled back one unit of each model to the previous firmware image and they did behave the same way.
Despite this CPU spike right after boot caused no issue, I wonder if normal network behaviour could be the cause for this spike, but I do not think so. I have considered the following aspects:
Right after boot STP starts running, generating BPDUs and cycling all ports in the switch through the Bloking, Listening, Learning and Forwarding states. However, even with 802.1D this process takes no longer than 1 minute with default timers. Furthermore, I was checking the switches through SSH, so all STP computations were already done by the time I could connect to the switch.
Right after boot the MAC address table is empty and broadcast is needed for the first frames to be forwarded. But I doubt this broadcasting would take 100% CPU, much less for 5 minutes, in a 24 port switch.
All switches acting as L2 devices, no L3 functionality enable, so I discard routing and other L3 processes.
Am I missing something "normal" in network operations that may explain this CPU usage for 5 minutes after reboot while keeping connectivity? Maybe it is some kind of background process the switch runs right after reboot?