Troubleshooting High CPU usage on a Catalyst 6500 due to ARP Input.
Faced with a Cat 6500 with dual Sup 2T's that responded to a
show running-config
with
Unable to read Configuration.Try Again later
and general cli slowness, it became aparrent that the CPU load on the supervisors was massive:
cat-6500-sw1#sh proc cpu sorted
CPU utilization for five seconds: 99%/83%; one minute: 99%; five minutes: 99%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
11 595538792 824019705 722 9.09% 8.54% 9.10% 0 ARP Input
The next step is to isolate the source of the arp requests. This is a fairly manual process dependant on the number of interfaces you have – but here's how I went about it:
cat-6500-sw1#clear counters
Clear "show interface" counters on all interfaces [confirm]
cat-6500-sw1#
cat-6500-sw1#show int stat
Vlan101
Switching path Pkts In Chars In Pkts Out Chars Out
Processor 0 0 110 7040
Route cache 0 0 0 0
Distributed cache 90801 19878330 112007 79166896
Total 90801 19878330 112117 79173936
Vlan102
Switching path Pkts In Chars In Pkts Out Chars Out
Processor 13 832 21 1344
Route cache 0 0 0 0
Distributed cache 14980 5568056 21858 19621765
Total 14993 5568888 21879 19623109
Vlan103
Switching path Pkts In Chars In Pkts Out Chars Out
Processor 0 0 0 0
Route cache 0 0 0 0
Distributed cache 0 0 0 0
Total 0 0 0 0
Vlan104
Switching path Pkts In Chars In Pkts Out Chars Out
Processor 14 896 14 896
Route cache 0 0 0 0
Distributed cache 1126 493152 2449 387180
Total 1140 494048 2463 388076
Vlan105
Switching path Pkts In Chars In Pkts Out Chars Out
Processor 411216 34901876 79195 5068480
Route cache 0 0 0 0
Distributed cache 45 3904 105 10566
Total 411261 34905780 79300 5079046
Seems strange that Vlan105 is sending such a large amount of packets to the general purpose CPU.
cat-6500-sw1#sh int accounting Vlan105
Vlan105 Test Network
Protocol Pkts In Chars In Pkts Out Chars Out
Other 7049 422940 0 0
IP 231708 21413675 120 12268
ARP 142076 8524560 87541 9804592
There's no way that interface should be sending that many ARP requests. There's either some nasty in there, or a misconfiguration.
cat-6500-sw1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
cat-6500-sw1(config)#int vlan 105
cat-6500-sw1(config-if)#shutdown
cat-6500-sw1(config-if)#end
And to verify, after a few moments the CPU utilisation had dropped back to normal.
cat-6500-sw1#sh proc cpu
CPU utilization for five seconds: 4%/2%; one minute: 85%; five minutes: 96%