12-15-2010 10:25 PM - edited 03-04-2019 10:48 AM
Hello All,
We were facing high CPU utilization on a Cisco 2821 ( 12.4(22)T ) router. After some troubleshooting we disabled 'ip virtual-reassembly' command on both inside interface and outside interface (internet facing interface). After monitoring the CPU statistics for last 48 hours we observed that avearge CPU utlilization has came down to normal but there are some spikes of more than 70 % utilization for a very limited time period ( based on show process cpu history output ). We also observed following message in logging which has stoped after disabling ip virtual-reassembly,
.Dec 11 12:37:14.746: %IP_VFR-4-FRAG_TABLE_OVERFLOW: Dialer1: the fragment table
has reached its maximum threshold 16
I just want to know that the high CPU utilization was due to above mentioned feature or it could be any other reason ?
Thanks & Regards
12-16-2010 01:33 AM
Hello,
Firstly, %IP_VFR-4-FRAG_TABLE_OVERFLOW: is a warning message printed as the result of having the Virtual Fragmentation Reassembly feature turned on. It indicates that the number of datagrams being reassembled at any one time has reached it maximum limit i.e threshold of max-datagram 16.
To avoid it either virtual reassembly must be turned off (as you have done) or Increase the maximum number of datagrams that can be reassembled by entering the 'ip virtual-reassembly max-reassemblies #" command, with number being the maximum number of datagrams that can be reassembled at a time.
For CPU utilization, we need to check what exactly caused it - processes or interrupts.
Please capture following outputs at the time when there was high CPU -
1. Show tech
2. Show proc cpu sort
3. Show stack
Refer to this link for more information - http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a00800a70f2.shtml
On analyzing these details, we may be able to judge if high cpu and virtual reassembly are related.
Regards,
Rahul
12-22-2010 05:55 AM
Hello Rahul,
Thanks for your response. After disabling ip virtual-reassembly we didn't observe high cpu utilization yet. Although the 'show process cpu history' shows max CPU % above 80% but the average CPU % is not going above 30% now. So can we conclude that the 'average utilization' above 80% was due to ip virtual-reassembly or it has nothing to do with high CPU utilization.
Thanks & Regards,
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