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Cisco 6509 CPU Usage

Posted on 2011-04-29
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We have allot of users that are complaining about slow performance.  Our   two 6509es that have memory around 80% and CPU is averaging about 65%.  That seems high to me.  any suggestions on where to look?
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Question by:japplewhaite
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koudry earned 500 total points
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Hello,

The Cisco 6500 platforms, are high end devices designed for core network and equipped to hand huge amount of traffic without being stressed. However, I have seen a scenario when a Cisco 7606-S router crashed before my eyes.  Both 6500 and 7600 are quite similar.

In my case, the 7600 router crashed because of high traffic generated using SmartBits test engine and packet fragmentation.  Fragmentation occurs when the packet received by the router has a higher MTU than the MTU configured on the outgoing interface.  

A typical scenario is, you are receiving a gigabit traffic on a gigabit port, the input port has an MTU of 2000 bytes and the output interface has an MTU of 1500 (default).  The CPU fragments each packet into 2 to be able to send it via the output interface. This fragmentation makes the CPU works harder, so the CPU usage goes up, in my case, it went to 100% and the RSP crashed.

To avoid fragmentation, check and make sure that MTU is the same from end to end.

The other feature that can drive your CPU usage up even on high end routers, is VPN.  A platform like 6500 is equipped with a dedicated VPN hardware.  The 6509-E has 9 slots one of which can be a VPN concentrator.  However, as the number of VPN tunnels goes up, CPU usage goes up to and performance goes down.

This is just a thought based on my experience.

Thanks.
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by:Jan Springer
Jan Springer earned 1000 total points
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sh proc cpu sorted

   and

sh proc mem sorted

when the loads are high
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by:Fidelius
Fidelius earned 500 total points
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Hello,

Also if you are monitoring your 6500 via SNMP and have a big routing table (especialy if it is a BGP peer) it can use quite a lot of CPU. In that case you can limit SNMP readout to only necessary OIDs, or filter out routing OIDs from polling.

Regards!
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by:Jan Springer
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Yes, with the SUP processors, BGP can spike the CPU.  The RSPs are much better at handling that.
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