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I have created a bar chart in Java that will display the current percent load on a each destination port per SPAN session. I have a refresh time of 30 seconds using the show interfaces command to update the values of the chart, and I was wondering if I put the refresh time to 5 or 10 seconds, would this cause any performance issues on the Cisco switch?

Edit:

I have no experience with Cisco switches, otherwise I would be off figuring this out myself.

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  • You should use SNMP for this. CLI parsing can add up to a lot of unnecessary CPU load.
    – Ricky
    Mar 16, 2016 at 23:06
  • Ya was thinking the cli command would effect performance, I'll have a look into the SNMP protocol.
    – noobCoder
    Mar 17, 2016 at 15:06

2 Answers 2

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The SPAN feature has no impact on the performance. SPAN port features vary among switch vendors. Because of this, the impact SPAN has on switch operation can vary.

a packet received on a port is transmitted on the internal switching bus. Every line card in the switch starts storing this packet in its internal buffers. At the same time, the Encoded Address Recognition Logic (EARL) receives the header of the packet and computes a result index that it sends to all the line cards via the result bus. Whether one or several ports eventually transmit the packet has absolutely no influence on the switch operation.

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  • This is true, and probably good information for the OP, but I don't think it addresses the question of the impact of doing a show interfaces command.
    – Ron Maupin ♦
    Mar 16, 2016 at 16:32
  • ohh sorry. I forgot to mention that command wouldn't impact on performance.
    – Nishad Morey
    Mar 16, 2016 at 16:56
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The answer to this will greatly depend on the particular switch model and current switching load. This could impact low-end models which have a heavy load, but it may be negligible on high-end switches. For instance, a switch doing a lot of things which eat CPU processing, like QoS marking, DHCP snooping, dynamic ARP inspection, IGMP snooping, etc., may have its CPU utilization impacted if it is a low-end device.

There is no one-size-fits-all answer to your question.

Also, are you using the load-interval command on the interface(s)? This will determine the time period over which the interface statistics are calculated. If you leave it at the default of 60 seconds, your 30 second refresh will include statistics over a 60 second time period.

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  • I currently have the load-interval set to 30 seconds for the destination ports but i feel i will need lower as this tool is for a company and they want live data of the destination ports.
    – noobCoder
    Mar 16, 2016 at 16:35
  • I'm always amazed the amount of money which clueless managers will spend on useless information.
    – Ron Maupin ♦
    Mar 16, 2016 at 16:37
  • the thing is, this for my final year project for college, so ain't costing them anything. i guess i'll leave the time at 30 seconds and see if they want me to change it.
    – noobCoder
    Mar 16, 2016 at 16:44

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