RMON Statistics Collection

Most network engineers familiar with RMON know to use the “alarms” and “events” to monitor things like the CPU loading hitting a certain threshold or looking for the number of incoming packets on an interface. Cisco Catalyst switches support some RMON features that allow you to collect more information about packets that arrive on your interfaces. If you want to enable this, then you have two options:

  • Native Mode (Analyze packets that are destined for your interface).
  • Promiscuous Mode (Analyze all packets that you encounter on the segment).

You can enable this for switch ports (layer 2) or routed ports (layer 3), but it’s impossible to enable it on SVI (switch virtual interface) interfaces. Here’s an example of how you can enable it on your Catalyst switch:

Switch(config)#interface fastEthernet 0/1
Switch(config-if)#rmon ?
  collection   Configure Remote Monitoring Collection on an interface
  native       Monitor the interface in native mode
  promiscuous  Monitor the interface in promiscuous mode

First, you need to decide whether you want to use the native or promiscuous mode. I’ll select promiscuous:

Switch(config-if)#rmon promiscuous

The second step is to configure how often and how much statistics we want to collect:

Switch(config-if)#rmon collection history 1 ?
  buckets   Requested buckets of intervals. Default is 50 buckets
  interval  Interval to sample data for each bucket. Default is 1800 seconds
  owner     Set the owner of this RMON collection

The “1” is the RMON collection control index. You can pick any value you like. By default RMON will sample data every 1800 seconds..this is a little too long for my example, so I’ll reduce it to 5 seconds:

Switch(config-if)#rmon collection history 1 interval 5

Now let’s see if my switch has collected anything:

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Forum Replies

  1. I have Question:

    ¿Can I configure to Rmon RMON Statistics Collection in a trunk interface in environment real without interruption of my operation?.

    this Trunk carry all traffic of my WLAN/LAN Network, arrive to BW Admin, please I need I known stadistics of packets of my trunk because BW Exinda is exceeding the threshold of allowed packets per second.

    Thanks a lot for your feedback,

    Best regards from Colombia :)!

  2. Hello Juan

    Enabling RMON should not disrupt the traffic on your interface. However, enabling it does use additional system and CPU resources. I suggest you initially implement it during low traffic periods and monitor CPU usage to verify that you won’t have a problem.

    I hope this has been helpful!


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