[mrtg] Re: Monitoring VLANs on CISCO

[Will Saxon]

You can't do it by vlan on a switch. You have to monitor a vlan as a whole, by querying the router. The switch does not store the information you are looking for.

Per port monitoring is easy, you just have to know the interface index for the port you want to monitor.

Using snmpwalk, do this:

snmpwalk -v 1 -c read_community_string <switch ip address> ifName

This usually returns a mapping of interface indexes to ports. I am not familiar with the 2900 series you have, but I suspect that the ports and ifIndexes are the same, so that port 1 is ifIndex 1. There will be some other ifIndexes that correspond to virtual ports (like VLAN ports). These 'virtual' vlan ports exist only for bookkeeping as far as I can tell - there are traffic counters and whatnot for them, but the values are _always_ zero. You can monitor them all day - you will have an empty graph.

Now that I wrote all this I just remembered that we have a 3924, which is an IOS based switch. It seems that this switch understands a little more about what passes through it than the 2820 and 1900 stackables I am used to seeing. 

Try using snmpwalk and modifying your community string to look like <community>@<vlan number>. Are you able to see anything different with that? I don't know a whole lot about these switches, but when I reworked the cammer script to do a bit user tracking for us, I learned that some of the Cisco devices present different information based on a vlan you pass it in the community string.

Ultimately, all of this stuff is based on what the Cisco device will spit out via snmp. The devices don't always work the same from model to model (at least, that has been my experience). I have found that the Cisco forums can be helpful - you might ask a similar question there and a TAC guy may be able to help you more.

-Will

-----Original Message-----
From: David Roze [mailto:droze at digiweb.ie]
Sent: Friday, March 14, 2003 12:01 PM
To: mrtg at list.ee.ethz.ch
Subject: [mrtg] Re: Monitoring VLANs on CISCO


I mean I can see the incoming and outgoing traffic on the ethernet interface.
Now, I'd like to be able to monitor on the same way the traffic for each 
Vlan, or even better each port.
Each port is connected to a different subnetwork, so I could monitor all of 
them.
Thanks for your help

David

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