We have a hosted environment, basically a PIX 515 as a firewall which uplinks into a 3560 which runs multiple vlans to other 2960's where servers are connected. Let's say I have 10 VLANs, 10, 20, 30, etc. and I want to monitor how much BW each VLAN is pushing out of it's respective VLAN. Assume that VLANs do not (cannot) talk to each other, only to the gateway and out the PIX.
What I did was setup MRTG on each VLAN interface on the 3560 thinking, "oh I need to go to the Internet, go through my gateway." and then I would be able to see how much traffic is passed through each virtual interface in and out of each subnet(VLAN). This isn't quite the case however. I am getting graphs, but the graphs are not representing this information. They are representing something but not what I think they are. If I get MRTG going on an individual switchport (so gi0/5), I get what looks like accurate bandwidth information that I could consider valid. This doesn't quite work though when you have a subnet with more than one machine. For clarification, let me also explain it this way:
Switch core1 has VLAN interfaces 10 and 20. Switch lan1 has ports gi0/1 - 5 in VLAN 10 and gi0/6 - 10 in VLAN 20. Ports gi0/23 - 24 on both switches make up an etherchannel between core1 and lan1 and the port channel is a trunk carrying vlan info. core1 gi0/1 uplinks to the PIX which goes to the world. I configure MRTG on interface 'VLAN
10' on switch: core1 thinking I will see traffic exiting and entering the vlan (for machines on lan1:gi0/1 -5). This is not the case.
How can I get some insight into traffic usage per VLAN?
Thanks!
Adam