netflow without a Layer3 Interface?

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Aug 17th, 2010
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Hi all,

I have somthing strange showing up via Netflow.

I have a 4503-E running 12.2(53)SG1. It has 5 VLANs:

Vlan 240:

Vlan 241:

Vlan 242: no IP, no SVI, but has traffic on it.

Vlan 243:

Vlan 503: Vlan used to peer with my WAN: 198.98.x.x/30

Here is my NetFlow config for it:

ip flow ingress infer-fields
ip flow ingress layer2-switched
ip flow-cache timeout inactive 10
ip flow-cache timeout active 15
ip flow-export source Vlan240
ip flow-export version 5
ip flow-export destination 2055
ip route-cache flow infer-fields

Strangely, in Orion, which we use for Network monitoring / Netflow collecting, it's showing traffic between two hosts in the subnet, 242.101 and 242.50. It's also reporting that traffic as being on VLAN 503. These hosts are valid hosts, but they are in-fact on VLAN 242, and I've verified that the subnet does not exist in my network any other place. These hosts are able to communicate without issues or latency, so it doesn't seem to be impacting the traffic, it looks to be just how its being reported.

Any idea why: A. This traffic is being reported and B. Is being reported on the wrong interface?

I was under the impression that Netflow required a Layer3 interface? I could be wrong.

I'm attaching a screenshot from the netflow collector just for grins and giggles.

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Giuseppe Larosa Tue, 08/17/2010 - 11:27
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Hello RtJensen4,

netflow export packets use SNMP ifindex to indicate input and output interface of an observed flow.

Have you configured snmp ifindex persist?

if not, has the system been rebooted recently?


you can use, if supported,

show snmp mib ifmib ifindex 

to see a list of current ifindex values for existing interfaces including SVI

Hope to help


rtjensen4 Wed, 08/18/2010 - 05:16
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Ok thanks. I think that must be the case. The device hasn't been restarted, but

I did upgrade to a new version of the Netflow collector, so maybe that's the issue.


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