Missing out traffic with Cisco 4500

NetFlow Analyzer | January 25, 2011 | 2 min read

Just found a limitation and a bug when using Flexible NetFlow (FNF) in Cisco 4500 running IOS XE and thought of sharing this. We were working with one of our customers to enable Flexible NetFlow on Cisco 4500. Everything went well but the record command was not being accepted. We did go over the issue over a couple of days and after we checked with Cisco TAC, we found our solution. Let me tell you what happened.

The Flexible NetFlow configuration requires 3 steps:

1. Creating a flow exporter and a flow record
2. Creating a flow monitor and associating the exporter and record to this monitor
3. Associating the flow monitor to an interface.

The Flexible NetFlow in Cisco 4500 requires you to configure your own “record” since there are no predefined ones available like in some other IOS. The creating of a flow record involves the below:

flow record nfa-record
    match ipv4 protocol
    match ipv4 source address
    match ipv4 destination address
    match transport source-port
    match transport destination-port
    collect interface input
    collect interface output
    collect counter bytes
    collect counter packets

Note the bold, we will get to that soon. After you have created this record and associated it to the monitor, the monitor has to be attached to every L3 interface which needs to be monitored. This is where the limitation and the bug pops up. When trying to associate the monitor to an interface, lets see what happens:

switch(config-if)# flow monitor NFAmonitor input

“intf output: cannot be supported as match field. Flow record fields can’t be supported with the monitor type

What does this mean? Simply that the statement collect interface output used when creating the record is not supported. This is a serious limitation as it is this line which helps capture the output interface for  a traffic conversation. You need to remove that line from the flow record configuration and then attach the flow record to the interface. Result: Only the IN traffic across the interface is captured and not the OUT traffic.

Now, let us have a look at the bug. I would not call it a bug actually, but an issue of providing a clearer statement. The scenario is:

switch(config-if)# flow monitor NFAmonitor output

“could not be added to interface due to invalid sub-traffic type: 0”

Makes no sense? This, I hope Cisco will change to a show the same error as with input monitor. But more than that, I hope Cisco will arrange to get records with collect interface output statement working on Cisco 4500 as the lack of support for this takes away the capability to view OUT traffic for an interface. Let us hope for the best.

Regards.
Praveen Manohar.

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  1. dg

    Cisco removes the basic netflow function and forces you to upgrade your license and after youve paid them extra for what was intgrated, its STILL crippled. I’ll do whatever it takes to avoid Cisco.

  2. Terry Hunt

    I was wondering if there were any updatre on this. I am the customer you were working with. It sure would be nice to get usable Netflow information from my 4506 again.