Happy Friday everyone!

The other day I was working with a new customer getting NetFlow export enabled on his Cisco 3800 routers. When I was explaining the concept of using the ip flow ingress command in enabling NetFlow per interface, he said:

“I have to do that on all my interfaces?!? I have like ten sub-interfaces to do.”

In reality, ten interfaces isn’t really hard to configure, but it can be… cumbersome.

So if you are enabling NetFlow on a device with LOTS of vlans, a nice quick way to get things up and running is to use the int range command.

You can do this by typing the following:

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If you checked your routers NetFlow configs, you’ll most likely find that you’re exporting NetFlow v5 templates. If you’re not sure, do a show run | i ip flow and look for:

ip flow-export version 5

With NetFlow v5, all your traffic is measured based on the ingress of an interface. What goes in, must come out, right?

This is not always the case…

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I’m doing some more work lately with Wireshark and Scrutinizer v7. I thought that the topic of egress vs. ingress might be interesting to some readers.  NOTE: Egress is only available in Cisco NetFlow v9 and not NetFlow v5.

IPFIX or NetFlow v9?
In theory, ingress and egress should work the same in IPFIX, which is based on NetFlow v9, but they are certainly different. Although they are very similar, don’t let any company tell you they are exactly the same. Many collectors that work with NetFlow v9 will puke when they receive IPFIX. Scrutinizer handles both with ease. Nortel supports IPFIX, as does/did Avici, which is now Soapstone Networks, Inc. Other vendors, such as Adtran and Enterasys, support NetFlow v9.

One annoying area where IPFIX and NetFlow v9 differ is in the labeling of fields: NetFlow v9 has ‘IN_BYTES’ and IPFIX labels the same field ‘octetDeltaCount’.  IPFIX probably renamed it because when talking about egress flows, IN_BYTES is sort of misleading.

Ingress vs. egress differences
NetFlow v9 Ingress is collected on traffic going into (i.e. inBound) an interface.  This is how NetFlow v5 collects data. To figure out outBound traffic volume, ingress must be collected on all interfaces and the reporting software then displays outbound traffic. What goes in must go out, right?  Ya, usually.

NetFlow v9 Egress is collected on traffic going out (i.e. outBound) of an interface.  Generally, it is used in combination with Ingress, but it doesn’t have to be. I’ll dive into this a bit more.

Why collect with egress?
Why collect with egress, if ingress worked so well with NetFlow v5? Because hardware such as WAN optimizers compress data.  Traffic compression with Cisco NetFlow means that what comes in 100 bytes might go out as 50 bytes. If only using ingress flows, the NetFlow reporting software will show 100 bytes outbound, even if it was compressed to 50 bytes. GASP!!! This is because it was calculated using ingress flows.

Tell me the truth!
If the router is exporting both ingress and egress and the NetFlow monitor can report on both without overstating utilization, you can see how much of each flow is being compressed. It’s pretty slick, but it requires that the NetFlow collector understand what is known as the flow “Direction”. If the field in the NetFlow v9 packet is a 0, then it is an ingress collected flow.  If the field is a 1, then it is an egress collected flow.

Ingress Flow with IPv6 (the same with IPv4)

Egress Flow with IPv6 (the same with IPv4)

The network traffic reports produced by the NetFlow analyzer need to be intelligent when dealing with ingress and egress flows. I feel that dynamically figuring out flow direction in mixed NetFlow v9 ingress egress environments is crucial, especially if the customer has hundreds of routers. If you are just setting up ingress, I would keep this blog in mind: “ip route-cache flow or ip flow ingress… Which do I use?”

Something else to think about
NetFlow traffic analysis is going to be taken to another level as Flexible NetFlow matures. Perhaps we’ll see it take advantage of what NetFlow v9 calls ‘OUT_BYTES’. (IPFIX, needing to be different, calls this same field ‘postOctetDeltaCount’.)

Now you might ask: how is it related to ingress or egress?  Stay tuned…

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For some reason, this week I’ve been bombarded with questions regarding configuring the 6509 Catalyst for NetFlow.

Being a switch/router hybrid model, the configurations are a little different from standard CISCO routers models, like the 2811, but not too much.

I would also recommend checking out this great resource directly from CISCO to configure the 6509 Catalyst for NetFlow.

With most CISCO routers, there are two sets of commands used to enable NetFlow. However, with the 6509, there are technically three sets of commands.

To enable NetFlow on the router, you need the following:

ip flow-export source (insert interface name here)
ip flow-export version 5
ip flow-export destination (netflow collector ip address) (port to export flows to)
ip flow ingress layer2-switched vlan (insert vlans X,Y,X)

ip flow-cache timeout active 1

Once those are in place, we now need to configure NetFlow for the switched traffic:

mls nde sender version 5
mls flow ip interface-full
mls nde interface
mls aging long 64
mls aging normal 64

After you have configured these globals, you now can configure each of the interfaces themselves for NetFlow:

ip route-cache flow
ip flow ingress

I have discussed the usage of the ip route-cache flow and ip flow ingress commands before. You might want to take look for more details.

That wasn’t so bad, was it?

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If you’ve ever configured a router for NetFlow, you may have had to work with either, or both, of these commands.

When configuring NetFlow on your router, you have two sets of configurations to setup. First, being your global commands that define which version of NetFlow is being used, where the flows will be exported, and on what port.

After configuring the global commands, however, you also need to configure the interfaces that will be using NetFlow. To enable flows on an interface, you have two commands that are very similar in nature, but used in different circumstances.

For more information regarding NetFlow configurations, check out this Activating NetFlow Guide.

So, back to the original question: “Do I use ip route-cache flow or ip flow ingress?”

Deciding which interfaces you want to monitor will answer this question.

If you are interested in monitoring flows on a physical interface, you would use ip route-cache flow. By enabling ip route-cache flow on the physical interface, it will in turn enable flows on all subsequent sub-interfaces.

But let’s say that you are not interested in seeing flows on sub-interfaces x,y and z; but you do want to see flows on subs a, b and c, from that same interface. This is where the command comes into use.

So as a quick summary:
ip route-cache flow will enable flows on the physical interface and all sub-interfaces associated with it.

ip flow ingress will enable flows on individual sub-interfaces, as opposed to all of them on the same interface.

Cisco’s article on Netflow and subinterface support offers a wealth of information on this subject.

**NOTE** With NetFlow v5, we only had the option to monitor inbound statistics using the ip flow ingress command. However, with the release of NetFlow v9, we now have the option to monitor traffic leaving each interface via ip flow egress. Check out this blog which tackles the question: Which one is better to use? Ingress or Egress?

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