Cisco IOS IP SLA Traffic Generator

IP SLA is a great tool that you can use to make things like static routing more reliable but did you know you can also use it as a traffic generator?

When you configure IP SLA with the correct number of packets and payload sizes, you can use it to create certain traffic streams. This can be very useful when you want to practice QoS since you don’t have to mess around with traffic generator tools.

Before we look at the configuration, let’s do some calculations. Imagine we want to send 16 kbps of traffic from one router to another over Ethernet. How many packets should we send and what should the payload size be? Let’s take a look at the different header sizes first:

Ethernet Ip Udp Payload Sizes

Here is an example of a frame, an Ethernet header has 14 bytes, IP is 20 bytes and UDP is 8 bytes.

Calculate total frame size

Total frame size = L2 header + L3 header + L4 header + payload

My routers will be connected using Ethernet so that’s 14 bytes. IP adds another 20 bytes and UDP requires 8 bytes. The reason that I use UDP is that I will configure IP SLA to use UDP jitter.

14 + 20 + 8 = 42 bytes.

To keep the calculation simple, I’ll use a payload of 58 bytes so that the total packet will be 42 + 58 = 100 bytes.

Calculate Bandwidth

Bandwidth = frame size x number of packets

We know our frame size is 100 bytes so how many packets should we send per second? Our goal is to generate 16 kbps of traffic, that’s 16.000 bits per second. This is how we calculate it:

Number of packets = bandwidth / frame size

Before we can do this, we need to convert our 16.000 bits to bytes:

16.000 bits / 8 = 2000 bytes.

Our packet size is 100 bytes and we need 2000 bytes per second to reach 16 kbps:

2000 / 100 = 20

We need to send 20 packets per second with a frame size of 100 bytes to hit 16 kbps!

So far so good? Let’s start the configuration…

Configuration

To demonstrate IP SLA we will use two routers connected to each other with Ethernet, R1 and R2:

R1 R2

Let’s start with the configuration of R1:

ip sla 1
 udp-jitter 192.168.12.2 17001 num-packets 20
 request-data-size 58
 threshold 500
 timeout 500
 frequency 1
ip sla schedule 1 life forever start-time now

As calculated we will send 20 packets per second with a payload size of 58. This will be good for 16 kbps but I will show you how to verify this in a bit. I used destination port 17002 (pick whatever you like) for this instance.

Let me give you another example for a bandwidth rate of 32 kbps:

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Forum Replies

  1. Hi Rene
    The image file named as “ethernet-ip-udp-payload-sizes.png” in this post is broken. It can not be displayed properly. If it is not my broadband problem, then your image file gets something wrong. Please check it.

  2. Hi Hongfei,

    Thanks for letting me know, I just re-uploaded this image.

    Rene

  3. IP SLA is great to generate some traffic so that you can test QoS with different traffic types and DSCP values but it really isn’t suitable for high bandwidth since it uses the CPU to generate traffic. For some more information, take a look at this article:

    https://www.cisco.com/en/US/technologies/tk648/tk362/tk920/technologies_qas0900aecd8017bd5a.html

    UDP-Jitter Probe for VoIP (G.729a) Running Eng 3-Cisco IOS Release 15.1(4)M Default Parameters: Frequency 60secs), Codec Packet Size (32bytes), Codec Interval (20ms), Codec Number of Packets (1000)

    1921 2921
    ... Continue reading in our forum

  4. I was very motivated to try this interesting feature. But, it seems on GNS3, it’s a little bit harder. Depending on examples ; when I ran sh ip sla moni stat (on 3600) or sh ip sla stat (on 7200), I got one of the errors below :

    • Latest RTT: NoConnection/Busy/Timeout
    • Latest operation return code: No connection
    • Latest operation return code: Timeout
    • Latest operation return code: Socket connect error

    I tried also the stuff in the link below. I got almost the same errors, unfortunately.

    ... Continue reading in our forum

  5. Hello Rene,
    This is indeed great tool for practicing.

    When I lab your configuraion, it seems like the destination device is blocking port 1967 which used for establishing IP SLA control connection via UDP.

    should the policy-map that you had configured block this port and allow only 17001? or is it used only to capture the traffic?

    it seems odd that my device blocking that port num 1967 when there is no access-group applied to the interface, only service-policy.

    All I get from my SLA operation is a reply that indicates “port is unreachable”.

    What could be the cause for that to happen?

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