Peak Traffic Shaping on Cisco IOS

Cisco IOS routers support two types of shaping:

  • shape average
  • shape peak

In my first lesson I explained the basics of shaping and I demonstrated how to configure shape average. This time we will take a look at peak shaping which is often misunderstood and confusing for many networking students.










 

Shape Average

Here’s a quick recap of how shape average works:

shape token bucket bc be

We have a bucket and it can contain Bc and Be tokens. At the beginning of the Tc we will only fill the token bucket with the Bc but because it’s larger we can “save” tokens up to the Be level. The advantage of having a bigger bucket is that we can save tokens when we have periods of time where we send less bits than the configured shaping rate.

After a period of inactivity, we can send our Bc and Be tokens which allows us to burst for a short time. When we use a bucket that has Bc and Be, this is what our traffic pattern will look like:

shaping bc be explained

Above you can see that we start with a period where we are able to spend Bc and Be tokens, the next interval only the Bc tokens are renewed so we are only able to spend those. After awhile a period of inactivity allows us to fill our bucket again.

Shape Peak

Peak shaping uses the Be in a completely different way. We still have a token bucket that stores Bc + Be but will fill our token bucket with Bc and Be tokens each Tc and unused tokens will be discarded.

Here’s what our traffic pattern will look like:

Shaping Peak Visualized

Each Tc our Bc and Be tokens are renewed so we are able to spend them. A period of inactivity doesn’t mean anything.

Now you might be wondering why do we use this and what’s the point of it?

Depending on your traffic contract, an ISP might give you a CIR and PIR (Peak Information Rate). The rate is the guaranteed bandwidth that they offer you, the PIR is the maximum non-guaranteed rate that you could get when there is no congestion on the network. When there is congestion, this traffic might be dropped. ISPs typically use policing to enforce these traffic contracts.

The idea behind peak shaping is that we can configure shaping and take the CIR and PIR of the ISP into account.

When we send a lot of traffic, we will be spending the Bc and Be tokens each Tc and we are shaping up to the PIR. When there isn’t as much traffic to shape, we only spend Bc tokens and that’s when we are shaping up to the CIR.

Let’s look at an configuration example which will help to clarify things.

Configuration

I will use the following topology to demonstrate peak shaping:

shape peak topology iperf

Above we have two computers and two routers. The computers will be used to generate traffic with iPerf, I’ll configure peak shaping on R1. Let’s do a quick test with iPerf, time to start the server:

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Forum Replies

  1. Great explanation my friend. Thank you for making this tutorial very clear and concise.

     

    Kyle

  2. In the paragraph at the end, just wondering where you got the 1024 from for the sustained bits per interval?

    The following commands however should give you the exact same result:

    shape peak 128000
    shape average 256000 1024 0

  3. Hello Chris,

    If you configure shape average and configure only the shaping rate then you get a Bc and Be:

    R1(config-pmap-c)#shape average 256000

    This shows:

    R1#show policy-map interface Fa0/0 | include bc
          shape (average) cir 256000, bc 1024, be 1024
    

    So I only reconfigured it and set the Be to 0:

    R1(config-pmap-c)#shape average 256000 1024 0

    And we get:

    R1)#show policy-map interface Fa0/0 | include bc
          shape (average) cir 256000, bc 1024, be 0

  4. Hello Rene/Laz,
    I have a few questions and I am going to use the below topology as the reference.

    //cdn-forum.networklessons.com/uploads/default/original/2X/5/557c9567785977e39a8d57eb4a93324326547066.png

    In this topology, there are two internet connections as you see in the picture and HSRP is configured between them. One router is active for one VLAN and another router is active for another VLAN is HSRP. There are two different firewalls for two different purposes. In this scenario, a lot of packet discards are being observed in the Trust zone and Untrust zon

    ... Continue reading in our forum

  5. I just found out that GLBP will not help for load balancing in this case since only firewall will be ARPing to the GLBP Virtual IP. Therefore, we can take GLBP out of the picture. Instead, an equal cost static route will be effective. Thanks.

    Azm

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