Max-Burst-Size and Egress Rate Limiting

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Hello Everybody,

Can anyone explain the 'max-burst-size' variable as it relates to egress rate limiting in the command below?

configure ports <port_list> rate-limit egress [no-limit | <cir-rate> [Kbps| Mbps | Gbps] {max-burst-size <burst-size> [Kb | Mb]}]

More specifically, if I've been provided a Telco Ethernet WAN link with a CIR of 100Mbps over a 1Gbps bearer, what max-burst-size should I use and why?

Many thanks!
Stephen
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Stephen Elliott

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Posted 3 years ago

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Hernandez, Joshua, Employee

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Hello Stephen,

Great question!  Max-burst-size specifies the number of consecutive bits that are allowed to be in-profile at wire-speed.  This size can be specified in Kb, Mb, or Gb and will round down to the nearest supported size.  This is used when rate-limiting traffic is necessary egress on a port.  It maintains that should the port experience any burst of traffic at one time, it will not exceed the specified rate limit. 

If this is not configured the maximum size allowed is programmed into hardware.  It is recommended that the max-burst-size be configured much smaller than the expected rate of traffic. 

For example, if you configure a rate limit egress on this port at 100Mbps but max-burst.size is not configured a lower rate or at all, the achieved rate will be at the maximum allowed, which on a 1Gps link could be considerably higher than the Telco company may expect or allow. 

The role it plays it to keep traffic burst from exceeding the anticipated rate.  See also our GTAC article:

EXOS traffic above rate-limit
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Stephen Elliott

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That's excellent and very helpful - thanks Joshua.

It's still tricky to get a steer on exactly what max-burst-size to use although I think I've got my head round it now.

Our Telco suggests a 4ms burst window.  So I've calulated the amount of traffic that can be transmitted at the CIR during the window and have configured that as the max burst size.

So, for a 100Mbps CIR
0.004 (4ms) X 100,000,000 = 400,000 bits (400Kbits)

conf port 1:1 rate-limit egress 100 Mbps max-burst-size 400 Kb

It seems to work and I'm getting no local congestion drops although I can't easily tell if the telco is dropping traffic - if anyone else has a better method of calculation please feel free to comment.

: )
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Manoharan, Senthil Arasan, Employee

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Stephen,
First I will try to answer for your first query.

More specifically, if I've been provided a Telco Ethernet WAN link with a CIR of 100 Mbps over a   1 Gbps bearer, what max-burst-size should I use and why?

Max-burst-size depends on customer requirement.
For Example in ISP's shared data center  a customer X opted 100 Mbps bandwidth for their server but they need bandwidth  width up to 1 Gbps & they are connected to core switch whose link speed is 10 Gbps.
So ISP configure provide rate limit of 100 Mbps & max-burst-size of 1 Gbps.
Meaning is 100 Mbps of guaranteed bandwidth & they can use up to 1 Gbps.
Above 1 Gbps they cant use the bandwidth.i.e.Servers internet speed is limited up to max of 1 Gbps not more than that.
Remember one-thing max burst size should be greater than CIR value.

But ExOS CLI accepts max max burst size lower than CIR.
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Stephen Elliott

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Thanks Senthil but that's not quite my understanding.  Max-burst-size is a finite amount of data (Mbits or Kbits), not a throughput speed (in Kbps or Mbps) which is why I've tried to work out how much data I can send at CIR for a given time period.
Of course, this calulation comes out considerably less than the CIR.  (400Kbits for a 100Mbps CIR)
Happy to be corrected.
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Manoharan, Senthil Arasan, Employee

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Yes.
Max-burst-size is a finite amount of data which is the maximum volume of data send out on that port.ex  FTP.
Hope you understand.
(Edited)