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Concerning QoS

Hi

We have couple of sites were we are taken L3 VPN MPLS. We have procured 100 Mbps bandwidth, however telco has shaped the traffic at 75%. And then out of that 75% bandwidth it is distributed in ratio of 80:60:30:10 to different classes (80 provided to Voice, rest others).

My question and concern is, if we are paying for 100 Mbps, why is the telco shaping it at 75%. When I raised the same  concern to Telco, they replied that it is due to L2 overheads ( 25% are L2 overheads, something which I'm not able to digest). Further more, what is more strange is they are claiming that if they don't shape it at 75% of actual bandwidth, our voice class will suffer.

Please shed some light on the same and share your experience

PS: Sorry to post this in R&S section of the forums, however I didn't find any section for QoS.

Regards,

Smitesh

7 REPLIES
New Member

Re: Concerning QoS

What are the interface line rates?

Sent from Cisco Technical Support iPhone App

Concerning QoS

Missed to mention interface is GigE with hardcoded Bandwidth of 100Mbps.

Regards,

Smitesh

Silver

Concerning QoS

It seems a lot to have 25% overhead. I don't understand the ratios, are you saying that voice has 80/180 * 75 Mbps bandwidth? Why such strange ratios?

How much of the traffic are you expecting to be voice? Which codec are you using? No 802.1Q on the Ethernet interface?

I wonder if they are counting in the MPLS encapsulation into that. In my opinion that should not be your problem though. If you buy a 100 Mbps circuit it should be 100 or close to it, how the packets are transported is not of your concern.

Daniel Dib
CCIE #37149

Please rate helpful posts.

Daniel Dib CCIE #37149 Please rate helpful posts.

Concerning QoS

Hi All,

The policy-map is bit more complex,so I just wrote the consice and brief what it is doing.

Let me put my doubt in other words, say if bandwidth provided by telco is 100Mbps, also same is configured on interface with bandwidth command.

Now if I shape any class at 68 Mbps, while shaping is in action does it includes L2 overheads ?

Regards,

Smitesh

Super Bronze

Re: Concerning QoS

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

The problem is, (I believe) many shapers don't account for L2 overhead.  So, you need to shape slower than the nominal L2 bandwidth.  The question then, is how much slower.  The answer depends on the size of the your underlying packet as the percentage of L2 overhead increases as the packet size decreases.

If you looks at this Table 1 in http://www.cisco.com/web/about/security/intelligence/network_performance_metrics.html, you'll see the minimum size Ethernet packets actually use 84 bytes on the wire to send a payload of 46 bytes.  So for those, your overhead is 38/84 = 45.2%.  I.e. if your packets were all that small, even shaping at 25% slower would be "too fast".

Conversely, if you were sending maximum (standard) Ethernet packets, your payload is 1500 bytes with 1538 bytes on the wire.  In that case, your L2 overhead is only 38/1538 = 2.47%.

Usually, though, all your packets are not minimum nor maximum size, and so you can often shape for average packet size, but when supporting VoIP, you want to err on the side of too small.

Personally, I've found 15% usually works okay, but although 25% seems excessive, it may not be.  In fact, depending on your traffic, it may be insufficient.

The above is one of the reasons I'll often suggest running physical interfaces at the logical bandwidth (if possible).  I.e. if your agreed bandwidth is 100 Mbps, run the GigEthernet interface at 100 Mbps, not gig. Then use physical congestion on your interface to cause any necessary "backpressure".

BTW, another issue with shapers is they average transmission rate over some time period, which can lead to transient congestion on a physical interface.  The impact of this can be minimized by using a small Tc, generally 10 ms or less will work well.  What Tc is your Telco using?

PS:

Nice Wiki article with image of parts of an Ethernet frame, http://en.wikipedia.org/wiki/Ethernet_frame

Re: Concerning QoS

Thanks Joseph...

Adding to my OP,

Assuming that interface is GigE, having bandwidth command of 100Mbps and shaping applied 75% ( shaped at 75Mbps).

Also consider the following.

VoIP Class: Assigned priority bandwidth of 60 Mbps

Class ABC: Assigning 60% of remaining bandwidth

Class PQR: Assiging 30% of remaining bandwidth

Class XYZ: Assigning 10% of remaining bandwidth

Will we face VoIP packet drops in hardware queue considering G.729 codec used, if we increase the original shaping limit from 75% to 100 %

Message was edited by: smitesh kharecha

Super Bronze

Re: Concerning QoS

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

An "it depends" type of answer.

It depends on how often and how long you would exceed the committed data rate.

It depends on how your vendor enforces the committed data rate.

[edit]

PS:

Remember even 75% might be too "fast".

If you have a way to monitor packet loss, you can experiment with the shaped percentage.

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