Traffic Shaping on Switch - Is it supported and on which models?

[thomasjkuhn]

Hello All,

I will start off by saying that I am not a Cisco expert my any means, but I have asked a few Cisco engineers that I work with and have received conflicting information as to whether a Cisco switch such as a 3560 or 3750 is capable of handling the following situation:

I have seen similar questions and responses but this requires much slower speeds.  We are working on developing a system that would require some form of traffic shaping and port routing.  While some have mentioned just using a Linux box to perform all of these tasks,  I think that the capabilities are inherent (Based on everything I have read so far) in the higher end Cisco Switches. 

We need to interface to a Radio. This radio has an IP interface into which we can pass IP packets with a data payload. Unfortunately the radio has a very small buffer in which to handle any surges in incoming data. We would like to use a Cisco Switch to handle this situation.  The switch would receive the data from another port, route it to the proper destination port (static - preconfigured by operator). The radio will get the payload and assemble into a serial data stream and transmit out of the RF port.  Depending on the rate that the radio is set to transmit (2400bps – 56Kbps Fixed by the operator and not variable), the switch would have to shape/buffer the IP traffic going to the radio in the case that the data coming into the switch is coming in faster than the RF can transmit.  Attached is a diagram showing two use cases and please note that all devices are on the same subnet: 

The top portion of the diagram shows a computer streaming out data at approximately 2400bps into F0/1.  A static route would route that to F0/3.  An outbound traffic shaper would ensure that the traffic is fed to the radio at approximately 2400bps ensuring for smooth operation.

The bottom portion of the diagram shows a computer transmitting a file (or Large burst at line speed) into F0/2.  A static route would route that to F0/4.  An outbound traffic shaper would ensure the data is buffered because can only handle an incoming speed of approximately 19200bps.  It would be nice if the buffer could be sized to something like 5MB.

Now that I have put that all out there, is there a switch that can perform the outbound packet shaping?  I have heard conflicting information this can only be done by routing.  We want to keep all devices on the same subnet.

Update: The Switch would need to support a total of 40 egress ports to radios, and 1 ingress port for all the data.  The diagramm only shows 2 radios.

[Joseph W. Doherty]

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Posting

Some switches can shape.  (NB: some high end chassis switch features depends on what's actually installed in the chassis for different features.)  For example, 3650/3750 have two shaping features.  They can shape individual queues or "shape" the egress port.  I've never used the former feature.  I've used the latter feature, but it's not precise and I don't believe supports rates as low as you require.

There are MetroE switches, that often offer some additional features over "LAN" switches; they might have some additional shaping support.

If you look at the IOS manuals, both reference and usage guides, look at the QoS features.  Shaping, if supported, should be described there.

For your purposes, a small router, like the 800 series, will likely ofter better shaping and QoS features then most switched.  They can certainly handle those transmission rates.

[thomasjkuhn]

I just took a look at the Catalyst 3750 Switch Configuration Guide, and it does discuss limiting the bandwidth of the egress interface.  Unfortunatley it looks, as the previous pooster mentioned, like it has a low granularity.

srr-queue bandwidth limit weight1 

Specify the percentage of the port speed to which the port should be limited. The range is 10 to 90. By default, the port is not rate limited and is set to 100 percent.

Even if the port was configured to 10Mbps, 10% is 1Mbps.  still nowhere near 2400bps.

[Joseph W. Doherty]

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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.

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Posting

Correct; that's the port "shaper".

For queue shaping, see: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_52_se/configuration/guide/3750scg/swqos.html#wp1163879

This should be much more granular, but it is per queue.  If you mapped all traffic to one queue, you could shape it at the rate desired, you just wouldn't be able to prioritize it.

You could also map and shape multiple queues; just be careful of the aggregate rate.

[thomasjkuhn]

Ok,

I just finished reading that section in the PDF manual.  so, let me see if i understand...

To clarify, I do not need to prioritize any packets. I just need FIFO.

I would just need to configure the SSR per interface.  the example given:

Switch(config)# interface gigabitethernet2/0/1

Switch(config-if)# srr-queue bandwidth shape 8 0 0 0

And the bandwidth would be 1/Weight.  But what would the weight be in this case?  Wouldn't all the packets have the same weight?  Would I have to configure the other three queues?

[Joseph W. Doherty · Accepted solution]

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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.

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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

1/weight = percentage

The queue is shaped at that rate.

You need to configure whatever queues you intend to use.  (Remember, if using more than one, for shaping, be careful of the aggregate shaped bandwidth usage.) 

3560/3750 has default CoS/DSCP mappings to queue.  If all your traffic has same ToS markings, only one queue will be used.

[thomasjkuhn]

So for a 100Mbit port, the queue shape configuration for 2400bps would be:

Switch(config-if)# srr-queue bandwidth shape 43690 0 0 0

104857600(100Mbit Port) / 2400 = 43690?

Actually I just found you can set the port to 10Mbps, so the value could actually be 4370.

Am I on the right track?

[Joseph W. Doherty · Accepted solution]

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

100 Mbps / 2400 = 41666 (rounded down on my Windows' calculator)