3560 TX-queue buffer-sizes

[ArashAlizadeh92]

Hi,

We're having some issues with a 3560 CPE. It's uplink is a GE fiber link, customer port is FE RJ45.

We see a lot of TX-frames being dropped at the FE port, but none at the GE port. Even when the customer is only at ~50Mbit/s downstream traffic.

When customer sending ~50Mbit/s upstream, there are no TX-drops at the GE link.

Is this a normal behaviour? From what I know the physical medium shouldn't have any inpact on this since drops occurs in the port-asic, and not in physical transmission.

Do the buffer-sizes between GE and FE differ? What could we do to optimize the flow and reduce drops? QoS is set to off and no modifications on the queues have been made on the interfaces.

[Joseph W. Doherty]

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Posting

50 Mbps across a FE interface; suspect you've bursting which causes drops, enough of which will bring your average transmission rate down.

If QoS is truly disabled, not much you can do to tune.

If possible, move the host to copper gig.  Or, if traffic is TCP, you might reduce the host's RWIN.

[JohnTylerPearce]

Arash,

Is the customer port FE going into their infrastructure and the uplink from the CPE device 1Gbps? Also if this is correct they are being rate limited to 50Mbps right?




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[ArashAlizadeh92]

Hi John,

Infact, we dont use any shaping in the switch. Instead we do policeing in both directions at the PE.

If you're suggesting to use srr, please explain why and maybe give a cli example.

[Joseph W. Doherty]

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

With policing, it's very easy to drop packets out of bursts.  As to why it can be so different based on direction, traffic coming from a gig interface can overrun a policer setting easier than coming from a FE interface.

[JohnTylerPearce]

Arash,

I just want to get a clear understanding on what's going on. On the CPE (3560), A FE port is going to the customers network infrastructure and. 1Gbps link is going into the SP network, is this the case?


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[ArashAlizadeh92]

Yes this is the case indeed John.

[JohnTylerPearce]

I'm assuming that the policer is configured on the FE interface for 50Mbps? Do you know what speed the port is on the customer's equipment heading to the CPE FE link? If you have a policer configure (which is bursty by nature),  and the port on the customer's equipment is 1Gbps, or 10Gbps, it's going to cause lots of traffic to be dropped.

[ArashAlizadeh92]

Hi,

There is no policer in the CPE. Customer facing link is 100/full.

Policeing is made at the PE router and is set to 100Mbit/s. Customer FE link starts to drop even when there is around 50Mbit/s of downstream traffic.