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Gig DWDM connection between 2 x 3750's

We have a customer that has a gig DWDM connection between 2 cat3750's. The interfaces are SMF.

He is complaining of throughput issues. He has a pair of AIX servers transfering a file between them using we believe RPC (not 100% sure). If he does it across the gig link he gets 300mbps, if he does it across a single switch or across two switches locally connected he gets 700mbps. I have checked the cfgs, and fibre connections and interface errors. they have also used spare 3750's to do the test again. Not sure if the transfer suffers in both directions. They are not using oversized frames. We have not replaced the GBIC's yet.

The links have been tested by ISP, and show now issues, the tets results show now latency. The 3750 code is SEC.

Any ideas


Re: Gig DWDM connection between 2 x 3750's

Is MLS QoS enabled on the switch by any chance?

I expirienced trouble with 3750 using 1000-BASE-SX and 1000-BASE-LH lasers that gave poor peformance with file transfers.

This was the case on switches that had MLS QoS enabled for IP phones.

The symptoms are described in bug toolkit CSCeg29704. There is a workaround. Upgrading to 12.2(25)SED or higher resolves the problem.


Release Notes

After enabling QOS on 3750 and 3560 switches, certain application (mostly bursty

and TCP based) experience significant performance degradation due to unexpected

packet drops on some of the egress queues.

This is due to initial default egress queue threshold settings

(when qos enabled) not optimized for this type of traffic pattern.

This initial default queue threshold settings (when qos enabled)

thus need to be changed to accommodate these traffic.


Tune the egress queue thresholds parameters to

allocate more to the affected queues.

Specifically, egress queue 2 thresholds need to have the following settings:

Thresholds1 = 200

Thresholds2 = 200

Reserved = 50

Maximum = 400


mls qos queue-set output 1 threshold 2 200 200 50 400

mls qos queue-set output 2 threshold 2 200 200 50 400



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Community Member

Re: Gig DWDM connection between 2 x 3750's

Sorry, think it should be RCP.

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