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Throughput issue between a 3550 and a 6509

Hello guys, good evening.

Recently I've been experiencing a weird problem with one of my costumers.

Basically, there are users complaining about reduced throughput for a FTP file transfer. In a brief, I have isolated the problem between a 3550 and a 6509 using TTCP. When I run the ttcp with a 64k window size on the 3550, I get around 800kbp/s and about 15.000 to 18.000 retransmissions on the session. When I jump to the 6509 (which is directly connected to the 3550 with a fiber multi-mode SX connection) I'm getting about 250 kbp/s and about 55000 to 65000 retransmissions.

Some info:

3550 and 6509 are directly connected;

There are no duplex or speed mismatches on both sides;

There are no evidences of high utilization or interface errors on both sides (counter errors are clear);

There's no apparent packet loss, latency or jitter between the nodes;

I tried to remove all the QOS settings on the downstream interface and run the tests with no queueing features.... still the same result;

Etc... etc... etc....

I tried a bunch of things, but I still can't understand why the throughput is so different on a "directly connected" lan environment like this one.

Could anyone help me please?
I can provide any info/output that you guys may need.

Best regards!!!!

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