Investigating a slow transmission issue at a site with 3 switches. A 200 Series Switch and a C2960G hang off the SG300-10 which is currently running in layer 3 mode with qos enabled but not routing. Currently no VoIP and all classification and marking has been disabled. Doing a data transfer exceeding 50MBs I get approx 300 tail drops per each file transfer. For testing I am using a 2GB file and sending from PC directly connected to SG300 on vlan 1 to a server hanging off the other switches. So layer 2 switched, without any routing. If I send to a fast server where I exceed 50mbps the tail drops start rising fast. If I send to a much slower device at approx 1/3 the speed no tail drops occur. I have enabled qos, disabled qos, while enabled changed the queue values in each direction and in no way can I get the tail drops to go away yet the SG200-18 never drops a single packet, nor does the 2960G compact model, only this SG300. I was going to leave qos disabled since currenlty not using markings but it changes nothing. I have tried giving as much bandwidth to queue 1 as possible with no changes since default qos or "0" goes there.
I am also curious in that the default QoS give the highest bandwidth by default to queue 4 the highest priority queue and the smallest to queue 1. this is backwards from what I am used to in the Enterprise IOS QoS VoIP world. Do these operate differently or are the values just reversed by default for some reason ?? Need help as word is to replace this switch today for not keeping up yet its statistics on the data sheet show it should far exceed whats needed and what other devices can do at 20Gbps bandwidth throughput and 14.88mpps. These are all 1500 byte frames, no jumbo, pretty generic, think of default switch with qos basic mode enabled, but then any tweaking and disabling of qos still sees packet tail drops between these devices at 50mbs.
Appears the tail drops are now gone yet performance is still high ! Hate the fact that this is the fix but none the less just wanted it fixed. Thanks for the input. I went ahead and put back to layer 2 mode as well since no routing is needed but I will continue testing to see if tail drops return under certain configs or circumstances. QoS is disabled but still first xfer I've had yet with zero drops on it.
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