10g DAC slows client upload speed

[ely105]

I've noticed this problem for some time and haven't been able to figure it out.  I have an MS425 stack that is uplinked to a MX105 appliance.  The MX105 has a copper 1g uplink to Internet.  If I use a 1GBASET uplink from the MS425's to the MX my upload/download speed to internet is as expected 940/940.  But if I use a 10g DAC to connect the MS425's to the MX I get 940 download, but only about 500 upload (using speedtest).  I've tried multiple DAC cables.  Other than swapping the uplink cable/speed, everything else is the same.  Any reason why this is or how to solve?  i'd prefer to use the 10g DAC for uplink but not at internet throughput cost.

Thanks,

Mark

[Johnfnadez]

Have you checked if using another MTU solves this problem?

[PhilipDAth]

My bet - the 10Gb link is simply running the MX105 out of buffer space when it tries to squeeze it all into its 1Gb uplink.

This is a fairly common issue when running 10Gb traffic into 1Gb links (not just the Internet, also 1Gb clients attached to 10Gbe connected servers, especially 10Gbe connected NAS to 1Gbe connected servers).

[ely105]

I can understand this situation but it seems like the MX appliance should have some form of flow control?  i know it would only affect large flows of traffic in the 10g->1g direction but as access port speeds increase the devices should be a little more intelligent at this.  I mean i'd like to have my inter-vlan traffic processed at 10g, but in order to make sure my internet upload speed isn't impacted, I drop my switch uplink to 1g.  I know that not a lot of traffic saturates links so its only those few traffic types that might hit this and speedtest is not a real world app.  I need to see if I can test with dropbox, frame.io and any other service that might push the data rates on the upload side.

[cmr]

@ely105 does it affect real world traffic, or only speed testing?

[ely105]

if i'm overrunning the buffer on the MX then it's likely only speedtest that suffers this so far.  I need to try something like dropbox, frame.io or similar on trying to saturate the upload link and if it takes a hit on it as well.