Fast Downloads but slow uploads – Check your duplex settings

We were noticing a lot of backup jobs offsite were taking longer than normal to complete.  We opened up a ticket with our ISP and of course the ISP says nothing was wrong.  Fortunately we keep detailed change requests in cases like this and we knew no changes were made on our end. Adding to this confusion was a lot of noise such as delayed packets and in some cases we were seeing packet-loss, being this was a DR site this wasn’t a major issue but something we were monitoring nevertheless. After working with the ISP for several hours over the course of a few days we finally got word that in order to resolve a previous issue effecting other clients a change was made to our demarcation hand-off.

Issue:

  • Our internet hand-off is a Gigabit copper connection and knowing we were only purchasing 100MB the ISP artificially limited the Gigabit connection to 100MB.  Generally this is a no-no and used to be used aggressively but network administrators back in the early 1990s.  First there a few problems that happened here.

Solution:

  • Because the ISP set the speed to 100MB our Auto Negotiate port broke and was set to 100MB Duplex Half.  Once we set our hand-off to 100MB duplex full as like the ISP did on their side we were seeing the correct download and upload speeds.  This could of been avoided if our ISP had told us they were going to be upgrading their core to 1GB.

Other Notes:

  • The end result is a connection that is working but performs extremely poorly because of the duplex mismatch
  • Fiber connections do not have these types of options.

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