Switching in general

[jweber]

Hey guys,

I am a hardware engineer and we made a custom board with an integrated switch, but we need some help by switching in general. 

Our board works fine until we daisy chain our custom boards. That we have the problem that the device which  is switching is slowed down. 

For explanation: Our switch is connected to an ethernet modul and has two open ports, where you can connect your PC or another custom board. We see if we send 700 Mbit/s to our PC everything works fine. If we configure another custom board to send also 700 Mbit/s the connection slows down. Furthermore we have the same problem if we connect the ethernet module directly to an external switch (we tested different unmanaged switches from cisco and netgear). Also important: if we connect the two custom boards via an external ethernet switch everything works fine. 
We have seen in the registers of our custom board that the ethernet module and the switch have done autonegotiation and advise that they don't use flow control.
So my question is: Have switches problems with speed if they have to switch between devices with flow control and without flow control? Because the port of the switches advise on flow control. 

If you need further information I could try to explain and I hope that my problem gets clear.

Kind regards,
Jens

[jweber]

[MHM Cisco World]

what switch platform you use?

[jweber]

on our custom board we use an IC from Microchip, but that is hardware specific I guess. 

In the drawing I added, the two switches which are directly connected to the ZestETM1 are the ICs from Microchip. The switch that is connected to the both switches was an external switch. I tried Netgear GS108 and Cisco 110 Series and both setup works. 

But if I connect the two ZestETM1 directly to the Micochip IC or the Netgear GS108 or Cisco 110 Series all switches have the problem that the connection slows down. For me it is not important from which company the switch is  and maybe I'm wrong in this forum but I'm a very beginner in network and I try to understand in general what the problem could be.

If I disable flow control on the Microchip it works a bit better. But than the switch port is not working to capacity.

[Joseph W. Doherty]

"If you need further information I could try to explain and I hope that my problem gets clear."

After reading you OP, looking at your diagram, and your 2nd posting, it's still unclear to me what "works" and what doesn't.

In general, anytime ingress traffic aggregate bandwidth usage can exceed egress bandwidth capacity, traffic will be queued/buffered, but there's a finite amount of queuing/buffering resources.  When the latter is exceeded, traffic is dropped.  Depending on the "nature" of the traffic, and specifically how some of it has been dropped, overall transmission rate can be drastically reduced.

"Have switches problems with speed if they have to switch between devices with flow control and without flow control?"

Yes, they may.  The purpose of Ethernet flow control, when active, is for a receiver to inform the sender, pause sending me traffic (because, otherwise, I'm going to start to drop the traffic I cannot handle just now).

BTW, Ethernet flow control doesn't always mitigate an oversubscription issue, although it might "relocate" where the problem appears.

[jweber]

Yes thanks...I got reply from the manufactor of the ethernet module and the ethernet module does not support flow control. So we connected flow control and no-fc devices together. If you stack switches together and loop trough the ethernet module we get "pseudo" flow control for the ethernet module because there is flow control between the switches, so data can be paused and everything works fine. Thank you for helping me to understand the problem.