Ethernet Switches & Routers

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Contributor
Posts: 50
Registered: ‎01-07-2011

Flow Control

Can someone explain to me why Foundry/Brocade has Flow Control enabled by default???? I am having a hard time coming up with a solid reason for this. If I need Flow Control on my network, I will enable it where I need it.

I also had two PC's that would not link up after turning Flow Control off on a Brocade FWS they were connecting to. These same PCs were connected to a Dell switch (Flow Control is disabled by default) and had no issues.

Lastly, why does disabling Flow Control bounce the interface?

Thanks,

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: Flow Control

From the manual.

Flow control (802.3x) is a QoS mechanism created to manage the flow of data between two full-duplex Ethernet devices. Specifically, a device that is oversubscribed (is receiving more traffic than it can handle) sends an 802.3x PAUSE frame to its link partner to temporarily reduce the amount of data the link partner is transmitting. Without flow control, buffers would overflow, packets would be dropped, and data retransmission would be required.

All FastIron devices support asymmetric flow control, meaning they can receive PAUSE frames but  cannot transmit them. In addition, FCX devices also support symmetric flow control, meaning they  can both receive and transmit 802.3x PAUSE frames. For details about symmetric flow control, refer to “Configuring symmetric flow control on FCX devices” on page 45.

I can not find the default stat for flow control - though for FSX1600 and Turboiron I know they are off be default.

FWS *MAY* have it on be default as it is asymmetric - mean it does not send PAUSE frames so would need the client or other switch to send the frames.

Manual also states the enableing or disabling fkiw control will bounce the port - I do not know why but it is the expected result of using the commands.

Check the speed duplex setting for the FCX when you have turned off flow control as it looks like flow control plays with these settings.

Thanks

Michael.

Contributor
Posts: 50
Registered: ‎01-07-2011

Re: Flow Control

I still don’t want it on by default. I don’t know of another vendor that does that. If fact I know when Brocade does performance testing they will turn this “feature” off.

And I can also verify turning flow control off does bounce the interface. This is yet another thing I do not understand…

Occasional Contributor
Posts: 7
Registered: ‎12-22-2009

Re: Flow Control

I don’t know of another vendor that is disabling Flow Control by default.

The reason for the bouncing interface may be autonegotiation. Flow Control is related to a link (two ports). If you disable Flow Control how is the other side notified about it? Flow Control capabilities are part of the autonegotiation function.

Contributor
Posts: 50
Registered: ‎01-07-2011

Re: Flow Control

It is disabled by default on Cisco switches and auto neg works just fine.

Occasional Contributor
Posts: 7
Registered: ‎12-22-2009

Re: Flow Control

The default for a 2950 is for 10/100/1000 Mbps ports is receive off and send desired.

And I dont understand your "and auto neg works just fine" point. I only tried to tell you that changing the port capabilites leads to a renegotiation of the link.

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