Users Guide

• When DCB is disabled on an interface, PFC, ETS, and DCBX are also disabled.
• When DCBX protocol packets are received, interfaces automatically enable DCB and disable link level ow control.
• DCB is required for PFC, ETS, DCBX, and FCoE initialization protocol (FIP) snooping to operate.
Link-level ow control uses Ethernet pause frames to signal the other end of the connection to pause data transmission for a certain
amount of time as specied in the frame. Ethernet pause frames allow for a temporary stop in data transmission. A situation may arise
where a sending device may transmit data faster than a destination device can accept it. The destination sends a pause frame back to the
source, stopping the sender’s transmission for a period of time.
The globally assigned 48-bit Multicast address 01-80-C2-00-00-01 is used to send and receive pause frames. To allow full duplex ow
control, stations implementing the pause operation instruct the MAC to enable reception of frames with a destination address equal to this
multicast address.
The pause frame is dened by IEEE 802.3x and uses MAC Control frames to carry the pause commands. Ethernet pause frames are
supported on full duplex only. The only conguration applicable to half duplex ports is rx o tx o.
Note that if a port is over-subscribed, Ethernet Pause Frame ow control does not ensure no loss behavior.
The following error message appears when trying to enable ow control when half duplex is already congured:
Can’t configure flowcontrol when half duplex is configure, config ignored.
The following error message appears when trying to enable half duplex and ow control conguration is on:
Can’t configure half duplex when flowcontrol is on, config ignored.
Enabling Pause Frames
Enable Ethernet pause frames ow control on all ports on a chassis. If not, the system may exhibit unpredictable behavior.
NOTE
: If you disable rx flow control, Dell Networking recommends rebooting the system.
The ow control sender and receiver must be on the same port-pipe. Flow control is not supported across dierent port-pipes. (also refer
to iSCSI Optimization: Operation).
NOTE
: After you disable DCB, if link-level ow control is not automatically enabled on an interface, to enable ow control,
manually shut down the interface (shutdown command) and re-enable it (no shutdown command).
To enable pause frames, use the following command.
• Control how the system responds to and generates 802.3x pause frames on 10 and 40 Gig ports.
INTERFACE mode
flowcontrol rx [off | on] tx [off | on] [negotiate]
• rx on: enter the keywords rx on to process the received ow control frames on this port.
• rx off: enter the keywords rx off to ignore the received ow control frames on this port.
• tx on: enter the keywords tx on to send control frames from this port to the connected device when a higher rate of trac is
received.
• tx off: enter the keywords tx off so that ow control frames are not sent from this port to the connected device when a
higher rate of trac is received.
• negotiate: enable pause-negotiation with the egress port of the peer device. If the negotiate command is not used, pause-
negotiation is disabled.
NOTE
: The default is rx o.
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Interfaces