Service Manual

Changes in the ow-control values may not be reected automatically in show interface output. To display the change, apply
the new ow-control setting, shutdown the interface using the shutdown command, enable the interface using the no
shutdown
command, and use the show interface command to verify the changes.
Enabling Pause Frames
Enable Ethernet pause frames ow control on all ports on a chassis or a line card. If not, the system may exhibit unpredictable
behavior.
NOTE: Changes in the ow-control values may not be reected automatically in the show interface output. As a
workaround, apply the new settings, execute shut then no shut on the interface, and then check the running-cong
of the port.
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.
To enable pause frames, use the following command.
• Control how the system responds to and generates 802.3x pause frames on 1 and 10 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. 40 gigabit Ethernet interfaces do not support pause-negotiation.
Congure the MTU Size on an Interface
If a packet includes a Layer 2 header, the dierence in bytes between the link MTU and IP MTU must be enough to include the Layer
2 header.
For example, for VLAN packets, if the IP MTU is 1400, the Link MTU must be no less than 1422:
1400-byte IP MTU + 22-byte VLAN Tag = 1422-byte link MTU
The MTU range is from 592 to 12000, with a default of 1500. IP MTU automatically congures.
The following table lists the various Layer 2 overheads found in Dell Networking OS and the number of bytes.
Table 20. Layer 2 Overhead
Layer 2 Overhead Dierence Between Link MTU and IP MTU
Ethernet (untagged) 18 bytes
VLAN Tag 22 bytes
Untagged Packet with VLAN-Stack Header 22 bytes
Tagged Packet with VLAN-Stack Header 26 bytes
Link MTU and IP MTU considerations for port channels and VLANs are as follows.
Port Channels:
Interfaces
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