Administrator Guide

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 the Ethernet 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 following table lists the various Layer 2 overheads found in the Dell Networking OS and the number of bytes.
Table 24. 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:
• All members must have the same link MTU value and the same IP MTU value.
• The port channel link MTU and IP MTU must be less than or equal to the link MTU and IP MTU values congured on the channel
members.
Interfaces
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