Reference Guide

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 flow 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 defined 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 configuration applicable to half duplex
ports is rx off tx off.
Note that if a port is over-subscribed, Ethernet Pause Frame flow control does not ensure no loss
behavior.
The following error message appears when trying to enable flow control when half duplex is already
configured:
Can’t configure flowcontrol when half duplex is configure, config ignored.
The following error message appears when trying to enable half duplex and flow control configuration is
on:
Can’t configure half duplex when flowcontrol is on, config ignored.
MTU Size
The Aggregator auto-configures interfaces to use a maximum MTU size of 12,000 bytes.
If a packet includes a Layer 2 header, the difference 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 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 592-12000, with a default of 1554.
The table below lists out the various Layer 2 overheads found in Dell Networking OS and the number of
bytes.
Difference between Link MTU and IP MTU
Layer 2 Overhead Difference 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.
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Interfaces