Service Manual

Enabling Pause Frames
Enable Ethernet pause frames flow control on all ports on a chassis or a line card. If not, the system may
exhibit unpredictable behavior.
NOTE: Changes in the flow-control values may not be reflected 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-config of the port.
NOTE: If you disable rx flow control, Dell Networking recommends rebooting the system.
The flow control sender and receiver must be on the same port-pipe. Flow control is not supported
across different 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 flow control frames on this port.
– rx off: enter the keywords rx off to ignore the received flow 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 traffic is received.
– tx off: enter the keywords tx off so that flow control frames are not sent from this port to the
connected device when a higher rate of traffic 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.
Configure the MTU Size on an Interface
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 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 configures.
The following table lists the various Layer 2 overheads found in Dell Networking OS and the number of
bytes.
Table 34. Layer 2 Overhead
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
450
Interfaces