Reference Guide

472 | Interfaces
www.dell.com | support.dell.com
Configure 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 592-12000, with a default of 1500. On the E-Series, the user must enter the ip mtu
command to manually configure the IP MTU to compensate for the Layer 2 header. The C-Series and
S-Series automatically configure the IP MTU.
Table 22-6 lists the various Layer 2 overheads found in FTOS and the number of 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.
Parameters:
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.
threshold (C-Series and S-Series only): When tx on is configured, the
user can set the threshold values for:
Number of flow-control packet pointers: 1-2047 (default = 75)
Flow-control buffer threshold in KB: 1-2013 (default = 49KB)
Flow-control discard threshold in KB: 1-2013 (default= 75KB)
Pause control is triggered when either the flow control buffer threshold
or flow control packet pointer threshold is reached.
Table 22-6. 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
Command Syntax Command Mode Purpose