Administrator Guide

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 594 to 9216, with a default of 1554 IP MTU automatically configures.
The following table lists the various Layer 2 overheads in the Dell Networking OS and the number of bytes.
Table 39. 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
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 configured on the
channel members.
For example, if the members have a link MTU of 2100 and an IP MTU 2000, the port channel’s MTU values cannot be higher
than 2100 for link MTU or 2000 bytes for IP MTU.
VLANs:
● All members of a VLAN must have the same IP MTU value.
● Members can have different Link MTU values. Tagged members must have a link MTU 4–bytes higher than untagged
members to account for the packet tag.
● The VLAN link MTU and IP MTU must be less than or equal to the link MTU and IP MTU values configured on the VLAN
members.
For example, the VLAN contains tagged members with Link MTU of 1522 and IP MTU of 1500 and untagged members with Link
MTU of 1518 and IP MTU of 1500. The VLAN’s Link MTU cannot be higher than 1518 bytes and its IP MTU cannot be higher
than 1500 bytes.
Auto-Negotiation on Ethernet Interfaces
By default, auto-negotiation of speed and duplex mode is enabled on 10/100/1000 Base-T Ethernet interfaces. Only 10GE
interfaces do not support auto-negotiation.
When using 10GE interfaces, verify that the settings on the connecting devices are set to no auto-negotiation.
The local interface and the directly connected remote interface must have the same setting, and auto-negotiation is the easiest
way to accomplish that, as long as the remote interface is capable of auto-negotiation.
NOTE:
As a best practice, Dell Networking recommends keeping auto-negotiation enabled. Only disable auto-negotiation on
switch ports that attach to devices not capable of supporting negotiation or where connectivity issues arise from
interoperability issues.
For 10/100/1000 Ethernet interfaces, the negotiation auto command is tied to the speed command. Auto-negotiation is
always enabled when the speed command is set to 1000 or auto.
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Interfaces