Users Guide

Table 37. 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.
Set Auto-Negotiation Options
The negotiation auto command provides a mode option for configuring an individual port to forced master/ forced slave
once auto-negotiation is enabled.
CAUTION: Ensure that only one end of the node is configured as forced-master and the other is configured as forced-
slave. If both are configured the same (that is, both as forced-master or both as forced-slave), the show interface
command flaps between an auto-neg-error and forced-master/slave states.
Interfaces 450