Reference Guide

Table 31. 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.
Port-Pipes
A port pipe is a Dell Networking-specific term for the hardware path that packets follow through a system.
Port pipes travel through a collection of circuits (ASICs) built into line cards and RPMs on which various processing
events for the packets occur. One or two port pipes process traffic for a given set of physical interfaces or a port-set.
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.
NOTE: Starting with FTOS version 7.8.1.0, when you use a copper SFP2 module with catalog number GP-SFP2-1T in
the S25P model, you can manually set its speed with the speed command. When the speed is set to 10Mbps or
100Mbps, you can execute the duplex command.
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.
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