Administrator Guide

Layer 2 header, the difference between the link MTU and IP MTU (the ip mtu
command) must be enough bytes to include for the Layer 2 header.
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 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.
The following describes the difference between Link MTU and IP MTU.
Layer 2 Overhead Difference between Link MTU and IP MTU
Ethernet
(untagged)
18 bytes
VLAN Tag Tag 22 bytes
Untagged Packet
with VLAN-Stack
Header
22 bytes
Tagged Packet
with VLAN-Stack
Header
26 bytes
Related
Commands
mtu — sets the link MTU for an Ethernet interface.
IPv4 Routing
747