Reference Guide

IPv4 Routing | 699
Command
History
Usage
Information
When you enter no mtu command, FTOS reduces the ip mtu value to 1536 bytes. To
return the IP MTU value to the default, enter
no ip mtu.
You must compensate for Layer 2 header when configuring link MTU on an Ethernet
interface or FTOS may not fragment packets. If the packet includes a Layer 2 header,
the difference between the link MTU and IP MTU (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.
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.
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.
Version 8.3.19.0 Introduced on S4820T
Version 8.3.7.0 Introduced on S4810
Version 8.1.1.0 Introduced on E-Series ExaScale
pre-Version 6.1.1.0 Introduced for E-Series
Table 24-2. 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