Service Manual

Compensate for Layer 2 header when configuring link MTU on an Ethernet interface or
Dell Networking OS may not fragment packets. If the packet includes a 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 — set the link MTU for an Ethernet interface.
IPv4 Routing 986