What is MTU; when should I change it in TR7?

Short answer

MTU (Maximum Transmission Unit) is the size of the largest IP packet that can be sent from an interface in a single frame; on Ethernet, the standard value is 1500 bytes (RFC 894), and larger values such as 9000 are called jumbo frames. A packet larger than the MTU of a link on the path is either fragmented or dropped. In TR7, the MTU is set with the MTU field in the Details section of the interface add and edit forms. If the field is left empty or 0 is entered, TR7 does not touch the MTU: a new interface is created with the system's default MTU, and an existing value is preserved. An MTU change is applied without the interface being recreated; no upper limit is enforced in the form. Changing the MTU is necessary if jumbo frames are used in the data center network or if the traffic passes through a tunnel such as VXLAN; the value must be consistent with the MTU of the parent interface, of the switch and of the devices on the path.

Where It Is Used in TR7

Where	Field / label	What for
Network → Interfaces → Add Interface / Edit Interface → Details (closed by default)	MTU	Setting the MTU value of the interface; present in every type, including ETH
Network → Interfaces → interface preview → Details	MTU row (per device, the value read from the system)	Seeing the running MTU of the interface
Left Monitor Panel → Interfaces → View Settings	Primary Metric → Static group → MTU	Listing the interfaces with the MTU value entered in the configuration
Background network job	MTU is being updated for '…': <old>-><new> message	Tracking that the MTU change is being applied
WEB-CLI	sys-cmd ip show address (ip show address)	Seeing the running MTU of the interfaces with the mtu value in the output

When Is It Changed?

Situation	What is done
The data center network uses jumbo frames end to end	The same large MTU (e.g. 9000) is entered on the interface (and on the Bond/LACP, or on the parent interface of the VLAN); the switch ports must also carry the same value
The traffic passes through a VXLAN tunnel	The VXLAN header adds 50 bytes per packet on an IPv4 underlay (RFC 7348); the MTU of the VXLAN interface is kept smaller than that of the underlying interface by this amount, or the MTU of the underlying interface is increased
Large packets are lost, small packets get through	An MTU mismatch on the path is looked for; blocking ICMP can break Path MTU Discovery (RFC 1191)
Standard Ethernet network	No change is needed; the field is left empty

Common Misconceptions

• "Increasing the MTU always makes things faster." If not all the devices on the path support the same large MTU, large packets are dropped.
• "Clearing the field returns the MTU to the default." An empty or 0 value does not change the MTU; the current value is preserved. For a different MTU, a new value is entered.
• "An MTU change recreates the interface." The MTU is applied in place; the interface is not deleted and recreated (System behavior after saving).

How it is done in the documentation

• Add Interface · MTU
• Edit Interface · MTU
• Edit Interface · The System Behavior After Saving

Related questions

• What is VXLAN; how does it differ from VLAN?
• What is an interface; what is the difference between a physical (ETH) interface and a virtual interface?
• What are ICMP and ping; where are they used in TR7?