Bug ID 766169: Replacing all VLAN interfaces resets VLAN MTU to a default value

Last Modified: Sep 13, 2023

Affected Product(s):
BIG-IP LTM(all modules)

Known Affected Versions:
12.1.0, 12.1.1, 12.1.2, 12.1.3,,,,,,,, 12.1.4,, 12.1.5,, 13.0.0, 13.0.0 HF1, 13.0.0 HF2, 13.0.0 HF3, 13.0.1, 13.1.0,,,,,,,,, 13.1.1,,,,, 13.1.3,,,,, 14.0.0,,,,,, 14.0.1,, 14.1.0,,,,,, 14.1.2,,,,,,,, 15.0.0, 15.0.1,,,,

Fixed In:

Opened: Mar 27, 2019

Severity: 3-Major


When the last physical interface is removed from a VLAN, VLAN's MTU is reset to default value of 1500. However when replacing all interfaces belonging to a VLAN with a new ones (e.g., with a command 'tmsh modify net vlan [name] interfaces replace-all-with {...}'), even if it does not look like removing all interfaces, it is actually done by removing all interfaces immediately followed by adding new ones. Because all interfaces are removed first, the VLAN MTU is reset to the default value even if the original value is perfectly valid for newly added interfaces. As it is done automatically inside TMM, it is not reflected in the configuration. TMSH and Configuration Utility continue to report original value.


VLAN MTU is set to 1500 despite Configuration Utility and TMSH still reporting original value.


Issue is visible only when removing or replacing all interfaces in a VLAN which has MTU value different than default 1500. Added interfaces must also have MTU values larger than 1500.


There are two workarounds: -- Reset desired MTU value after each operation of replacing all interfaces in a VLAN. -- Avoid replace-all-with operation by adding new interfaces before removing unneeded ones.

Fix Information

VLAN MTU value is left unchanged after the last interface is removed. It is recalculated upon adding a new interface anyway, so there is no risk it will be too large.

Behavior Change

Guides & references

K10134038: F5 Bug Tracker Filter Names and Tips