Configurable Default Network MTU in OSIE

Summary

Allow administrators to configure the default MTU used when creating new networks in OSIE, while still supporting higher MTU values for environments that run jumbo frames in the backend.

Current Behaviour

In our RHOSO environment we run a 9000 MTU backend fabric (configured via global_physnet_mtu).

Currently:

• OSIE does not allow MTU to be configured when creating a network

• When a network is created without specifying an MTU, it automatically inherits the maximum MTU (9000) from the backend

This results in all newly created networks defaulting to jumbo frames.

Problem

Running jumbo frames by default can introduce unnecessary complexity and compatibility issues for tenants and more importantly, the user is not able to reduce the MTU.

Many workloads and network integrations assume 1500 MTU.
While jumbo frames are valuable in certain cases (storage networks, high-performance workloads, etc.), enabling them by default across all tenant networks can lead to issues.

Desired Behaviour

Allow cloud administrators to configure a default MTU value for networks created via OSIE.

Example behaviour:

Backend fabric MTU:
9000

Default network MTU (OSIE setting):
1500

Allowed MTU range:
1500 → 9000

When a user creates a network in OSIE:

• If no MTU is specified → network is created with 1500 MTU

• Admins (or optionally users) can increase MTU up to the backend maximum

Optional Enhancement

In addition to setting a default MTU, it would be useful to have a toggle to control whether tenants can modify MTU when creating networks.

For example:

Admin settings:

Default network MTU: 1500
Allow tenant MTU override: Enabled / Disabled

This would allow providers to:

• Maintain safe defaults

• Still allow advanced users to enable jumbo frames when required

Thanks!