MTUs in neutron

I’m not overly familiar with neutron but it appears that in the past correctly setting MTU values was easier said than done. This has been massively improved in recent releases, but getting us to this point required a lot of work and, in particular, a lot of changes to the MTU-related configuration options available over the course of Liberty and Newton. The below table tries to collect these changes in a coherent table.

Configuration Option Version Commit Comments
segment_mtu Liberty - -
path_mtu Liberty - -
segment_mtu Mitaka b2d0985d Default set to 1500
global_physnet_mtu Mitaka ae45cd57 New name for segment_mtu
path_mtu Mitaka 7a4633a9 Default set to 1500
global_physnet_mtu Newton - -
path_mtu Newton 38531ca5 Default set to 0

These options are described in the docs for both Mitaka and Newton.

In addition, it’s also worth noting that the default for global_physnet_mtu in TripleO was changed from 1400 to 1500 as part of bug 1590101.

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