Jumbo frames between instances using mtu-selection-and-advertisement feature


This blueprint describes a way to use Jumbo frames between instances using mtu-selection-and-advertisement[1] openstack kilo feature.

Problem description

For network providers it may make sense to control(increase) mtu value in openstack environment. Increasing the frame size makes a certain large amount of data transferable with less effort, reducing CPU utilization (mostly due to interrupt reduction) and increasing throughput by reducing the number of frames needing processing and reducing the total overhead byte count of all the frames sent.

Currently, we are already supporting Jumbo frames between instances using ‘network_device_mtu’ nova & neutron configuration options and mtu value is based on interface’s mtu value what assigned to private network. This mtu value will be default value for all cluster and private networks if we have it more then one. mtu-selection-and-advertisement feature implementation is supposed to support separate mtu configuration for each private network. Fuel doesn’t support multiple private networks right now and we are not going to implement it in scope of 7.0 release, but, I’m thinking that this feature may be in the scope of 8.0 and later release. So, it may be useful to switch our Jumbo frames between instances approach from current implementation to implementation using mtu-selection-and-advertisement preliminarily.

Proposed change

I propose to replace our current Jumbo frames between instances implementation with implementation using mtu-selection-and-advertisement. These changes will touch only puppet part and do not require any changes in nailgun side. Currently, we have way to calculate mtu value for private network in case if we have gre traffic. These calculations are not obligatory when we switch to mtu-selection-and-advertisement solution, cause neutron performing it itself. But, we should not wide out these calculations cause we should use it to control mtu of dhcp/l3 neutron agents tap interfaces (cause current feature implementation do not allow to do it). Also, we should extend current calculation process to properly calculate mtu for vxlan environments. Remaining to move mtu value for private network (mtu value of interface what assigned to private network) to the appropriated configuration files.

Implementation will be include following steps:

  • fix mtu calculations to support vxlan environments
  • to enable mtu-selection-and-advertisement feature set in neutron.conf file on controller nodes: advertise_mtu = True
  • to advertise proper mtu using neutron’s L2 mechanism drivers(VLAN) to VMs set in ml2_conf.ini file on all nodes: physical_network_mtus = physnetX:private_net_mtu
  • to advertise proper mtu using neutron’s L3 mechanism drivers(GRE) set in ml2_conf.ini file on all nodes: path_mtu = private_net_mtu
  • keep ‘network_device_mtu = 65000’ in nova.conf configuration files on compute nodes to set appropriated mtu for veth devices
  • keep ‘network_device_mtu = $calculated_mtu’ option in neutron.conf file on controller nodes to avoid traffic’s fragmentation on virtual interfaces what are placed in namespaces ( dhcp/l3 agents tap interfaces)



Data model impact


REST API impact


Upgrade impact


Security impact


Notifications impact


Other end user impact


Performance Impact

Network bandwidth between instances will be significantly increased.

Plugin impact


Other deployer impact

Openstack Kilo feature will be used.

Developer impact

Feature may require additional bugfixing and improvements during testing procedure.

Infrastructure impact




Primary assignee:
Valyavskiy Viacheslav <slava-val-al>

Work Items

  • Fix puppet to configure mtu between instances using mtu-selection-and-advertisement feature

Acceptance criteria

User is able to use mtu-selection-and-advertisement feature(in clusters where network provider is neutron). Feature will be switched on automatically during the deployment process and its parameters will be based on node’s private interface mtu value.


Devops tool should be extended to prepare test environment with custom mtu values for the virtual bridges to test passing of jumbo frames between instances.

Documentation Impact

Ability to change mtu values between instances should be documented in Fuel Deployment Guide.