[18.08] data-port + veth + linux bridge handling does not set up MTU properly

Bug #1791101 reported by Chris MacNaughton on 2018-09-06
16
This bug affects 2 people
Affects Status Importance Assigned to Milestone
Charm Helpers
Medium
Unassigned
OpenStack neutron-gateway charm
Medium
Unassigned
OpenStack neutron-openvswitch charm
Medium
Unassigned

Bug Description

The file created to connect the veth to the bridge in https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/network/ovs/__init__.py#L35-L40 needs additional lines to deal with networks using differing MTUs. On a jumbo-frame network, the guests connected through the gateway will be unable to receive large packets without adding in:

    pre-up ip link set veth-br0 mtu 9000
    pre-up ip link set veth-br-ex mtu 9000
    pre-up ip link set br0 mtu 9000

Note: A linux bridge interface MTU is set to the lowest MTU setting of any interface that is a member of that bridge. veth interfaces get 1500 setting from the linux kernel if not set explicitly which causes the linux bridge with MTU setting of 9000 to drop MTU to 1500.

tags: added: cpe-onsite
description: updated
summary: - When attaching neutron-gateway to abridge onto a jumbo-framed network,
- the veth is 1500
+ [18.08] data-port + veth + linux bridge handling does not set up MTU
+ properly
James Page (james-page) wrote :

Note that bridge adopts the lowest MTU of all connected devices, so does not need to be set.

Changed in charm-helpers:
status: New → Triaged
Changed in charm-neutron-gateway:
status: New → Triaged
Changed in charm-neutron-openvswitch:
status: New → Triaged
Changed in charm-helpers:
importance: Undecided → Medium
Changed in charm-neutron-gateway:
importance: Undecided → Medium
Changed in charm-neutron-openvswitch:
importance: Undecided → Medium

The default MTU on a veth is 1500 - so the moment we assign one to the bridge, the bridge goes down to 1500, rather than the veth taking the higher MTU from the bridge

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