Open Stack

We have 3 node cluster on Mikata openstack and we are using DVR
network. Recently we build two puppetmaster server on openstack and
put them behind Lbaasv2 load-balancer to share load but i found my
none of client able to talk to correctly with puppetmaster server on
openstack. After lots of research found openstack VM use mtu 1400 and
my rest of puppet agent server who are not on openstack they used mtu
1500.

as soon as i change puppet agent MTU size to 1400 everything started
working. But just surprised why openstack use 1400 for VM there must
be a reason like vxlan or GRE.

So for experiment i change mtu 1500 on puppetmaster server on
openstack but it didn't help. How do i fix this issue?