MTU considerations

The Networking service uses the MTU of the underlying physical
network to calculate the MTU for virtual network components including
instance network interfaces. By default, it assumes a standard 1500-byte
MTU for the underlying physical network.

The Networking service only references the underlying physical
network MTU. Changing the underlying physical network device MTU
requires configuration of physical network devices such as switches and
routers.

Jumbo frames

The Networking service supports underlying physical networks using
jumbo frames and also enables instances to use jumbo frames minus any
overlay protocol overhead. For example, an underlying physical network
with a 9000-byte MTU yields a 8950-byte MTU for instances using a VXLAN
network with IPv4 endpoints. Using IPv6 endpoints for overlay networks
adds 20 bytes of overhead for any protocol.

The Networking service supports the following underlying physical
network architectures. Case 1 refers to the most common architecture. In
general, architectures should avoid cases 2 and 3.

Case 1

For typical underlying physical network architectures that implement
a single MTU value, you can leverage jumbo frames using two options, one
in the neutron.conf file and the other in the
ml2_conf.ini file. Most environments should use this
configuration.

For example, referencing an underlying physical network with a
9000-byte MTU:

1. In the neutron.conf file:

   [DEFAULT]
   global_physnet_mtu = 9000

2. In the ml2_conf.ini file:

   [ml2]
   path_mtu = 9000

Case 2

Some underlying physical network architectures contain multiple
layer-2 networks with different MTU values. You can configure each flat
or VLAN provider network in the bridge or interface mapping options of
the layer-2 agent to reference a unique MTU value.

For example, referencing a 4000-byte MTU for provider2,
a 1500-byte MTU for provider3, and a 9000-byte MTU for
other networks using the Open vSwitch agent:

1. In the neutron.conf file:

   [DEFAULT]
   global_physnet_mtu = 9000

2. In the openvswitch_agent.ini file:

   [ovs]
   bridge_mappings = provider1:eth1,provider2:eth2,provider3:eth3

3. In the ml2_conf.ini file:

   [ml2]
   physical_network_mtus = provider2:4000,provider3:1500
   path_mtu = 9000

Case 3

Some underlying physical network architectures contain a unique
layer-2 network for overlay networks using protocols such as VXLAN and
GRE.

For example, referencing a 4000-byte MTU for overlay networks and a
9000-byte MTU for other networks:

1. In the neutron.conf file:

   [DEFAULT]
   global_physnet_mtu = 9000

2. In the ml2_conf.ini file:

   [ml2]
   path_mtu = 4000

   Note

   Other networks including provider networks and flat or VLAN
   self-service networks assume the value of the
   global_physnet_mtu option.

Instance network interfaces
(VIFs)

The DHCP agent provides an appropriate MTU value to instances using
IPv4, while the L3 agent provides an appropriate MTU value to instances
using IPv6. IPv6 uses RA via the L3 agent because the DHCP agent only
supports IPv4. Instances using IPv4 and IPv6 should obtain the same MTU
value regardless of method.

Networks with enabled
vlan transparency

In case of networks with enabled vlan transparency, if additional
vlan tag is configured inside guest VM, MTU has to be lowered by 4 bytes
to make space for additional vlan tag in the packet’s header. For
example, if network’s MTU is set to 1500, value configured
for the interfaces in the guest vm should be manually set to
1496 or less bytes.