netplan: fix mtu if provided by network config for all rendered types

When network configuration for any interface defines maximum transmission
values (MTU) the netplan, eni and sysconfig renders will take into account
any device-level, or subnet-level mtu values.

When network configuration has conflicting device-level and ipv4 subnet
mtu values, the subnet-specific value is honored and a warning will be
logged about any ignored device-level setting.

LP: #1774666

2 files changed, 33 insertions, 0 deletions

diff --git a/doc/rtd/topics/network-config-format-v1.rst b/doc/rtd/topics/network-config-format-v1.rst
index 2f8ab54c..3b0148ca 100644
--- a/doc/rtd/topics/network-config-format-v1.rst
+++ b/doc/rtd/topics/network-config-format-v1.rst
@@ -130,6 +130,18 @@ the bond interfaces.
 The ``bond_interfaces`` key accepts a list of network device ``name`` values
 from the configuration.  This list may be empty.
 
+**mtu**: *<MTU SizeBytes>*
+
+The MTU key represents a device's Maximum Transmission Unit, the largest size
+packet or frame, specified in octets (eight-bit bytes), that can be sent in a
+packet- or frame-based network.  Specifying ``mtu`` is optional.
+
+.. note::
+
+  The possible supported values of a device's MTU is not available at
+  configuration time.  It's possible to specify a value too large or to
+  small for a device and may be ignored by the device.
+
 **params**:  *<Dictionary of key: value bonding parameter pairs>*
 
 The ``params`` key in a bond holds a dictionary of bonding parameters.
@@ -268,6 +280,21 @@ Type ``vlan`` requires the following keys:
 - ``vlan_link``: Specify the underlying link via its ``name``.
 - ``vlan_id``: Specify the VLAN numeric id.
 
+The following optional keys are supported:
+
+**mtu**: *<MTU SizeBytes>*
+
+The MTU key represents a device's Maximum Transmission Unit, the largest size
+packet or frame, specified in octets (eight-bit bytes), that can be sent in a
+packet- or frame-based network.  Specifying ``mtu`` is optional.
+
+.. note::
+
+  The possible supported values of a device's MTU is not available at
+  configuration time.  It's possible to specify a value too large or to
+  small for a device and may be ignored by the device.
+
+
 **VLAN Example**::
 
    network:
diff --git a/doc/rtd/topics/network-config-format-v2.rst b/doc/rtd/topics/network-config-format-v2.rst
index 335d236a..ea370ef5 100644
--- a/doc/rtd/topics/network-config-format-v2.rst
+++ b/doc/rtd/topics/network-config-format-v2.rst
@@ -174,6 +174,12 @@ recognized by ``inet_pton(3)``
 Example for IPv4: ``gateway4: 172.16.0.1``
 Example for IPv6: ``gateway6: 2001:4::1``
 
+**mtu**: *<MTU SizeBytes>*
+
+The MTU key represents a device's Maximum Transmission Unit, the largest size
+packet or frame, specified in octets (eight-bit bytes), that can be sent in a
+packet- or frame-based network.  Specifying ``mtu`` is optional.
+
 **nameservers**: *<(mapping)>*
 
 Set DNS servers and search domains, for manual address configuration. There