Set DEFAULT_NETWORK_MTU to 1500 and use it

No more magic numbers.

Change-Id: I5f557c005434e4b922b692ecdffa0b9c4255a1cd

neutron/common/config.py

@@ -28,6 +28,7 @@ from oslo_service import wsgi
 
 from neutron._i18n import _, _LI
 from neutron.api.v2 import attributes
+from neutron.common import constants
 from neutron.common import utils
 from neutron import policy
 from neutron import version
@@ -176,7 +177,7 @@ core_opts = [
                help=_("This will choose the web framework in which to run "
                       "the Neutron API server. 'pecan' is a new experiemental "
                       "rewrite of the API server.")),
-    cfg.IntOpt('global_physnet_mtu', default=1500,
+    cfg.IntOpt('global_physnet_mtu', default=constants.DEFAULT_NETWORK_MTU,
                deprecated_name='segment_mtu', deprecated_group='ml2',
                help=_('MTU of the underlying physical network. Neutron uses '
                       'this value to calculate MTU for all virtual network '

neutron/common/constants.py

@@ -191,7 +191,7 @@ RPC_NAMESPACE_STATE = None
 RPC_NAMESPACE_RESOURCES = None
 
 # Default network MTU value when not configured
-DEFAULT_NETWORK_MTU = 0
+DEFAULT_NETWORK_MTU = 1500
 IPV6_MIN_MTU = 1280
 
 ROUTER_MARK_MASK = "0xffff"

neutron/plugins/ml2/config.py

@@ -16,6 +16,7 @@
 from oslo_config import cfg
 
 from neutron._i18n import _
+from neutron.common import constants
 
 ml2_opts = [
     cfg.ListOpt('type_drivers',
@@ -39,7 +40,7 @@ ml2_opts = [
                        "entrypoints to be loaded from the "
                        "neutron.ml2.extension_drivers namespace. "
                        "For example: extension_drivers = port_security,qos")),
-    cfg.IntOpt('path_mtu', default=1500,
+    cfg.IntOpt('path_mtu', default=constants.DEFAULT_NETWORK_MTU,
                help=_('Maximum size of an IP packet (MTU) that can traverse '
                       'the underlying physical network infrastructure without '
                       'fragmentation. For instances using a '