From 38531ca59ce7bff2752252238056378d74a5588e Mon Sep 17 00:00:00 2001
From: Ihar Hrachyshka <ihrachys@redhat.com>
Date: Thu, 21 Apr 2016 15:40:38 +0200
Subject: [PATCH] Revert "ML2: Configure path_mtu to default to 1500 bytes"

This reverts commit 7a4633a9ca213ee03cbcd45027189e0b3e7df2f5.

Revert to using 0 as the default value for path_mtu.

In most situations, underlying MTU does not differ for tunnel backed and
vlan/flat tenant networks, in which case the only configuration expected
from operators is setting global_physnet_mtu to the appropriate MTU
value as reflecting all data paths that tenant traffic may take between
nodes.

But with the non-zero value set for path_mtu, if an operator would like
to raise the global underlying MTU used by neutron to support Jumbo
frames, both global_physnet_mtu and path_mtu need a bump, which is not
ideal.

So switch back to using a zero value for path_mtu, effectively making it
not participating in MTU calculation, unless explicitly overridden.

Left the original release note intact since it reflects the state for
Mitaka.

Added a release note for the change.

Conflicts:
	neutron/plugins/ml2/config.py
	releasenotes/notes/1500-default-mtu-b0d6e4ab193b62a4.yaml

Change-Id: I97c4aa647efc85d7b6b45359e43e2a2ae2514a69
---
 neutron/plugins/ml2/config.py                            | 3 +--
 .../notes/path-mtu-back-to-zero-e4f9e8bdd8317ad4.yaml    | 9 +++++++++
 2 files changed, 10 insertions(+), 2 deletions(-)
 create mode 100644 releasenotes/notes/path-mtu-back-to-zero-e4f9e8bdd8317ad4.yaml

diff --git a/neutron/plugins/ml2/config.py b/neutron/plugins/ml2/config.py
index 8ea7cbd7004..4e2e43d8b0a 100644
--- a/neutron/plugins/ml2/config.py
+++ b/neutron/plugins/ml2/config.py
@@ -16,7 +16,6 @@
 from oslo_config import cfg
 
 from neutron._i18n import _
-from neutron.common import constants
 
 ml2_opts = [
     cfg.ListOpt('type_drivers',
@@ -40,7 +39,7 @@ 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=constants.DEFAULT_NETWORK_MTU,
+    cfg.IntOpt('path_mtu', default=0,
                help=_('Maximum size of an IP packet (MTU) that can traverse '
                       'the underlying physical network infrastructure without '
                       'fragmentation. For instances using a '
diff --git a/releasenotes/notes/path-mtu-back-to-zero-e4f9e8bdd8317ad4.yaml b/releasenotes/notes/path-mtu-back-to-zero-e4f9e8bdd8317ad4.yaml
new file mode 100644
index 00000000000..0815e71423b
--- /dev/null
+++ b/releasenotes/notes/path-mtu-back-to-zero-e4f9e8bdd8317ad4.yaml
@@ -0,0 +1,9 @@
+---
+upgrade:
+  - In case you rely on the default ML2 path_mtu value of 1500 to cap MTU used
+    for new network resources, please set it explicitly in your ml2_conf.ini
+    file.
+fixes:
+  - The default value for ML2 path_mtu option is changed from 1500 to 0,
+    effectively disabling its participation in network MTU calculation unless
+    it's overridden in the ml2_conf.ini configuration file.