From 7e7d5dc8613d14aba50d74343deb841bc1faf5e7 Mon Sep 17 00:00:00 2001
From: "Sean M. Collins" <sean@coreitpro.com>
Date: Mon, 4 Jan 2016 16:14:39 -0800
Subject: [PATCH] Make Neutron attempt to advertise MTUs by default

It seems like a useful option, and it could be a more sane default,
rather than pushing the responsibility to enable this onto deployers and
provisioning tools[1]. In fact, it may be worth doing some more work to
take away some of the hassle of configuring all these things correctly,
and see if it can be automatically determined.

[1]: https://specs.openstack.org/openstack/fuel-specs/specs/7.0/jumbo-frames-between-instances.html

Related-Bug: #1527675
Change-Id: I5cbbc4660f8c4e15e59f8f5ce0419501bdd27348
---
 neutron/common/config.py                         |  9 +++++----
 ...dvertise_mtu_by_default-d8b0b056a74517b8.yaml | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)
 create mode 100644 releasenotes/notes/advertise_mtu_by_default-d8b0b056a74517b8.yaml

diff --git a/neutron/common/config.py b/neutron/common/config.py
index 64911f156fe..c2bba5a87c6 100644
--- a/neutron/common/config.py
+++ b/neutron/common/config.py
@@ -153,10 +153,11 @@ core_opts = [
     cfg.IntOpt('send_events_interval', default=2,
                help=_('Number of seconds between sending events to nova if '
                       'there are any events to send.')),
-    cfg.BoolOpt('advertise_mtu', default=False,
-                help=_('If True, effort is made to advertise MTU settings '
-                       'to VMs via network methods (DHCP and RA MTU options) '
-                       'when the network\'s preferred MTU is known.')),
+    cfg.BoolOpt('advertise_mtu', default=True,
+                help=_('If True, advertise network MTU values if core plugin '
+                       'calculates them. Currently, the only way to advertise '
+                       'MTU to running instances is using corresponding DHCP '
+                       'option.')),
     cfg.StrOpt('ipam_driver',
                help=_("Neutron IPAM (IP address management) driver to use. "
                       "If ipam_driver is not set (default behavior), no IPAM "
diff --git a/releasenotes/notes/advertise_mtu_by_default-d8b0b056a74517b8.yaml b/releasenotes/notes/advertise_mtu_by_default-d8b0b056a74517b8.yaml
new file mode 100644
index 00000000000..1f88901abee
--- /dev/null
+++ b/releasenotes/notes/advertise_mtu_by_default-d8b0b056a74517b8.yaml
@@ -0,0 +1,16 @@
+---
+features:
+  - By default, the DHCP agent provides a network MTU value to instances using
+    the corresponding DHCP option if core plugin calculates the value. For ML2
+    plugin, calculation mechanism is enabled by setting [ml2] path_mtu option
+    to a value greater than zero.
+upgrade:
+  - To disable, use [DEFAULT] advertise_mtu = False.
+other:
+  - For overlay networks managed by ML2 core plugin, the calculation algorithm
+    subtracts the overlay protocol overhead from the value of [ml2] path_mtu.
+    The DHCP agent provides the resulting (smaller) MTU to instances using
+    overlay networks.
+  - The [DEFAULT] advertise_mtu option must contain a consistent value on all
+    hosts running the DHCP agent.
+  - Typical networks can use [ml2] path_mtu = 1500.