when setting local_mtu > 1500 in undercloud.conf, undercloud install failed

[Bob Fournier]

If local_mtu in undercloud.conf is set to a value greater than 1500, the undercloud install will fail:

2018-05-22 11:24:19,743 DEBUG: An exception occurred
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 2336, in install
    _post_config(instack_env, upgrade)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 2002, in _post_config
    network = _ensure_neutron_network(sdk)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 2052, in _ensure_neutron_network
    provider_physical_network=PHYSICAL_NETWORK, mtu=mtu)
  File "/usr/lib/python2.7/site-packages/openstack/network/v2/_proxy.py", line 1086, in create_network
    return self._create(_network.Network, **attrs)
  File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 194, in _create
    return res.create(self)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 732, in create
    self._translate_response(response)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 664, in _translate_response
    exceptions.raise_from_response(response, error_message=error_message)
  File "/usr/lib/python2.7/site-packages/openstack/exceptions.py", line 205, in raise_from_response
    http_status=http_status, request_id=request_id
BadRequestException: BadRequestException: Unknown error

The issue is that the configured mtu is exceeding the max mtu for the physical network (ctlplane) as can be seen in .instack_undercloud/install-undercloud.log

2018-05-29 16:50:04,647 DEBUG: RESP: [400] Content-Type: application/json Content-Length: 142 X-Openstack-Request-Id: req-08b5917a-e400-4e20-a69f-d59374490e84 Date: Tue, 29 May 2018 20:50:04 GMT Connection: keep-alive
RESP BODY: {"NeutronError": {"message": "Invalid input for operation: Requested MTU is too big, maximum is 1500.", "type": "InvalidInput", "detail": ""}}

This problem only occurs in Queens or later, it was introduced when the mtu was set on the neutron network here with this change https://review.openstack.org/#/c/552693/.

Steps to reproduce
==================
1. Set local_mtu in undercloud.conf to 2000, for example.
2. run "openstack undercloud install"

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/571307

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/571307
Committed: https://git.openstack.org/cgit/openstack/instack-undercloud/commit/?id=41c401ca65ff32b2a3a1cdd07aabfdcc1bc508ab
Submitter: Zuul
Branch: master

commit 41c401ca65ff32b2a3a1cdd07aabfdcc1bc508ab
Author: Bob Fournier <email address hidden>
Date: Wed May 30 16:45:28 2018 -0400

    Allow local_mtu to be set to value greater than default (1500)

    The neutron global_physnet_mtu must be set to the configured local_mtu
    in order to set local_mtu to a value greater than 1500. Otherwise
    the neutron configuration will fail during the undercloud install.

    Change-Id: Iaadff350a14a2cfb4bf545065f6d12eab49ba125
    Closes-Bug: #1774271

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/queens
Review: https://review.openstack.org/571459

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/instack-undercloud 9.1.0 release.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/571459
Committed: https://git.openstack.org/cgit/openstack/instack-undercloud/commit/?id=2ca340b6f1b7db663d3b185d0802a2884132fb8d
Submitter: Zuul
Branch: stable/queens

commit 2ca340b6f1b7db663d3b185d0802a2884132fb8d
Author: Bob Fournier <email address hidden>
Date: Wed May 30 16:45:28 2018 -0400

    Allow local_mtu to be set to value greater than default (1500)

    The neutron global_physnet_mtu must be set to the configured local_mtu
    in order to set local_mtu to a value greater than 1500. Otherwise
    the neutron configuration will fail during the undercloud install.

    Change-Id: Iaadff350a14a2cfb4bf545065f6d12eab49ba125
    Closes-Bug: #1774271
    (cherry picked from commit 41c401ca65ff32b2a3a1cdd07aabfdcc1bc508ab)

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/instack-undercloud 8.4.3 release.