yahoo-eng-team team mailing list archive

["Dr. Jens Rosenboom" <j.rosenboom@xxxxxxxx>]

Public bug reported:

When creating a tenant network with type vxlan, the MTU is automatically
set to 1450:

# neutron net-show net2
+---------------------------+--------------------------------------+
| Field                     | Value                                |
+---------------------------+--------------------------------------+
| admin_state_up            | True                                 |
| availability_zone_hints   |                                      |
| availability_zones        | nova                                 |
| created_at                | 2016-04-08T11:11:42                  |
| description               |                                      |
| id                        | f44e9e2c-8a60-46c7-98bb-3f1824fc09e9 |
| ipv4_address_scope        |                                      |
| ipv6_address_scope        |                                      |
| mtu                       | 1450                                 |
| name                      | net2                                 |
| provider:network_type     | vxlan                                |
| provider:physical_network |                                      |
| provider:segmentation_id  | 65633                                |
| router:external           | False                                |
| shared                    | False                                |
| status                    | ACTIVE                               |
| subnets                   | 288021c1-7073-41c5-a233-529226971dd3 |
| tags                      |                                      |
| tenant_id                 | e3c253d3e8344a8796e70bc4f96b6166     |
| updated_at                | 2016-04-08T11:11:42                  |
+---------------------------+--------------------------------------+

This is the maximum Ethernet MTU possible for the tenant assuming the
encapsulated packet has to fit into an IP MTU of 1500 on the tunnel
network.

Now neutron tells DHCP to set --dhcp-option-force=option:mtu,1450 but
the DHCP option refers to the IP layer MTU, see
https://tools.ietf.org/html/rfc2132 section 5.1. So a correctly behaving
client will set its interface (Ethernet) MTU to 1454, implying an IP MTU
of 1450.

But now it will send Ethernet frames of size 1454, which encapsulated
have size 1504, and thus get dropped on the tunnel network.

The correct behaviour here would be to advertise an MTU reduced by 4 via
DHCP.

** Affects: neutron
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1567923

Title:
  Neutron advertises too high MTU for vxlan

Status in neutron:
  New

Bug description:
  When creating a tenant network with type vxlan, the MTU is
  automatically set to 1450:

  # neutron net-show net2
  +---------------------------+--------------------------------------+
  | Field                     | Value                                |
  +---------------------------+--------------------------------------+
  | admin_state_up            | True                                 |
  | availability_zone_hints   |                                      |
  | availability_zones        | nova                                 |
  | created_at                | 2016-04-08T11:11:42                  |
  | description               |                                      |
  | id                        | f44e9e2c-8a60-46c7-98bb-3f1824fc09e9 |
  | ipv4_address_scope        |                                      |
  | ipv6_address_scope        |                                      |
  | mtu                       | 1450                                 |
  | name                      | net2                                 |
  | provider:network_type     | vxlan                                |
  | provider:physical_network |                                      |
  | provider:segmentation_id  | 65633                                |
  | router:external           | False                                |
  | shared                    | False                                |
  | status                    | ACTIVE                               |
  | subnets                   | 288021c1-7073-41c5-a233-529226971dd3 |
  | tags                      |                                      |
  | tenant_id                 | e3c253d3e8344a8796e70bc4f96b6166     |
  | updated_at                | 2016-04-08T11:11:42                  |
  +---------------------------+--------------------------------------+

  This is the maximum Ethernet MTU possible for the tenant assuming the
  encapsulated packet has to fit into an IP MTU of 1500 on the tunnel
  network.

  Now neutron tells DHCP to set --dhcp-option-force=option:mtu,1450 but
  the DHCP option refers to the IP layer MTU, see
  https://tools.ietf.org/html/rfc2132 section 5.1. So a correctly
  behaving client will set its interface (Ethernet) MTU to 1454,
  implying an IP MTU of 1450.

  But now it will send Ethernet frames of size 1454, which encapsulated
  have size 1504, and thus get dropped on the tunnel network.

  The correct behaviour here would be to advertise an MTU reduced by 4
  via DHCP.

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