docs/doc/source/planning/openstack/the-ethernet-mtu.rst

The Ethernet MTU

The of an Ethernet frame is a configurable attribute in . Changing its default size must be done in coordination with other network elements on the Ethernet link.

In the context of , the refers to the largest possible payload on the Ethernet frame on a particular network link. The payload is enclosed by the Ethernet header (14 bytes) and the CRC (4 bytes), resulting in an Ethernet frame that is 18 bytes longer than the size.

The original IEEE 802.3 specification defines a valid standard Ethernet frame size to be from 64 to 1518 bytes, accommodating payloads ranging in size from 46 to 1500 bytes. Ethernet frames with a payload larger than 1500 bytes are considered to be jumbo frames.

For a network, the frame also includes a 4-byte ID header, resulting in a frame size 22 bytes longer than the size.

For a network, the frame is either 54 or 74 bytes longer, depending on whether IPv4 or IPv6 protocol is used. This is because, in addition to the Ethernet header and CRC, the payload is enclosed by an IP header (20 bytes for Ipv4 or 40 bytes for IPv6), a header (8 bytes), and a header (8 bytes).

In , you can configure the size for the following interfaces and networks:

• The management and network interfaces on the controller. The size for these interfaces is set during initial installation; for more information, see the installation guide for your system. To make changes after installation, see : Change the MTU of an OAM Interface <changing-the-mtu-of-an-oam-interface-using-the-cli>.
• Data interfaces on compute nodes. For more information, see Change the MTU of a Data Interface <changing-the-mtu-of-a-data-interface>.
• Data networks. For more information, see : Data Networks <data-network-management-data-networks>.

In all cases, the default size is 1500. The minimum value is 576, and the maximum is 9216.

Note

You cannot change the for a cluster-host interface. The default of 1500 must always be used.

Because data interfaces are defined over physical interfaces connecting to data networks, it is important that you consider the implications of modifying the default size:

• The sizes for a data interface and the corresponding Ethernet interface on the edge router or switch must be compatible. You must ensure that each side of the link is configured to accept the maximum frame size that can be delivered from the other side. For example, if the data interface is configured with a size of 9216 bytes, the corresponding switch interface must be configured to accept a maximum frame size of 9238 bytes, assuming a tag is present.

  The way switch interfaces are configured varies from one switch manufacturer to another. In some cases you configure the size directly, while in some others you configure the maximum Ethernet frame size instead. In the latter case, it is often unclear whether the frame size includes headers or not. In any case, you must ensure that both sides are configured to accept the expected maximum frame sizes.

• For a network, the additional IP, , and headers are invisible to the data interface, which expects a frame only 18 bytes larger than the . To accommodate the larger frames on a network, you must specify a larger nominal on the data interface. For simplicity, and to avoid issues with stacked tagging, some third party vendors recommend rounding up by an additional 100 bytes for calculation purposes. For example, to attach to a data network with an of 1500, a data interface with an of 1600 is recommended.

• A data network can only be associated with a compute node data interface with an of equal or greater value.

• The size of a compute node data interface cannot be modified to be less than the size of any of its associated data networks.

• The size of a data network is automatically propagated to new project networks. Changes to the data network are not propagated to existing project networks.

• The Neutron L3 and agents automatically propagate the size of their networks to their Linux network interfaces.

• The Neutron agent makes the option interface-mtu available to any client request from a virtual machine. The request response from the server is the current interface's size, which can then be used by the client to adjust its own interface size.