3143d86b69
Updates for patchset 2 review comments Changed link depth of main Planning index and added some narrative guidance Added planning/openstack as sibling of planning/kubernetes Related additions to abbrevs.txt Added max-workers substitution to accomodate StarlingX/vendor variants Signed-off-by: Ron Stone <ronald.stone@windriver.com> Change-Id: Ibff9af74ab3f2c00958eff0e33c91465f1dab6b4 Signed-off-by: Stone <ronald.stone@windriver.com>
107 lines
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107 lines
5.0 KiB
ReStructuredText
Executable File
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.. jow1404333732592
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.. _os-planning-the-ethernet-mtu:
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================
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The Ethernet MTU
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================
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The |MTU| of an Ethernet frame is a configurable attribute in |prod-os|.
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Changing its default size must be done in coordination with other network
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elements on the Ethernet link.
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In the context of |prod-os|, the |MTU| refers to the largest possible payload
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on the Ethernet frame on a particular network link. The payload is enclosed by
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the Ethernet header \(14 bytes\) and the CRC \(4 bytes\), resulting in an
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Ethernet frame that is 18 bytes longer than the |MTU| size.
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The original IEEE 802.3 specification defines a valid standard Ethernet frame
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size to be from 64 to 1518 bytes, accommodating payloads ranging in size from
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46 to 1500 bytes. Ethernet frames with a payload larger than 1500 bytes are
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considered to be jumbo frames.
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For a |VLAN| network, the frame also includes a 4-byte |VLAN| ID header,
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resulting in a frame size 22 bytes longer than the |MTU| size.
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For a |VXLAN| network, the frame is either 54 or 74 bytes longer, depending on
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whether IPv4 or IPv6 protocol is used. This is because, in addition to the
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Ethernet header and CRC, the payload is enclosed by an IP header \(20 bytes for
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Ipv4 or 40 bytes for IPv6\), a |UDP| header \(8 bytes\), and a |VXLAN| header
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\(8 bytes\).
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In |prod-os|, you can configure the |MTU| size for the following interfaces and
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networks:
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.. _the-ethernet-mtu-ul-qmn-yvn-m4:
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- The management and |OAM| network interfaces on the controller. The |MTU|
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size for these interfaces is set during initial installation; for more
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information, see the |prod-os| installation guide for your system. To make
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changes after installation, see |sysconf-doc|: :ref:`Change the MTU of an
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OAM Interface <changing-the-mtu-of-an-oam-interface-using-the-cli>`.
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- Data interfaces on compute nodes. For more information, see :ref:`Change
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the MTU of a Data Interface <changing-the-mtu-of-a-data-interface>`.
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- Data networks. For more information, see |datanet-doc|: :ref:`Data Networks
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<data-network-management-data-networks>`.
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In all cases, the default |MTU| size is 1500. The minimum value is 576, and the
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maximum is 9216.
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.. note::
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You cannot change the |MTU| for a cluster-host interface. The default |MTU|
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of 1500 must always be used.
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Because data interfaces are defined over physical interfaces connecting to data
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networks, it is important that you consider the implications of modifying the
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default |MTU| size:
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.. _the-ethernet-mtu-ul-hsq-2f4-m4:
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- The |MTU| sizes for a data interface and the corresponding Ethernet
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interface on the edge router or switch must be compatible. You must ensure
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that each side of the link is configured to accept the maximum frame size
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that can be delivered from the other side. For example, if the data
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interface is configured with a |MTU| size of 9216 bytes, the corresponding
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switch interface must be configured to accept a maximum frame size of 9238
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bytes, assuming a |VLAN| tag is present.
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The way switch interfaces are configured varies from one switch
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manufacturer to another. In some cases you configure the |MTU| size
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directly, while in some others you configure the maximum Ethernet frame
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size instead. In the latter case, it is often unclear whether the frame
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size includes |VLAN| headers or not. In any case, you must ensure that both
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sides are configured to accept the expected maximum frame sizes.
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- For a |VXLAN| network, the additional IP, |UDP|, and |VXLAN| headers are
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invisible to the data interface, which expects a frame only 18 bytes larger
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than the |MTU|. To accommodate the larger frames on a |VXLAN| network, you
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must specify a larger nominal |MTU| on the data interface. For simplicity,
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and to avoid issues with stacked |VLAN| tagging, some third party vendors
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recommend rounding up by an additional 100 bytes for calculation purposes.
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For example, to attach to a |VXLAN| data network with an |MTU| of 1500, a
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data interface with an |MTU| of 1600 is recommended.
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- A data network can only be associated with a compute node data interface
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with an |MTU| of equal or greater value.
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- The |MTU| size of a compute node data interface cannot be modified to be
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less than the |MTU| size of any of its associated data networks.
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- The |MTU| size of a data network is automatically propagated to new project
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networks. Changes to the data network |MTU| are *not* propagated to
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existing project networks.
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- The Neutron L3 and |DHCP| agents automatically propagate the |MTU| size of
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their networks to their Linux network interfaces.
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- The Neutron |DHCP| agent makes the option interface-mtu available to any
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|DHCP| client request from a virtual machine. The request response from the
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server is the current interface's |MTU| size, which can then be used by the
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client to adjust its own interface |MTU| size.
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.. .. only:: partner
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.. include:: ../../_includes/the-ethernet-mtu.rest
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