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+.. _mtu_doc:
+
+MTU in VPP
+==========
+
+Maximum Transmission Unit is a term used to describe the maximum sized
+“thingy” that can be sent out an interface. It can refer to the maximum
+frame size that a NIC can send. On Ethernet that would include the
+Ethernet header but typically not the IGF. It can refer to the maximum
+packet size, that is, on Ethernet an MTU of 1500, would allow an IPv4
+packet of 1500 bytes, that would result in an Ethernet frame of 1518
+bytes.
+
+
+VPP allows setting of the physical payload MTU. I.e. not including L2
+overhead. Setting the hardware MTU will program the NIC. This MTU will
+be inherited by all software interfaces.
+
+VPP also allows setting of the payload MTU for software interfaces.
+Independently of the MTU set on the hardware. If the software payload
+MTU is set higher than the capability of the NIC, the packet will be
+dropped.
+
+In addition VPP supports setting the MTU of individual network layer
+protocols. IPv4, IPv6 or MPLS. For example an IPv4 MTU of 1500 (includes
+the IPv4 header) will fit in a hardware payload MTU of 1500.
+
+*Note we might consider changing the hardware payload MTU to hardware
+MTU*. That is, the MTU includes all L2 framing. Then the payload MTU can
+be calculated based on the interface’s configuration. E.g. 802.1q tags
+etc.
+
+There are currently no checks or warnings if e.g. the user configures a
+per-protocol MTU larger than the underlying payload MTU. If that happens
+packets will be fragmented or dropped.
+
+Data structures
+^^^^^^^^^^^^^^^
+
+The hardware payload MTU is stored in the max_packet_bytes variable in
+the vnet_hw_interface_t structure.
+
+The software MTU (previously max_l3_packet_bytes) is in
+vnet_sw_interface_t->in mtu[VNET_N_MTU].
+
+MTU API
+-------
+
+Set physical MTU
+^^^^^^^^^^^^^^^^
+
+This API message is used to set the physical MTU. It is currently
+limited to Ethernet interfaces. Note, this programs the NIC.
+
+::
+
+   autoreply define hw_interface_set_mtu
+   {
+    u32 client_index;
+    u32 context;
+    u32 sw_if_index;
+    u16 mtu;
+   };
+
+Set the L2 payload MTU
+^^^^^^^^^^^^^^^^^^^^^^
+
+:: note
+    (not including the L2 header) and per-protocol MTUs
+
+This API message sets the L3 payload MTU. E.g. on Ethernet it is the
+maximum size of the Ethernet payload. If a value is left as 0, then the
+default is picked from VNET_MTU_L3.
+
+::
+
+   autoreply define sw_interface_set_mtu
+   {
+    u32 client_index;
+    u32 context;
+    u32 sw_if_index;
+    /* $$$$ Replace with enum */
+    u32 mtu[4]; /* 0 - L3, 1 - IP4, 2 - IP6, 3 - MPLS */
+   };
+
+Get interface MTU
+^^^^^^^^^^^^^^^^^
+
+The various MTUs on an interface can be queried with the
+sw_interface_dump/sw_interface_details calls.
+
+::
+
+   define sw_interface_details
+   {
+     /* MTU */
+     u16 link_mtu;
+
+     /* Per protocol MTUs */
+     u32 mtu[4]; /* 0 - L3, 1 - IP4, 2 - IP6, 3 - MPLS */
+   };
+
+MTU CLI
+-------
+
+::
+
+   set interface mtu [packet|ip4|ip6|mpls] <value> <interface>