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Dell EMC VxRail Multirack Deployment Guide
B
Maximum Transmission Unit (MTU) considerations
Virtual extensible LAN (VXLAN), as a network virtualization overlay (NVO) technology, places some additional
overhead on the original data frames. This overhead manifest in the form of additional identifier information
required for the functioning of VXLAN itself. As a rule, fragmentation, splitting of a frame or data packet
because it is too large for a transport network, should be avoided. Fragmentation and reassembly put an
additional burden on the network switches and VxRail node resources, which result in transport inefficiency.
Ethernet has a standard maximum transmission unit (MTU) of 1500 bytes. The total size of an Ethernet frame
is 1518, or 1522, with the additional optional 802.1q tag, with six bytes each for the source and destination
MAC address, two bytes for the Ethertype, and four bytes for the frame check sequence (FCS). This means
that a server can send a payload of 1500 byte or less, which includes all the header information, from Layer 3
and above. VXLAN reduces the MTU to 1450 bytes through its default configuration. This is because VXLAN
adds either 50 or 54 bytes of overhead as part of identifier information. Figure 23 shows a standard 54-byte
VXLAN packet with an 802.1Q tag.
Original L2 Frame
Outer
MAC
Inner IP
UDP
VXLAN
SMAC
802.1Q
Etype
CRC
(New)
DMAC
14B 20B 8B 8B 18B 4B
Payload
SMAC
802.1Q
Etype
DMAC
Payload
CRC
20B + 8B + 8B + 18B* = 54
Bytes of Total Overhead
VXLAN Frame Format, Including Byte Count (50/54 Byte Overhead for Comparison)
VxRail configures virtual switches to accommodate jumbo frames, data units that are larger than the
traditional 1500 bytes typical of Ethernet frames, and in the case of vSAN 9000 bytes. With a need to support
MTUs of 9000 bytes from the VxRail nodes, and with the additional bytes from the VXLAN encapsulation, a
value around 9050 or 9054 bytes needs to be allowed to avoid fragmentation.
Dell EMC Networking provides a maximum MTU value of 9216 bytes. Jumbo frame MTU of 9000 bytes from
the VxRail node can be accommodated by the network switches without introducing fragmentation. The
jumbo frame MTU needs to be configured consistently across all points in the network, including the ESXi
virtual switches.