VIAVI TestCenter: Why is IMIX adding extra four bytes when testing with devices that have VLAN or MPLS?

VIAVI TestCenter: Why is IMIX adding extra four bytes when testing with devices that have VLAN or MPLS?

• Detail Descriptions:   • There is no additional four bytes added when Stream block frame Size configured with Fixed or other frame size option (except iMix) and with VLAN or MPLS added. • However, Stream block with iMix frame size with VLAN or MPLS always adding extra four bytes to the data when it received • Explanation/Resolution:  • Fixed frame sizes and other frame sizes already included the VLAN or MPLS header and will not notice any different in the packet size. • The concern is only with IMIX. • iMix Frame size is mainly focused on the IP length and is where you set the length values. • Also displayed is the length of the packet if the default Ethernet frame is considered. • The default Ethernet does not include four byte VLAN or MPLS. • A default Ethernet frame can be one of two things. • 1. ETH, IP, Padding FCS • 64 byte is considered the minimum ethernet frame size, so padding is required if the IP length is not large enough to create a 64 byte frame. • 2. ETH, IP, FCS    • No padding required if IP length creates a frame that meet or exceeds the minimum ethernet frame size. • Since only the default Ethernet size is indicated in the IMIX editor, the user need to be aware of this when setting the IP length for VLAN or MPLS traffic. • VLAN Examples: • IP length is set for 40 bytes (Default Etherent size shows as 64 bytes) • ETH-VLAN-IP-Padding-FCS = 14+4+40+2+4 • Since normally there would be 6 bytes of padding, adding a VLAN to this will still produce a 64 byte frame.  • IP length is set for 46 bytes (Default Etherent size shows as 64 bytes) • ETH-VLAN-IP-FCS = 14+4+46+4 • Although the default Ethernet size shows as 64 bytes, due to the VLAN, the actual length will be 68 bytes.