Processing Labeled IPv4 Datagrams which are Too Big If a labeled IPv4 datagram is "too big", and the DF bit is not set in its IP header, then the LSR MAY silently discard the datagram. Note that discarding such datagrams is a sensible procedure...
According the diagram 802.q will make another 4 byte overhead so it will 1522switch also 1500 system mtu so it will forward 1518(frame overhead 18 byte )
As a workaround, you can enable a port for trunking in order to support baby giants. When you enable a port for 802.1q trunking, the switch automatically assumes that an extra four bytes of data are appended on, and increment the frame size of the L2...