baby giant
jargonA frame slightly over the classic 1518-byte Ethernet maximum: a tag or label added a few bytes to a standard packet.
One 802.1Q tag turns a full 1518-byte frame into 1522 bytes: the tag sits in the frame header and the 1500-byte payload is unchanged. MPLS labels differ: RFC 3032 counts each 4-byte label inside the payload, so a 1500-byte IP packet under two labels needs a link MTU of 1508.
A baby giant is a frame slightly larger than the classic 1518-byte Ethernet maximum, because a VLAN tag or an MPLS label stack added a few bytes to a packet of standard size. IEEE 802.3 itself names the 1522-octet Q-tagged frame and an envelope frame of up to 2000 octets. It is not a jumbo frame: a jumbo frame grows the payload by thousands of bytes, a baby giant grows the frame by a handful.
Where the bytes go matters. A VLAN tag rides in the frame header and leaves the IP MTU at 1500. MPLS labels sit inside the frame payload, 4 octets each (RFC 3032, sections 2.1 and 3.1), so a full 1500-byte IP packet under one label needs 1504 bytes of payload and under two needs 1508; on a link left at 1500 the labelled path carries 1496 or 1492 bytes of IP instead.
The failure is quiet and specific. A switch or interface not configured for the extra bytes drops these frames while passing everything else, so a trunk carrying tagged or labelled traffic loses full-size packets and passes small ones. The symptom is the familiar small-works-large-fails pattern, and the fix is a system or MPLS MTU adjustment rather than jumbo frame configuration.
Also known as: baby giants, baby giant frames