How many bits in length is the DEI field in the 802.1Q tag?

The DEI (Drop Eligible Indicator) field in the 802.1Q tagging protocol is 1 bit in length. This field is used to indicate whether the frame can be eligible for dropping in certain situations, such as during network congestion. The simplicity of it being a single bit allows for a straightforward implementation within the overall 802.1Q tag format, which includes the TPID (Tag Protocol Identifier) and the Priority Code Point (PCP), as well as the DEI itself and the VLAN identifier.

In the 802.1Q tag, the DEI field serves a specific purpose without requiring a larger size, as it only needs to convey a binary state—either the frame is marked as drop eligible or it is not. Other options indicate lengths (3, 5, and 8 bits) that do not align with the established standards of the 802.1Q protocol, which is designed for efficient tagging and prioritization of Ethernet frames in a VLAN environment.