The IEEE 802.1ad-2005 standard allows drop eligibility to be conveyed separately from priority in-service VLAN TAGs (S-TAGs). The S-TAG has a new format where the priority and discard eligibility parameters are conveyed in the 3-bit priority code point (PCP) field and in the DE bit, respectively (see Figure: DE Bit in the 802.1ad S-TAG).
The introduction of the DE bit allows the S-TAG to convey eight forwarding classes/distinct priorities, each with a drop eligible indication.
When DE bit is set to 0 (DE = FALSE), the related packet is not discard eligible. This is the case for the packets that are within the CIR limits and must be given priority in case of congestion. If the DEI is not used or backwards compliance is required, the DE bit should be set to zero on transmission and ignored on reception.
When the DE bit is set to 1 (DE = TRUE), the related packet is discard eligible. This is the case for the packets that are sent above the CIR limit. In case of congestion, these packets are the first ones to be dropped.