Page 428 7750 SR OS Services Guide
DEI in IEEE 802.1ad
IEEE 802.1ad-2005 standard allows drop eligibility to be conveyed separately from priority in
Service VLAN TAGs (STAGs) so that all of the previously introduced traffic types can be marked
as drop eligible. The Service VLAN TAG has a new format where the priority and discard
eligibility parameters are conveyed in the three bit Priority Code Point (PCP) field and
respectively in the DE Bit (Figure 62).
Figure 62: DE Bit in the 802.1ad S-TAG
The DE bit allows the S-TAG to convey eight forwarding classes/distinct emission 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 (but below the PIR). In case of congestion these
packets will be the first ones to be dropped.
To Next Page
Loading...
