Dei In Ieee 802.1Ad; Dei In Ieee 802.1Ah; Service Egress And Ingress Qos Policies; Figure 8: De Bit In The 802.1Ad S-Tag - Alcatel-Lucent 7950 Quality Of Service Manual

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
Octets
1
Bits
8
The introduction of 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.

DEI in IEEE 802.1ah

IEEE 802.1ah (PBB) standard provides a dedicate bit for DE indication in both the BVID and the
ITAG.
The backbone VLAN ID (BVID) is a regular 802.1ad STAG. Its DE bit may be used to convey the
related tunnel QoS throughout an Ethernet backbone.
The ITAG header offers also an I-DEI bit that may be used to indicate the service drop eligibility
associated with this frame.
These bits must follow the same rules as described in
7950 XRS Quality of Service Guide
(Figure 8).
PCP DE
6 5 4

Figure 8: DE Bit in the 802.1ad S-TAG

Service Egress and Ingress QoS Policies

2
VID
1 8
DEI in IEEE 802.1ad on page
1
OSSG267
135.
Page 135