Alcatel-Lucent 7450 Quality Of Service Manual page 202

Service Egress QoS Policy
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 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
IEEE 802.1ad Use Case
Figure 15
aware, 802.1ad access network connected via a regular SAP to a router PE.
In this example, PE1 can ensure coherent processing of the DE indication between the
802.1ad and the MPLS networks: for example, for packets ingressing the SAP connected to
802.1ad access, read the DE indication and perform classification, color-aware metering/
policing, marking of the related backbone QoS fields and selective discarding of the frames
throughout the queueing system based on their discard eligibility. In addition, packets
egressing the SAP towards the 802.1ad access provide proper DE indication by marking the
new DE bit in the STAG.
202
illustrates an example of a topology where the new DE feature may be used: a DE
DEI in IEEE 802.1ad.
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