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Bits Preserved, Cleared, and Rewritten
For every incoming packet, the ingress classifier
decodes the ingress CoS bits into a forwarding class and packet loss
priority (PLP) combination. The egress CoS information depends on
which type of rewrite marker is active, as follows:
- For Multiprotocol Label Switching (MPLS) EXP and IEEE
802.1 rewrite markers, values are derived from the forwarding class
and PLP values in rewrite rules. MPLS EXP and IEEE 802.1 markers
are not preserved because they are part of the Layer 2 encapsulation.
- For IP precedence and DiffServ code point (DSCP) rewrite
markers, the marker alters the first three bits on the type-of-service
(ToS) byte while leaving the last three bits unchanged.
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