When deploying an Alcatel-Lucent MPLS network with QoS, how are the EXP bits set?

Question

Options

• AThe first 3 bits of the DSCP value are copied directly into the EXP field on the first network
• BThe EXP bits are set on the first network egress, based on a combination of the SAP- ingress
• CThe 3 priority bits are copied directly from the dot1p header into the EXP field of the first MPLS
• DThe EXP bits are set based on a default map that translates DSCP codes into internal forwarding

• A
  77% (37)
• C
  8% (4)
• D
  15% (7)

Explanation

In Alcatel-Lucent (Nokia SR OS) MPLS QoS, two mechanisms govern how EXP bits are set. Option A is correct because IP Precedence - which is exactly the first 3 (most significant) bits of the 6-bit DSCP value - maps directly into the 3-bit MPLS EXP field at the network ingress label push. Option B is also correct because in the ALU architecture, traffic is classified at the SAP (Service Access Point) ingress where it's assigned an internal Forwarding Class (FC) and profile; when the MPLS label is pushed at the network egress, the EXP bits are set based on that FC/profile combination from the SAP-ingress QoS policy.

Option C is wrong because dot1p bits (802.1Q Layer 2 priority) are not directly copied into the EXP field - they go through a classification and mapping process, not a straight copy. Option D is wrong because it describes only the DSCP-to-internal-FC translation step, which determines forwarding behavior, not specifically how the EXP bits in the MPLS header are ultimately encoded at label imposition.

Memory tip: Think of it as a two-path rule - either the top 3 DSCP bits ride directly into EXP (A = "copy the class"), or the full SAP-ingress policy pipeline determines EXP at the egress label push (B = "classify then mark"). C and D each describe only one partial step and get the mechanism wrong.

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