Which of the following CANNOT be used as external marking bits?

IP Precedence (IPv4 ToS field), Dot1p (802.1Q PCP field), and EXP bits (MPLS TC field) are all standardized fields within packet headers used to carry Quality of Service (QoS) information across a network. These fields are "external" because they are visible to and interpreted by different network devices. In contrast, a Forwarding Class (FC) is an internal construct within the Nokia Service Router Operating System (SR OS). After a packet is classified based on its external markings or other criteria, it is mapped to an internal FC. This FC then determines the packet's per-hop behavior, such as which queue it enters. The FC itself is not a field in the packet header and is not transmitted to other routers.

A. IP Precedence: These are the first three bits of the IPv4 Type of Service (ToS) byte, used for external QoS marking between IP-aware devices.
B. Dot1p: These are the three Priority Code Point (PCP) bits in an IEEE 802.1Q VLAN tag, used for external Layer 2 QoS marking.
D. EXP: These are the three Experimental (now Traffic Class) bits in an MPLS label header, used for external QoS marking within an MPLS domain.

1. Nokia 7750 SR OS, Quality of Service Guide, Release 21.10.R1, Section: "QoS Policy Entities", Subsection: "Forwarding Classes".

Quote/Concept: This section describes Forwarding Classes as internal designations for traffic. It states, "The forwarding class is an identifier that is used to associate a packet with a specific queue." This confirms its role as an internal mapping mechanism, not an external header field.

2. Nokia 7750 SR OS, Quality of Service Guide, Release 21.10.R1, Section: "Network QoS Policies", Subsection: "Network Policy Egress Marking".

Quote/Concept: This section details how egress policies can be configured to set or "re-mark" external QoS bits. It explicitly lists ip-prec, dot1p, and mpls-exp as target fields for re-marking actions, confirming they are external markings. The Forwarding Class is used to decide what to re-mark the external bits to, but is not itself the external marking.

3. Nokia 4A0-107 Courseware, Module 3: Quality of Service, Section: "Classification and Marking".

Quote/Concept: The official course material distinguishes between classification (interpreting external markings like Dot1p, IP Precedence, DSCP, EXP) and internal treatment (mapping to a Forwarding Class). It clarifies that the FC is the result of classification and dictates internal handling, while the other options are the input criteria for classification or the output of a re-marking action.