Service Provider Layer 3 VPN Traffic Marking

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In a Layer 3 VPN service, the Service Provider (SP) network must differentiate between the customer's traffic and its own network's traffic. To achieve this, the SP can use different QoS mechanisms to mark the packets and treat them accordingly.

Out of the options provided, two PDU fields that a service provider can use to mark the Layer 3 VPN traffic without interfering with the customer marking are:

A. MPLS EXP bits: The MPLS EXP bits are used in the MPLS header to mark the priority level of the packet. These bits can be used by the SP to mark the VPN traffic, as long as the customer's marking is preserved when the packet traverses the SP network. This can be achieved by mapping the customer's marking to a corresponding MPLS EXP value and using a Differentiated Services Code Point (DSCP) to MPLS EXP mapping on the SP routers. By doing this, the SP can ensure that the customer's marking is preserved while still being able to mark the packets for its own QoS requirements.

C. DSCP: DSCP is a field in the IP header that is used to mark the priority level of the packet. This field is defined by the customer and can be used by the SP to mark the VPN traffic without interfering with the customer's marking. The SP can apply its own DSCP values to the VPN traffic, as long as it maps the customer's DSCP values to the corresponding SP DSCP values. This can be achieved through a DSCP to DSCP mapping on the SP routers.

Therefore, options A and C are the correct answers to this question. Option B (discard-class) and option D (DS-TE) are not relevant for marking Layer 3 VPN traffic. Option E (802.1p) is used for marking Layer 2 Ethernet frames and is not applicable for Layer 3 VPN traffic.