150-130 · Question #119
You want to classify and mark traffic at the edge of your network before it is sent to the core and routed where Layer 3 QoS is trusted and enabled. Which type of marking should you configure to…
The correct answer is D. Mark the DSCP field according to application priority. See the full explanation below for the reasoning.
Question
You want to classify and mark traffic at the edge of your network before it is sent to the core and routed where Layer 3 QoS is trusted and enabled. Which type of marking should you configure to ensure your application traffic is serviced correctly in the core of your network?
Options
- AMark the 802.lp field according to application priority
- BMark the PCP field according to application priority
- CMark the EXP field according to application priority
- DMark the DSCP field according to application priority
How the community answered
(37 responses)- A3% (1)
- B11% (4)
- C3% (1)
- D84% (31)
Community Discussion
7
Goran A.Nov 1, 2025D is the move here. DSCP lives in the IP header, which is what your Layer 3 core actually inspects and trusts for QoS decisions, so marking it at the edge means the core can read and honor that classification all the way through without any re-marking gymnastics.
Toby R.Nov 4, 2025Right, just worth adding that the edge marking only holds up if you actually enforce a trust boundary there, because if you blindly honor whatever DSCP value the end host slapped on the packet you lose control of the whole policy.
Yusuf A.Oct 12, 2025This one got me for a second because I almost picked the 802.1p option since my senior kept drilling Layer 2 marking into me that week, but then I remembered he also said that once traffic hits the routed core, Layer 2 headers get stripped at every hop so DSCP is the one that survives end to end in the IP header. Went with D and moved on, and yeah it was right.
Goran A.Oct 13, 2025Good call trusting DSCP, though the deeper reason to tattoo on your brain is that 802.1p lives in the 802.1Q tag and that tag evaporates the moment a router strips the Ethernet frame, so any QoS marking you care about beyond a single L2 segment has to ride in the IP header DSCP field, full stop.
Toby R.Oct 21, 2025I honestly almost went with C because I had just finished studying MPLS and the EXP bits were fresh in my head, but then I remembered the question says Layer 3 QoS is trusted in the core, and DSCP lives in the IP header which works at Layer 3 end to end regardless of the transport. EXP is MPLS-specific so it would not carry your markings reliably across the whole path the way DSCP does.
Viktor S.Oct 18, 2025D is right, but let me make you earn it instead of just memorizing it. The core is running Layer 3 QoS, so it is making forwarding decisions based on IP header fields. DSCP lives in the IP header, specifically in the Differentiated Services field, and that marking survives across routed hops as long as the routers are configured to trust it, which the question tells you they are. Now, A and B are the same thing dressed up differently, 802.1p is just the old name for the Priority Code Point field in the Ethernet 802.1Q tag, and that is a Layer 2 marking that gets stripped at every router hop, so it is completely useless once you leave the edge. C is the tempting one if you have any MPLS in your brain, EXP bits (now officially called TC bits) mark traffic inside MPLS labels and only matter if the core is an MPLS network, which this question does not say. Mark your traffic at the edge with DSCP so the routed core actually has something to work with.
Toby R.Oct 20, 2025Good breakdown, and the trust boundary piece is worth hammering home too, because if the ingress device is set to untrust, it rewrites everything to DSCP 0 before the packet even reaches that routed core you just marked for.