You are designing the QoS features for a large enterprise network that includes DMVPN. In which situation should you use the QoS pre-classify feature?

Question

Options

• AWhen you are marking packets with the ToS bits
• BWhen the QoS policy cannot be based on DSCP bits
• CWhen you are marking packets with the DSCP bits
• DWhen your service provider requires the DSCP bits be set

• A
  3% (1)
• B
  77% (23)
• C
  7% (2)
• D
  13% (4)

Explanation

QoS pre-classify solves a tunneling problem, not a marking problem. In DMVPN, packets get encrypted and encapsulated before leaving the tunnel interface, hiding the original IP headers. When your QoS policy needs to match on fields like source/destination IP, protocol, or port numbers that will be buried inside the encrypted payload, pre-classify tells the router to evaluate and store the packet's classification before encryption occurs - enabling you to apply the right queuing treatment even though those fields are no longer visible on the wire. This is precisely the scenario in option B: when the original packet's inner fields are inaccessible post-encapsulation and your policy can't rely solely on DSCP bits carried in the outer tunnel header.

Options A and C are wrong because marking (ToS or DSCP) is an action in a QoS policy, not the problem pre-classify solves - you don't need pre-classify just to set markings. Option D is wrong because satisfying a service provider's DSCP requirement is about outbound marking, which again is unrelated to the pre-classification of encrypted inner headers.

Memory tip: The "pre" in pre-classify literally means before the tunnel. If your policy would go "blind" after the packet is wrapped in encryption, you need pre-classify to peek at the original headers first.

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