QoS Design Practice Questions

Answers and explanations appear when you work through the set above.

1. What is a key operational distinction between Traffic Policing and Traffic Shaping?

   • Policing queues excess traffic in a buffer, while shaping drops the excess traffic immediately.
   • Shaping queues excess packets to smooth the rate; policing drops or re-marks the excess.
   • Policing can be applied only outbound, while shaping can be applied in either direction.
   • Shaping can be applied only inbound, while policing can be applied only outbound.

2. In a Single Rate Three Color Marker (srTCM) policer, which bucket size defines the additional burst allowed beyond the committed burst before traffic is classed as violating?

   • Committed Burst Size (Bc)
   • Excess Burst Size (Be)
   • Peak Information Rate (PIR)
   • Maximum Transmission Unit (MTU)

3. Which switch ASIC lookup memory structure provides high-speed, parallel searches for complex decision-making, such as Access Control Lists (ACLs) and QoS policies?

   • DRAM (Dynamic Random Access Memory)
   • CAM (binary Content Addressable Memory, exact match)
   • TCAM (Ternary Content Addressable Memory)
   • NVRAM (Non-Volatile RAM)

4. In QoS policing, what are the three output classifications generated by a Two-Rate Three-Color Marker (trTCM) algorithm based on traffic rates?

   • Low, Medium, High priority
   • Conform, Exceed, Violate
   • Transmit, Queue, Drop
   • Green, Yellow, Blue

5. What is the binary value and decimal equivalent of the Expedited Forwarding (EF) Per-Hop Behavior (PHB) used for low-latency voice traffic?

   • Binary 001010, Decimal 10 (AF11)
   • Binary 101110, Decimal 46 (EF)
   • Binary 111000, Decimal 56 (CS7)
   • Binary 000000, Decimal 0 (BE)

6. Where is the optimal placement for a QoS Trust Boundary in an enterprise network design?

   • As close to the core switch as possible to minimize packet classification overhead.
   • As close to the endpoint as possible, such as the IP phone or access switch port.
   • On the egress WAN router interface, where congestion actually happens.
   • At the Internet edge firewall, where untrusted traffic enters.

7. Which functional difference correctly distinguishes Traffic Policing from Traffic Shaping?

   • Policing buffers excess packets in memory to smooth bursts, whereas Shaping drops the excess immediately.
   • Policing drops or re-marks packets above the rate, whereas Shaping queues the excess to smooth bursts.
   • Policing can only be applied in the outbound egress direction, whereas Shaping applies exclusively to ingress traffic.
   • Policing increases packet serialization delay, whereas Shaping introduces zero delay.

8. How do Class of Service (CoS) and Differentiated Services Code Point (DSCP) compare in QoS frame/packet classification?

   • CoS uses 6 bits in the IPv4 ToS byte (64 values); DSCP uses 3 bits in the 802.1Q tag (8 values).
   • CoS operates at Layer 3 in the IP header, while DSCP operates at Layer 2 in the Ethernet frame.
   • CoS uses 3 bits in the 802.1Q tag (8 values); DSCP uses 6 bits in the IP ToS/DS byte (64 values).
   • CoS and DSCP are both rewritten to zero by any Ethernet switch the frame crosses, whatever its trust setting.

9. In traffic policing, what happens when a burst of traffic arrives and there are not enough tokens in the single token bucket (Bc)?

   • The packets are held in a buffer until enough new tokens are added to the bucket.
   • Excess packets exceeding the token bucket capacity are immediately conforming and forwarded without delay.
   • The non-conforming packets are dropped or re-marked, according to the configured police action.
   • The switch sends an ICMP Source Quench message back to the originating host.