Cisco CCNA (200-301)IP ServicesHard
A network administrator is implementing Quality of Service (QoS) on a Cisco router to prioritize voice traffic. The voice traffic is identified by DSCP EF (Expedited Forwarding). The requirement is to ensure that voice traffic is always given preferential treatment and has a strict maximum bandwidth allocation of 5 Mbps on an interface with a total bandwidth of 100 Mbps. Which QoS mechanism and configuration concept would best meet these requirements?
- AClass-Based Weighted Fair Queuing (CBWFQ) with a 'bandwidth' command.
- BTraffic shaping with a 'shape' command.
- CWeighted Fair Queuing (WFQ) with a bandwidth percentage.
- DLow Latency Queuing (LLQ) with a 'priority' command.
Show answer & explanationAnswer & explanation
Correct answer: D. Low Latency Queuing (LLQ) with a 'priority' command.
Low Latency Queuing (LLQ) is specifically designed for real-time traffic like voice. The 'priority' command within LLQ ensures that the specified class of traffic (voice in this case) is given strict priority and is serviced before any other queues, guaranteeing low latency and jitter. It also allows for a maximum bandwidth reservation (policing) which meets the 'strict maximum bandwidth allocation' requirement.
Why the other options are wrong
- A. CBWFQ allows for bandwidth guarantees, but it does not provide strict priority queuing; it uses fair queuing algorithms. It can't guarantee that voice traffic is always serviced first.
- B. Traffic shaping buffers excess traffic to conform to a rate, but it doesn't provide strict priority over other traffic types or guarantee low latency for real-time applications.
- C. WFQ provides fair allocation but does not offer strict priority or explicit maximum bandwidth limits for a specific class.
Low Latency Queuing (LLQ)
A QoS queuing mechanism that combines strict priority queuing for real-time traffic (like voice) with CBWFQ for other traffic classes.
- Uses 'priority' command for strict priority queue.
- Guarantees low latency and jitter for mission-critical applications.
- Includes implicit policing to ensure the priority queue doesn't starve other queues.
Memory trick: For 'Low Latency,' think 'LLQ' – it gives the VIPs (voice) a fast pass!