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Module 42
Queuing Mechanisms (Deep Dive)
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# 📘 CCNA 200-301 - Video 42: Queuing Mechanisms (Deep Dive) ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - Why queuing is necessary - Different queuing mechanisms (FIFO, PQ, CQ, WFQ, CBWFQ, LLQ) - How each queuing mechanism works - When to use each queuing type - Configuration and verification of queuing - Queuing best practices --- ## 🧠 Core Concepts ### 1. Why Queuing? **Definition:** Queuing is the process of temporarily storing packets in buffers when the output interface is busy, then transmitting them in an organized order based on configured policies. **Analogy:** Think of queuing like different checkout lanes at a supermarket: - **FIFO:** One line, first-come-first-served - **Priority Queuing:** Express lane for customers with few items (priority traffic) - **Custom Queuing:** Dedicated lanes for different types of customers (each gets guaranteed service) - **WFQ:** Fair distribution, no one waits too long ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WHY QUEUING IS NEEDED │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Without Queuing: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Incoming Traffic: Voice, Video, Data, Backup │ │ │ │ │ │ │ │ │ ▼ │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ No Queue │ │ │ │ │ │ Packets dropped when interface busy │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ │ ▼ │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ Interface │ │ │ │ │ │ (Transmitting at line rate) │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ Result: All traffic treated equally, important packets dropped │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ With Queuing: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Incoming Traffic: Voice, Video, Data, Backup │ │ │ │ │ │ │ │ │ ▼ │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ Queuing System │ │ │ │ │ │ │ │ │ │ │ │ High Priority: Voice → Voice → Voice │ │ │ │ │ │ Medium Priority: Video → Video │ │ │ │ │ │ Low Priority: Data → Data → Backup │ │ │ │ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ │ ▼ │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ Interface │ │ │ │ │ │ (Transmitting at line rate) │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ Result: Priority traffic transmitted first, fair allocation │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. Queuing Mechanisms Overview | Mechanism | Type | Description | Pros | Cons | |-----------|------|-------------|------|------| | **FIFO** | First-In-First-Out | Single queue, no prioritization | Simple, low overhead | No QoS | | **PQ** | Priority Queuing | Multiple queues, strict priority | Low latency for priority traffic | Starvation possible | | **CQ** | Custom Queuing | Multiple queues, round-robin | Guaranteed bandwidth | Complex configuration | | **WFQ** | Weighted Fair Queuing | Flow-based, automatic | Fair, no configuration | Limited control | | **CBWFQ** | Class-Based WFQ | Class-based, guaranteed bandwidth | Granular control | Moderate complexity | | **LLQ** | Low-Latency Queuing | Priority + CBWFQ | Voice + data | Priority queue limits | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QUEUING MECHANISMS COMPARISON │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ FIFO (First-In-First-Out): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ [Packet1] → [Packet2] → [Packet3] → [Packet4] → Transmit │ │ │ │ │ │ │ │ • Single queue │ │ │ │ • No prioritization │ │ │ │ • Default on high-speed interfaces (2 Mbps+) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ PQ (Priority Queuing): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ High │ [Voice] [Voice] → Transmit first │ │ │ │ Medium │ [Video] → Transmit when high empty │ │ │ │ Normal │ [Data] → Transmit when high/medium empty │ │ │ │ Low │ [Backup] → Transmit when all others empty │ │ │ │ │ │ │ │ • 4 priority levels (High, Medium, Normal, Low) │ │ │ │ • Strict priority │ │ │ │ • Risk: Low priority queues may starve │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ CQ (Custom Queuing): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Queue 1: [Voice] → [Voice] → (up to 1500 bytes) │ │ │ │ Queue 2: [Video] → [Video] → (up to 1500 bytes) │ │ │ │ Queue 3: [Data] → [Data] → (up to 1500 bytes) │ │ │ │ Queue 4: [Backup] → (up to 1500 bytes) │ │ │ │ │ │ │ │ • Round-robin service │ │ │ │ • Each queue gets configured byte count │ │ │ │ • Guaranteed bandwidth per queue │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ WFQ (Weighted Fair Queuing): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Flow 1: │Data│Data│ (weight based on IP precedence) │ │ │ │ Flow 2: │Voice│Voice│ (higher weight → more bandwidth) │ │ │ │ Flow 3: │Video│Video│ │ │ │ │ │ │ │ │ • Automatic flow classification │ │ │ │ • Fair sharing of bandwidth │ │ │ │ • Default on low-speed interfaces (2 Mbps or less) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. FIFO (First-In-First-Out) **Description:** Simplest queuing method where packets are transmitted in the order they arrive. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ FIFO OPERATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Incoming Order: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ A → B → C → D → E │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ │ ▼ │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ FIFO Queue │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ [A] [B] [C] [D] [E] │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ │ ▼ │ │ Transmitted Order: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ A → B → C → D → E │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Characteristics: │ │ • Default on high-speed interfaces (2 Mbps or greater) │ │ • Single queue │ │ • No prioritization │ │ • Simple but no QoS guarantees │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **FIFO Configuration:** ```cisco ! FIFO is default on high-speed interfaces interface GigabitEthernet0/0 no fair-queue ! Disable WFQ, enable FIFO ``` --- ### 4. PQ (Priority Queuing) **Description:** Four priority queues (High, Medium, Normal, Low). Higher priority queues are always serviced first. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PQ OPERATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Classification: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ High: Voice traffic (RTP) │ │ │ │ Medium: Video traffic │ │ │ │ Normal: Data traffic (HTTP, HTTPS) │ │ │ │ Low: Backup traffic │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Queues: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ High │ Voice │ Voice │ Voice │ ← Always serviced first │ │ │ │ Medium │ Video │ Video │ │ ← Serviced when high empty │ │ │ │ Normal │ Data │ Data │ Data │ ← Serviced when high/medium empty│ │ │ │ Low │ Backup│ Backup│ │ ← Serviced only when others empty│ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Characteristics: │ │ • Strict priority (high always serviced first) │ │ • Risk: Low priority queues may starve │ │ • 4 queues (High, Medium, Normal, Low) │ │ • Legacy mechanism (replaced by LLQ) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **PQ Configuration:** ```cisco ! Define priority list priority-list 1 protocol ip high tcp 22 ! SSH high priority priority-list 1 protocol ip medium tcp 80 ! HTTP medium priority priority-list 1 protocol ip normal tcp 21 ! FTP normal priority priority-list 1 default low ! Apply to interface interface Serial0/0/0 priority-group 1 ``` --- ### 5. CQ (Custom Queuing) **Description:** Multiple queues (up to 16) serviced in round-robin fashion, each with configurable byte count. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ CQ OPERATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Queues with Byte Counts: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Queue 1: [Voice] [Voice] [Voice] (1500 bytes) │ │ │ │ Queue 2: [Video] [Video] [Video] (1500 bytes) │ │ │ │ Queue 3: [Data] [Data] [Data] (1500 bytes) │ │ │ │ Queue 4: [Backup] (500 bytes) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Round-Robin Service: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Cycle 1: Queue 1 (1500 bytes) → Queue 2 (1500 bytes) │ │ │ │ → Queue 3 (1500 bytes) → Queue 4 (500 bytes) │ │ │ │ │ │ │ │ Cycle 2: Queue 1 (1500 bytes) → Queue 2 (1500 bytes) │ │ │ │ → Queue 3 (1500 bytes) → Queue 4 (500 bytes) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Bandwidth Allocation: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Each queue gets guaranteed bandwidth based on byte count │ │ │ │ • Bandwidth % = Queue Byte Count / Total Byte Count × 100 │ │ │ │ • Example: Total = 5000 bytes, Queue 1 = 1500 → 30% bandwidth │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **CQ Configuration:** ```cisco ! Define custom queue list queue-list 1 protocol ip 1 tcp 22 ! SSH to queue 1 queue-list 1 protocol ip 2 tcp 80 ! HTTP to queue 2 queue-list 1 protocol ip 3 tcp 21 ! FTP to queue 3 queue-list 1 default 4 ! Default to queue 4 queue-list 1 queue 1 byte-count 1500 ! Queue 1 gets 1500 bytes per cycle queue-list 1 queue 2 byte-count 1500 ! Queue 2 gets 1500 bytes per cycle queue-list 1 queue 3 byte-count 1500 ! Queue 3 gets 1500 bytes per cycle queue-list 1 queue 4 byte-count 500 ! Queue 4 gets 500 bytes per cycle ! Apply to interface interface Serial0/0/0 custom-queue-list 1 ``` --- ### 6. WFQ (Weighted Fair Queuing) **Description:** Automatically classifies traffic into flows based on source/destination IP, port, protocol. Each flow gets fair share of bandwidth, with weights based on IP precedence. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WFQ OPERATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Flow Classification: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Flow 1: Source IP A, Dest IP B, Port 80 │ │ │ │ Flow 2: Source IP A, Dest IP C, Port 80 │ │ │ │ Flow 3: Source IP B, Dest IP D, Port 22 │ │ │ │ Flow 4: Source IP C, Dest IP E, Port 443 │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Weight Calculation: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Weight = 4096 / (IP Precedence + 1) │ │ │ │ • Lower weight = more bandwidth │ │ │ │ │ │ │ │ IP Precedence 0 (Best Effort): Weight = 4096 │ │ │ │ IP Precedence 5 (Voice): Weight = 4096 / 6 = 682 │ │ │ │ │ │ │ │ Higher precedence = lower weight = more bandwidth │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Service: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Fair sharing among active flows │ │ │ │ • Low-volume flows get priority │ │ │ │ • Prevents one flow from dominating bandwidth │ │ │ │ • Default on low-speed interfaces (2 Mbps or less) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **WFQ Configuration:** ```cisco ! Enable WFQ on interface interface Serial0/0/0 fair-queue fair-queue queue-limit 64 ! Default queue limit fair-queue congestive-discard-threshold 64 ! WFQ is default on low-speed interfaces ``` --- ### 7. CBWFQ (Class-Based Weighted Fair Queuing) **Description:** Extends WFQ by allowing user-defined classes with guaranteed bandwidth. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ CBWFQ OPERATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Class Definitions: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Class Voice: 30% bandwidth (strict) │ │ │ │ Class Video: 25% bandwidth (guaranteed) │ │ │ │ Class Data: 20% bandwidth (guaranteed) │ │ │ │ Default: 25% bandwidth (remaining) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Queue Structure: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Voice Queue │ Video Queue │ Data Queue │ Default Queue │ │ │ │ (30%) │ (25%) │ (20%) │ (25%) │ │ │ │ [Voice][Voice] │ [Video][Video]│ [Data][Data] │ [BE][BE] │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Service: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Each class gets guaranteed bandwidth │ │ │ │ • Unused bandwidth shared among classes │ │ │ │ • No priority queue (use LLQ for voice) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **CBWFQ Configuration:** ```cisco ! Class maps class-map VOICE match ip dscp ef class-map VIDEO match ip dscp af41 class-map DATA match ip dscp af31 ! Policy map policy-map CBWFQ-POLICY class VOICE bandwidth percent 30 class VIDEO bandwidth percent 25 class DATA bandwidth percent 20 class class-default bandwidth percent 10 fair-queue ! Apply to interface interface Serial0/0/0 service-policy output CBWFQ-POLICY ``` --- ### 8. LLQ (Low-Latency Queuing) **Description:** Combines strict priority queue (for voice/video) with CBWFQ (for other traffic). This is the recommended queuing mechanism for voice and video deployments. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ LLQ OPERATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Queue Structure: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Priority Queue (LLQ) │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ Voice: Always serviced first (strict priority) │ │ │ │ │ │ Max 33% of bandwidth (prevents starvation) │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ CBWFQ Queues │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ Video: 25% guaranteed bandwidth │ │ │ │ │ │ Data: 20% guaranteed bandwidth │ │ │ │ │ │ Default: Remaining bandwidth │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Service Order: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ 1. Service Priority Queue (Voice) until empty │ │ │ │ 2. Service CBWFQ queues in round-robin │ │ │ │ 3. Any unused priority bandwidth shared with CBWFQ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Advantages: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Low latency for voice/video │ │ │ │ • Guaranteed bandwidth for other classes │ │ │ │ • Prevents voice starvation │ │ │ │ • Recommended for converged networks │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **LLQ Configuration:** ```cisco ! Class maps class-map VOICE match ip dscp ef class-map VIDEO match ip dscp af41 class-map DATA match ip dscp af31 ! Policy map with LLQ policy-map LLQ-POLICY class VOICE priority percent 30 ! Strict priority queue class VIDEO bandwidth percent 25 ! Guaranteed bandwidth class DATA bandwidth percent 20 ! Guaranteed bandwidth class class-default bandwidth percent 10 fair-queue ! Apply to interface interface Serial0/0/0 service-policy output LLQ-POLICY ``` --- ### 9. Queuing Mechanism Selection Guide ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QUEUING SELECTION GUIDE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ QUESTION 1: Is there congestion? │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ NO → Use FIFO (no queuing needed) │ │ │ │ YES → Continue │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ │ ▼ │ │ QUESTION 2: Do you need to differentiate traffic? │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ NO → Use WFQ (fair sharing without classification) │ │ │ │ YES → Continue │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ │ ▼ │ │ QUESTION 3: Do you have voice/video traffic? │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ YES → Use LLQ (priority queue + CBWFQ) │ │ │ │ NO → Use CBWFQ (guaranteed bandwidth per class) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 10. Queuing Verification Commands | Command | Purpose | |---------|---------| | `show policy-map interface [int]` | Display queuing statistics | | `show queueing interface [int]` | Display queue statistics | | `show interfaces [int]` | Display interface drops | | `show running-config | section policy-map` | Display policy map config | | `debug policy-map` | Debug QoS policy | **Example Outputs:** ```cisco Router# show policy-map interface serial 0/0/0 Serial0/0/0 Service-policy output: LLQ-POLICY Class-map: VOICE (match-all) 1234 packets, 123456 bytes 5 minute offered rate 1000 bps, drop rate 0 bps Match: dscp ef (46) Strict Priority Output Queue: Conversation 264 Bandwidth 30 (%) Bandwidth 300 (kbps) Burst 7500 (Bytes) (pkts matched/bytes matched) 1234/123456 (total drops/bytes drops) 0/0 Class-map: VIDEO (match-all) 5678 packets, 567890 bytes 5 minute offered rate 2000 bps, drop rate 0 bps Match: dscp af41 (34) Queueing Output Queue: Conversation 265 Bandwidth 25 (%) Bandwidth 250 (kbps) Max Threshold 64 (packets) (pkts matched/bytes matched) 5678/567890 (depth/total drops/no-buffer drops) 0/0/0 Class-map: class-default (match-any) 9012 packets, 901234 bytes 5 minute offered rate 5000 bps, drop rate 0 bps Match: any Queueing Output Queue: Conversation 266 Bandwidth 10 (%) Bandwidth 100 (kbps) Max Threshold 64 (packets) (pkts matched/bytes matched) 9012/901234 (depth/total drops/no-buffer drops) 0/0/0 ``` --- ### 11. Queuing Best Practices ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QUEUING BEST PRACTICES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ 1. USE LLQ FOR VOICE AND VIDEO │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Voice requires strict priority (low latency) │ │ │ │ • Limit priority queue to 33% of bandwidth │ │ │ │ • Use CBWFQ for other classes │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 2. ALLOCATE BANDWIDTH BASED ON BUSINESS NEEDS │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Voice: 30% (priority) │ │ │ │ • Video: 25% (guaranteed) │ │ │ │ • Mission-critical data: 20% (guaranteed) │ │ │ │ • Best effort: remaining │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 3. AVOID OVER-SUBSCRIPTION │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Total guaranteed bandwidth should not exceed link capacity │ │ │ │ • Leave bandwidth for best-effort traffic │ │ │ │ • Example: 33% + 25% + 20% = 78% (leave 22% for BE) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 4. MONITOR DROPS │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Check drop counters regularly │ │ │ │ • Adjust bandwidth allocations if drops occur │ │ │ │ • Investigate priority queue drops (possible misconfiguration) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 5. APPLY QUEUING ON CONGESTED INTERFACES │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • WAN interfaces (slower links) │ │ │ │ • Uplinks to distribution │ │ │ │ • Interfaces facing internet │ │ │ │ • No need for queuing on under-utilized links │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: FIFO Configuration ```cisco ! Enable FIFO on high-speed interface interface GigabitEthernet0/0 no fair-queue ! Disable WFQ ! FIFO is default when no other queuing configured ``` --- ### Lab 2: PQ Configuration ```cisco ! Priority list configuration priority-list 1 protocol ip high tcp 22 ! SSH high priority priority-list 1 protocol ip high udp 53 ! DNS high priority priority-list 1 protocol ip medium tcp 80 ! HTTP medium priority priority-list 1 protocol ip medium tcp 443 ! HTTPS medium priority priority-list 1 protocol ip normal tcp 21 ! FTP normal priority priority-list 1 default low interface Serial0/0/0 priority-group 1 ``` --- ### Lab 3: CQ Configuration ```cisco ! Custom queue list queue-list 1 protocol ip 1 tcp 22 ! SSH queue 1 queue-list 1 protocol ip 1 udp 53 ! DNS queue 1 queue-list 1 protocol ip 2 tcp 80 ! HTTP queue 2 queue-list 1 protocol ip 2 tcp 443 ! HTTPS queue 2 queue-list 1 protocol ip 3 tcp 21 ! FTP queue 3 queue-list 1 default 4 ! Default queue 4 queue-list 1 queue 1 byte-count 1500 ! 1500 bytes per cycle queue-list 1 queue 2 byte-count 1500 queue-list 1 queue 3 byte-count 1500 queue-list 1 queue 4 byte-count 500 interface Serial0/0/0 custom-queue-list 1 ``` --- ### Lab 4: CBWFQ Configuration ```cisco ! Class maps class-map match-any CRITICAL match ip dscp af31 af32 af33 class-map match-any BUSINESS match ip dscp af21 af22 af23 class-map match-any SCAVENGER match ip dscp cs1 ! Policy map policy-map CBWFQ-WAN class CRITICAL bandwidth percent 30 class BUSINESS bandwidth percent 25 class SCAVENGER bandwidth percent 5 class class-default bandwidth percent 20 fair-queue interface Serial0/0/0 service-policy output CBWFQ-WAN ``` --- ### Lab 5: LLQ Configuration (Recommended) ```cisco ! Class maps class-map VOICE match ip dscp ef class-map VIDEO match ip dscp af41 class-map CRITICAL match ip dscp af31 class-map BUSINESS match ip dscp af21 ! Policy map with LLQ policy-map LLQ-WAN class VOICE priority percent 30 class VIDEO bandwidth percent 25 class CRITICAL bandwidth percent 20 class BUSINESS bandwidth percent 10 class class-default fair-queue interface Serial0/0/0 service-policy output LLQ-WAN ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **FIFO** | Default on high-speed interfaces, no prioritization | | **PQ** | 4 queues, strict priority, starvation possible | | **CQ** | Round-robin, guaranteed bandwidth per queue | | **WFQ** | Automatic flow classification, fair sharing | | **CBWFQ** | User-defined classes, guaranteed bandwidth | | **LLQ** | Priority queue + CBWFQ, best for voice/video | | **Priority Queue Limit** | Should not exceed 33% of bandwidth | ### Common Exam Scenarios: **Scenario 1:** "Which queuing mechanism is recommended for a network with voice, video, and data traffic?" - **Answer:** LLQ (Low-Latency Queuing) **Scenario 2:** "What is the risk of using PQ (Priority Queuing) for voice traffic?" - **Answer:** Low priority queues may starve (never get service) **Scenario 3:** "Which queuing mechanism is the default on low-speed interfaces (2 Mbps or less)?" - **Answer:** WFQ (Weighted Fair Queuing) ### Mnemonics: **Queuing Mechanisms:** **"F.P.C.W.C.L." - FIFO, PQ, CQ, WFQ, CBWFQ, LLQ** **LLQ Components:** **"P + G" - Priority + Guaranteed** - **P**riority queue: Voice/video (low latency) - **G**uaranteed bandwidth: Other classes **Priority Queue Limit:** **"33 is the key"** - Priority queue should not exceed 33% of bandwidth --- ## 📝 Summary (1-Minute Revision) ``` QUEUING MECHANISMS: FIFO (First-In-First-Out): ├── Single queue, no prioritization ├── Default on high-speed interfaces └── Use: No congestion or simple networks PQ (Priority Queuing): ├── 4 queues (High, Medium, Normal, Low) ├── Strict priority (high always first) ├── Risk: Starvation of low priority queues └── Legacy (replaced by LLQ) CQ (Custom Queuing): ├── Up to 16 queues, round-robin ├── Configurable byte count per queue ├── Guaranteed bandwidth per queue └── Legacy (replaced by CBWFQ) WFQ (Weighted Fair Queuing): ├── Automatic flow classification ├── Fair sharing among flows ├── Default on low-speed interfaces (≤2 Mbps) └── Weight based on IP precedence CBWFQ (Class-Based WFQ): ├── User-defined classes ├── Guaranteed bandwidth per class ├── Fair sharing within classes └── No priority queue LLQ (Low-Latency Queuing): ├── Priority queue + CBWFQ ├── Strict priority for voice/video ├── Limit priority queue to 33% └── Recommended for converged networks VERIFICATION: ├── show policy-map interface ├── show queueing interface └── show interfaces ``` --- ## 🧪 Practice Questions **1. Which queuing mechanism is the default on high-speed interfaces (2 Mbps or greater)?** - A) FIFO - B) WFQ - C) CBWFQ - D) LLQ <details> <summary>Answer</summary> <b>A) FIFO</b> - FIFO is the default queuing mechanism on high-speed interfaces. </details> **2. Which queuing mechanism provides a strict priority queue for low-latency traffic?** - A) FIFO - B) PQ - C) WFQ - D) LLQ <details> <summary>Answer</summary> <b>D) LLQ</b> - Low-Latency Queuing provides a strict priority queue for voice/video. </details> **3. What is the risk of using Priority Queuing (PQ)?** - A) High latency for priority traffic - B) Low priority queues may starve - C) Complex configuration - D) No QoS guarantees <details> <summary>Answer</summary> <b>B) Low priority queues may starve</b> - Strict priority can cause starvation of lower queues. </details> **4. Which queuing mechanism automatically classifies traffic into flows?** - A) FIFO - B) PQ - C) WFQ - D) CQ <details> <summary>Answer</summary> <b>C) WFQ</b> - Weighted Fair Queuing automatically classifies traffic into flows. </details> **5. What is the recommended maximum percentage for a priority queue in LLQ?** - A) 10% - B) 25% - C) 33% - D) 50% <details> <summary>Answer</summary> <b>C) 33%</b> - Priority queue should not exceed 33% to prevent starvation of other traffic. </details> **6. Which queuing mechanism uses round-robin with configurable byte counts?** - A) FIFO - B) PQ - C) CQ - D) WFQ <details> <summary>Answer</summary> <b>C) CQ</b> - Custom Queuing uses round-robin with configurable byte counts. </details> **7. Which queuing mechanism is the default on low-speed interfaces (2 Mbps or less)?** - A) FIFO - B) WFQ - C) CBWFQ - D) LLQ <details> <summary>Answer</summary> <b>B) WFQ</b> - WFQ is the default queuing mechanism on low-speed interfaces. </details> **8. Which queuing mechanism allows user-defined classes with guaranteed bandwidth?** - A) FIFO - B) PQ - C) WFQ - D) CBWFQ <details> <summary>Answer</summary> <b>D) CBWFQ</b> - Class-Based Weighted Fair Queuing allows user-defined classes. </details> **9. What does LLQ combine?** - A) FIFO + PQ - B) PQ + CQ - C) Priority queue + CBWFQ - D) WFQ + CBWFQ <details> <summary>Answer</summary> <b>C) Priority queue + CBWFQ</b> - LLQ combines a strict priority queue with CBWFQ. </details> **10. Which command displays queuing statistics on an interface?** - A) `show queue` - B) `show policy-map interface` - C) `show queuing` - D) `show interface stats` <details> <summary>Answer</summary> <b>B) `show policy-map interface`</b> - Displays queuing and QoS statistics. </details> **11. Which queuing mechanism has 4 priority levels (High, Medium, Normal, Low)?** - A) FIFO - B) PQ - C) CQ - D) WFQ <details> <summary>Answer</summary> <b>B) PQ</b> - Priority Queuing uses 4 priority levels. </details> **12. Which queuing mechanism is recommended for voice and video traffic?** - A) FIFO - B) PQ - C) CBWFQ - D) LLQ <details> <summary>Answer</summary> <b>D) LLQ</b> - Low-Latency Queuing is recommended for voice and video. </details> --- ## 📌 Next Steps After completing Video 42, you should be ready for: - **Video 43:** Congestion Avoidance (RED, WRED) - **Video 44:** Traffic Policing and Shaping **Lab Practice:** 1. Configure FIFO on a high-speed interface 2. Configure LLQ with priority queue for voice 3. Configure CBWFQ for data classes 4. Verify with `show policy-map interface` 5. Test different traffic types 6. Monitor drop counters --- **Ready for Video 43?** Share the link or say "next" and I'll continue with Congestion Avoidance (RED, WRED). I'll continue with **Video 43: Congestion Avoidance (RED, WRED)** based on the standard CCNA 200-301 curriculum. ---