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Module 41
QoS (Quality of Service) Overview
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# 📘 CCNA 200-301 - Video 41: QoS (Quality of Service) Overview ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - What QoS is and why it's needed - QoS models (Best Effort, IntServ, DiffServ) - QoS components (Classification, Marking, Queuing, Shaping, Policing) - QoS tools and mechanisms - QoS on Cisco devices - Network congestion and delay types --- ## 🧠 Core Concepts ### 1. What is QoS? **Definition:** Quality of Service (QoS) is a set of technologies that manage network resources to ensure predictable and acceptable performance for critical applications by prioritizing certain types of traffic. **Analogy:** Think of QoS like a highway system with different lanes: - **Emergency vehicles (Voice)** get the express lane (highest priority) - **Buses (Video)** get dedicated lanes (high priority) - **Regular cars (Data)** use regular lanes - **Trucks (Background)** use slower lanes - **Traffic police (QoS)** manage and direct traffic to ensure critical vehicles get through ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WHY QoS IS NEEDED │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ WITHOUT QoS: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ All traffic treated equally: │ │ │ │ [Voice] [Data] [Video] [Voice] [Data] [Backup] [Voice] │ │ │ │ │ │ │ │ Results: │ │ │ │ • Voice choppy (packet loss, jitter) │ │ │ │ • Video freezing │ │ │ │ • Critical applications slow │ │ │ │ • Network congestion affects everything │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ WITH QoS: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Priority traffic gets preferential treatment: │ │ │ │ [Voice][Voice][Video][Data][Backup][Voice][Voice] │ │ │ │ ↑Priority ↑Next ↑Lowest │ │ │ │ │ │ │ │ Results: │ │ │ │ • Voice clear and consistent │ │ │ │ • Video smooth │ │ │ │ • Critical apps perform well │ │ │ │ • Non-critical apps get remaining bandwidth │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. Common Network Problems Solved by QoS | Problem | Description | QoS Solution | |---------|-------------|--------------| | **Packet Loss** | Network drops packets during congestion | Prioritization, Queuing | | **Latency** | Delay in packet delivery (milliseconds) | Priority queuing, Low-latency queuing | | **Jitter** | Variation in packet delay | Consistent queuing, Traffic shaping | | **Bandwidth** | Insufficient capacity | Bandwidth allocation, Rate limiting | | **Out-of-Order** | Packets arrive in wrong order | Proper queuing mechanisms | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ NETWORK PERFORMANCE METRICS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ BANDWIDTH: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Maximum data transfer rate (Mbps/Gbps) │ │ │ │ • Limited resource - needs allocation │ │ │ │ • QoS allocates bandwidth to critical traffic │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ LATENCY (Delay): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Time for packet to travel from source to destination │ │ │ │ • Types: │ │ │ │ - Propagation delay (distance) │ │ │ │ - Serialization delay (putting on wire) │ │ │ │ - Queuing delay (waiting in queue) │ │ │ │ - Processing delay (router CPU) │ │ │ │ • Voice requirement: < 150 ms │ │ │ │ • Video requirement: < 200 ms │ │ │ │ • Gaming requirement: < 50 ms │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ JITTER: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Variation in packet delay (difference between min and max) │ │ │ │ • Voice requirement: < 30 ms │ │ │ │ • Solved by consistent queuing and priority │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ PACKET LOSS: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Percentage of packets dropped │ │ │ │ • Voice requirement: < 1% │ │ │ │ • Data: can tolerate loss (TCP retransmits) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. QoS Models | Model | Description | Complexity | Scalability | |-------|-------------|------------|-------------| | **Best Effort** | No QoS; all traffic treated equally | None | Excellent | | **IntServ (Integrated Services)** | Per-flow reservation (RSVP) | High | Poor | | **DiffServ (Differentiated Services)** | Traffic classes with different priorities | Medium | Excellent | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QoS MODELS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ BEST EFFORT (No QoS): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ All traffic = Same priority │ │ │ │ • Simple │ │ │ │ • No guarantees │ │ │ │ • Suitable for networks with no congestion │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ INTSERV (Integrated Services - RSVP): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Application requests resources: │ │ │ │ "I need 64 kbps with <50ms delay for this flow" │ │ │ │ │ │ │ │ • Per-flow reservation │ │ │ │ • RSVP protocol │ │ │ │ • End-to-end guarantee │ │ │ │ • Not scalable for large networks │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ DIFFSERV (Differentiated Services): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Traffic classified into classes: │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ Voice: Priority 1 (highest) │ │ │ │ │ │ Video: Priority 2 │ │ │ │ │ │ Critical Data: Priority 3 │ │ │ │ │ │ Best Effort: Priority 4 │ │ │ │ │ │ Background: Priority 5 (lowest) │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ • No per-flow state │ │ │ │ • Scalable │ │ │ │ • Most common QoS model │ │ │ │ • Used in enterprise and service provider networks │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 4. QoS Components ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QoS COMPONENTS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ 1. CLASSIFICATION & MARKING │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Classification: Identify traffic type │ │ │ │ • Based on: IP address, protocol, port, application │ │ │ │ • ACLs, NBAR (Network Based Application Recognition) │ │ │ │ │ │ │ │ Marking: Tag traffic for QoS treatment │ │ │ │ • CoS (Class of Service) - Layer 2 (802.1p) │ │ │ │ • DSCP (Differentiated Services Code Point) - Layer 3 │ │ │ │ • IP Precedence - Layer 3 (legacy) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 2. QUEUING & DROPPING │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Queuing: Manage packets waiting for transmission │ │ │ │ • FIFO (First-In-First-Out) - default │ │ │ │ • PQ (Priority Queuing) - strict priority │ │ │ │ • CQ (Custom Queuing) - bandwidth allocation │ │ │ │ • WFQ (Weighted Fair Queuing) - flow-based │ │ │ │ • CBWFQ (Class-Based WFQ) - class-based │ │ │ │ • LLQ (Low-Latency Queuing) - priority + CBWFQ │ │ │ │ │ │ │ │ Dropping: Manage congestion │ │ │ │ • Tail Drop - drop packets when queue full │ │ │ │ • RED (Random Early Detection) - proactive drop │ │ │ │ • WRED (Weighted RED) - class-based drop │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 3. SHAPING & POLICING │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Policing: Drop or remark traffic exceeding rate │ │ │ │ • Rate limiting │ │ │ │ • Drop excess packets │ │ │ │ • Applied inbound or outbound │ │ │ │ │ │ │ │ Shaping: Buffer traffic exceeding rate │ │ │ │ • Smooth traffic flow │ │ │ │ • Applied outbound only │ │ │ │ • Prevents packet loss │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 4. CONGESTION AVOIDANCE │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ • WRED (Weighted Random Early Detection) │ │ │ │ • Drop lower-priority packets before queues fill │ │ │ │ • Prevents TCP global synchronization │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 5. Classification and Marking **Layer 2 Marking - CoS (Class of Service):** | CoS | Priority | Application | DSCP Mapping | |-----|----------|-------------|--------------| | 0 | Best Effort | Default | 0 | | 1 | Background | Bulk data | 8 | | 2 | Spare | Spare | 16 | | 3 | Excellent Effort | Business apps | 24 | | 4 | Controlled Load | Streaming | 32 | | 5 | Voice | Voice, Video | 40 (EF) | | 6 | Network Control | Routing | 48 | | 7 | Network Control | Routing | 56 | **Layer 3 Marking - IP Precedence (3 bits):** | IPP | Name | Application | |-----|------|-------------| | 0 | Routine | Best Effort | | 1 | Priority | Priority | | 2 | Immediate | Immediate | | 3 | Flash | Flash | | 4 | Flash-Override | Flash Override | | 5 | Critical | Voice | | 6 | Internet | Network Control | | 7 | Network | Network Control | **Layer 3 Marking - DSCP (Differentiated Services Code Point):** | DSCP | Binary | Application | |------|--------|-------------| | EF (46) | 101110 | Voice (Expedited Forwarding) | | AF41 (34) | 100010 | Video (Assured Forwarding) | | AF31 (26) | 011010 | High-priority data | | AF21 (18) | 010010 | Medium-priority data | | AF11 (10) | 001010 | Low-priority data | | 0 | 000000 | Best Effort | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ DSCP AND IP PRECEDENCE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ IP Header - ToS (Type of Service) / DSCP Field: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ 0 1 2 3 4 5 6 7 │ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ │ │ DSCP (6 bits) │ ECN (2 bits) │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ DSCP to IP Precedence Mapping: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ DSCP (bits 0-2) = IP Precedence (3 bits) │ │ │ │ Example: DSCP 46 (101110) = IPP 5 (101) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Common DSCP Values: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • EF (Expedited Forwarding): 46 - Voice │ │ │ │ • AF41 (Assured Forwarding): 34 - Video │ │ │ │ • AF31: 26 - Critical Data │ │ │ │ • AF21: 18 - Business Data │ │ │ │ • AF11: 10 - Best Effort │ │ │ │ • CS0: 0 - Default │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 6. QoS Tools and Mechanisms **Marking Configuration:** ```cisco ! Classify traffic using ACL access-list 100 permit tcp any any eq 80 access-list 100 permit tcp any any eq 443 ! Class map class-map match-any WEB-TRAFFIC match access-group 100 ! Policy map with marking policy-map MARK-POLICY class WEB-TRAFFIC set dscp af21 class class-default set dscp default ! Apply to interface interface GigabitEthernet0/0 service-policy input MARK-POLICY ``` **Queuing Configuration (CBWFQ):** ```cisco ! Class maps for different traffic types class-map VOICE match dscp ef class-map VIDEO match dscp af41 class-map DATA match dscp af21 ! Policy map with queuing policy-map QUEUE-POLICY class VOICE priority percent 33 ! Strict priority, 33% bandwidth class VIDEO bandwidth percent 25 ! Guaranteed 25% bandwidth class DATA bandwidth percent 20 ! Guaranteed 20% bandwidth class class-default bandwidth percent 10 ! Remaining bandwidth ! Apply to interface interface GigabitEthernet0/0 service-policy output QUEUE-POLICY ``` --- ### 7. Queuing Mechanisms ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QUEUING MECHANISMS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ FIFO (First-In-First-Out): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ [Packet1][Packet2][Packet3][Packet4][Packet5] → Transmit │ │ │ │ • Default on high-speed interfaces │ │ │ │ • No priority │ │ │ │ • Simple but no differentiation │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ PQ (Priority Queuing): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ High: [Voice][Voice][Voice] → Transmit first │ │ │ │ Medium: [Video][Video] → Transmit when high empty │ │ │ │ Normal: [Data][Data] → Transmit when high/medium empty │ │ │ │ Low: [Backup][Backup] → Transmit only when all others empty │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ CBWFQ (Class-Based Weighted Fair Queuing): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Each class gets guaranteed bandwidth │ │ │ │ • Classes can have different weights │ │ │ │ • Fair sharing of remaining bandwidth │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ LLQ (Low-Latency Queuing): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Priority queue + CBWFQ │ │ │ │ • Priority queue (Voice) always serviced first │ │ │ │ • Other classes get guaranteed bandwidth │ │ │ │ • Most common for voice/video deployments │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 8. Policing vs. Shaping | Feature | Policing | Shaping | |---------|----------|---------| | **Action** | Drops or remarks excess traffic | Buffers excess traffic | | **Direction** | Inbound or outbound | Outbound only | | **Effect** | Hard rate limit, may drop | Smooth traffic flow | | **Buffer** | No buffer | Uses buffer | | **Delay** | No added delay | Adds delay | | **Best For** | Enforcing strict limits | Smoothing bursty traffic | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ POLICING vs. SHAPING │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ POLICING (Drop excess): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Traffic Rate │ │ │ │ ▲ │ │ │ │ 1M │ ████ │ │ │ │ 800K│ ████ │ │ │ │ 600K│ ████ │ │ │ │ 400K│ ████ │ │ │ │ 200K│ ████ │ │ │ │ └──────────────────────────────────► Time │ │ │ │ │ │ │ │ • Excess packets dropped │ │ │ │ • No delay │ │ │ │ • Traffic shaped to line rate │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ SHAPING (Buffer excess): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Traffic Rate │ │ │ │ ▲ │ │ │ │ 1M │ │ │ │ │ 800K│ ████ │ │ │ │ 600K│ ████ │ │ │ │ 400K│ ████ │ │ │ │ 200K│ ████ │ │ │ │ └──────────────────────────────────► Time │ │ │ │ │ │ │ │ • Excess packets buffered │ │ │ │ • Adds delay │ │ │ │ • Smooth traffic shape │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Policing Configuration:** ```cisco ! Police traffic to 1 Mbps, burst 8000 bytes policy-map POLICE-POLICY class class-default police 1000000 8000 conform-action transmit exceed-action drop interface GigabitEthernet0/0 service-policy input POLICE-POLICY ``` **Shaping Configuration:** ```cisco ! Shape traffic to 10 Mbps policy-map SHAPE-POLICY class class-default shape average 10000000 interface GigabitEthernet0/0 service-policy output SHAPE-POLICY ``` --- ### 9. QoS Verification Commands | Command | Purpose | |---------|---------| | `show policy-map` | Display policy map configuration | | `show policy-map interface [int]` | Display QoS statistics on interface | | `show class-map` | Display class map configuration | | `show queueing interface [int]` | Display queue statistics | | `show interface [int]` | Display interface statistics (drops) | | `debug policy-map` | Debug QoS policy (use cautiously) | **Example Outputs:** ```cisco Router# show policy-map interface gigabitEthernet 0/0 GigabitEthernet0/0 Service-policy output: QUEUE-POLICY Class-map: VOICE (match-all) 0 packets, 0 bytes 5 minute offered rate 0000 bps, drop rate 0000 bps Match: dscp ef (46) Strict Priority Output Queue: Conversation 264 Bandwidth 33 (%) Bandwidth 33 (kbps) Burst 825 (Bytes) (pkts matched/bytes matched) 0/0 (total drops/bytes drops) 0/0 Class-map: VIDEO (match-all) 0 packets, 0 bytes 5 minute offered rate 0000 bps, drop rate 0000 bps Match: dscp af41 (34) Queueing Output Queue: Conversation 265 Bandwidth 25 (%) Bandwidth 25 (kbps) Max Threshold 64 (packets) (pkts matched/bytes matched) 0/0 (depth/total drops/no-buffer drops) 0/0/0 Class-map: class-default (match-any) 0 packets, 0 bytes 5 minute offered rate 0000 bps, drop rate 0000 bps Match: any Queueing Output Queue: Conversation 266 Bandwidth 10 (%) Bandwidth 10 (kbps) Max Threshold 64 (packets) (pkts matched/bytes matched) 0/0 (depth/total drops/no-buffer drops) 0/0/0 ``` --- ### 10. QoS Best Practices ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ QoS BEST PRACTICES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ 1. CLASSIFY TRAFFIC NEAR THE SOURCE │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Mark traffic as close to the source as possible │ │ │ │ • Trusted devices can keep markings │ │ │ │ • Untrusted devices should have markings overwritten │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 2. USE DSCP FOR LAYER 3 MARKING │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • DSCP is the standard for IP networks │ │ │ │ • Provides 64 classes (vs. 8 for IP precedence) │ │ │ │ • Backward compatible with IP precedence │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 3. USE LLQ FOR VOICE AND VIDEO │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Voice requires low latency (LLQ priority queue) │ │ │ │ • Limit priority queue to 33% of bandwidth │ │ │ │ • Prevent starvation of other traffic │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 4. CONFIGURE END-TO-END QOS │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Consistent marking across all devices │ │ │ │ • Queuing policies on congested interfaces │ │ │ │ • Policing at network edges │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 5. MONITOR AND ADJUST │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Monitor drop rates │ │ │ │ • Adjust bandwidth allocations │ │ │ │ • Review QoS effectiveness regularly │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: Basic QoS Marking ```cisco ! Classify voice traffic class-map VOICE match dscp ef ! Classify video traffic class-map VIDEO match dscp af41 ! Classify critical data class-map CRITICAL match dscp af31 ! Policy to mark traffic policy-map MARKING class VOICE set dscp ef class VIDEO set dscp af41 class CRITICAL set dscp af31 class class-default set dscp default ! Apply marking on trusted interface interface GigabitEthernet0/0 service-policy input MARKING ``` --- ### Lab 2: LLQ with CBWFQ ```cisco ! Class maps class-map VOICE match dscp ef class-map VIDEO match dscp af41 class-map DATA match dscp af31 ! Policy with LLQ and CBWFQ policy-map WAN-QOS class VOICE priority percent 30 ! Strict priority (voice) class VIDEO bandwidth percent 25 ! Guaranteed bandwidth (video) class DATA bandwidth percent 20 ! Guaranteed bandwidth (critical data) class class-default bandwidth percent 5 ! Best effort fair-queue ! Fair queuing within default class ! Apply to WAN interface interface Serial0/0/0 service-policy output WAN-QOS ``` --- ### Lab 3: Traffic Shaping ```cisco ! Shape traffic to 10 Mbps on WAN link policy-map SHAPE-WAN class class-default shape average 10000000 service-policy WAN-QOS ! Apply child policy inside shape interface Serial0/0/0 service-policy output SHAPE-WAN ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **QoS Models** | Best Effort, IntServ (RSVP), DiffServ | | **QoS Components** | Classification, Marking, Queuing, Shaping, Policing | | **DSCP Values** | EF (46), AF41 (34), AF31 (26), AF21 (18), AF11 (10) | | **LLQ** | Priority queue + CBWFQ, used for voice/video | | **Policing vs. Shaping** | Policing (drops), Shaping (buffers) | | **WRED** | Prevents TCP global synchronization | ### Common Exam Scenarios: **Scenario 1:** "Voice traffic requires low latency and jitter. Which queuing mechanism should be used?" - **Answer:** LLQ (Low-Latency Queuing) with priority queue **Scenario 2:** "A WAN link needs to limit traffic to 10 Mbps to match service provider commit rate. Which QoS tool should be used?" - **Answer:** Shaping (buffers excess traffic) **Scenario 3:** "Which QoS marking uses 6 bits in the IP header?" - **Answer:** DSCP (Differentiated Services Code Point) ### Mnemonics: **QoS Models:** **"B.I.D." - Best Effort, IntServ, DiffServ** - **B**est Effort: No QoS - **I**ntServ: Per-flow (RSVP) - **D**iffServ: Class-based **DSCP Values:** **"E for Voice, A for Video"** - **E**F: 46 - Voice - **A**F41: 34 - Video - **A**F31: 26 - Critical Data --- ## 📝 Summary (1-Minute Revision) ``` QoS (Quality of Service): QoS MODELS: ├── Best Effort: No QoS ├── IntServ: RSVP, per-flow └── DiffServ: Class-based (most common) QoS COMPONENTS: ├── Classification: Identify traffic (ACL, NBAR) ├── Marking: Tag traffic (CoS, DSCP) ├── Queuing: Manage queues (FIFO, PQ, CBWFQ, LLQ) ├── Shaping: Buffer excess traffic ├── Policing: Drop excess traffic └── Congestion Avoidance: WRED DSCP VALUES: ├── EF (46): Voice ├── AF41 (34): Video ├── AF31 (26): High-priority data ├── AF21 (18): Medium-priority data ├── AF11 (10): Low-priority data └── 0: Best Effort QUEUING MECHANISMS: ├── FIFO: First-in-first-out ├── PQ: Strict priority ├── CBWFQ: Guaranteed bandwidth per class └── LLQ: Priority + CBWFQ (best for voice/video) POLICING vs. SHAPING: ├── Policing: Drops excess, inbound/outbound └── Shaping: Buffers excess, outbound only VERIFICATION: ├── show policy-map interface ├── show class-map └── show queueing interface ``` --- ## 🧪 Practice Questions **1. Which QoS model uses RSVP for per-flow reservations?** - A) Best Effort - B) IntServ - C) DiffServ - D) MPLS QoS <details> <summary>Answer</summary> <b>B) IntServ</b> - Integrated Services uses RSVP for per-flow resource reservations. </details> **2. What is the DSCP value for Expedited Forwarding (voice)?** - A) 34 - B) 26 - C) 46 - D) 18 <details> <summary>Answer</summary> <b>C) 46</b> - EF (Expedited Forwarding) is DSCP 46. </details> **3. Which queuing mechanism provides a strict priority queue for low-latency traffic?** - A) FIFO - B) CBWFQ - C) LLQ - D) WFQ <details> <summary>Answer</summary> <b>C) LLQ</b> - Low-Latency Queuing provides a strict priority queue. </details> **4. What is the difference between policing and shaping?** - A) Policing buffers, shaping drops - B) Policing drops, shaping buffers - C) Both drop excess traffic - D) Both buffer excess traffic <details> <summary>Answer</summary> <b>B) Policing drops, shaping buffers</b> - Policing drops excess traffic, shaping buffers it. </details> **5. Which QoS marking is used at Layer 2?** - A) DSCP - B) IP Precedence - C) CoS - D) MPLS EXP <details> <summary>Answer</summary> <b>C) CoS</b> - Class of Service (802.1p) is used for Layer 2 marking. </details> **6. What is the purpose of WRED?** - A) Increase bandwidth - B) Prevent TCP global synchronization - C) Prioritize voice traffic - D) Shape traffic <details> <summary>Answer</summary> <b>B) Prevent TCP global synchronization</b> - WRED drops packets randomly to prevent multiple TCP flows from slowing simultaneously. </details> **7. Which QoS tool is typically applied on WAN interfaces to match service provider CIR?** - A) Policing - B) Shaping - C) Marking - D) Queuing <details> <summary>Answer</summary> <b>B) Shaping</b> - Shaping buffers excess traffic to match the CIR (Committed Information Rate). </details> **8. What is the DSCP value for AF41 (video)?** - A) 46 - B) 34 - C) 26 - D) 18 <details> <summary>Answer</summary> <b>B) 34</b> - AF41 (Assured Forwarding Class 4, Low Drop) is DSCP 34. </details> **9. Which command displays QoS statistics on an interface?** - A) `show qos` - B) `show policy-map interface` - C) `show interface qos` - D) `show queue` <details> <summary>Answer</summary> <b>B) `show policy-map interface`</b> - Displays QoS policy statistics on the interface. </details> **10. What is the recommended maximum percentage for 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> **11. Which QoS mechanism adds delay to traffic to smooth bursts?** - A) Policing - B) Marking - C) Shaping - D) Classification <details> <summary>Answer</summary> <b>C) Shaping</b> - Shaping buffers traffic, adding delay to smooth out bursts. </details> **12. What is the maximum latency requirement for voice traffic?** - A) 50 ms - B) 100 ms - C) 150 ms - D) 200 ms <details> <summary>Answer</summary> <b>C) 150 ms</b> - Voice traffic requires less than 150 ms one-way latency. </details> --- ## 📌 Next Steps After completing Video 41, you should be ready for: - **Video 42:** Queuing Mechanisms (Deep Dive) - **Video 43:** Congestion Avoidance (WRED, RED) **Lab Practice:** 1. Configure classification and marking 2. Configure LLQ for voice and video 3. Configure traffic shaping on WAN link 4. Verify with `show policy-map interface` 5. Test different traffic types 6. Monitor drop rates and queue depths --- **Ready for Video 42?** Share the link or say "next" and I'll continue with Queuing Mechanisms (Deep Dive). I'll continue with **Video 42: Queuing Mechanisms (Deep Dive)** based on the standard CCNA 200-301 curriculum. ---