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Module 43
Congestion Avoidance (RED, WRED)
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# 📘 CCNA 200-301 - Video 43: Congestion Avoidance (RED, WRED) ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - What congestion avoidance is and why it's needed - Tail drop problems (TCP global synchronization) - RED (Random Early Detection) concepts - WRED (Weighted Random Early Detection) - WRED configuration and parameters - WRED verification --- ## 🧠 Core Concepts ### 1. What is Congestion Avoidance? **Definition:** Congestion avoidance is a proactive mechanism that manages network congestion by selectively dropping packets before queues become full, preventing the problems associated with tail drop. **Analogy:** Think of congestion avoidance like a traffic management system on a highway: - **Tail Drop:** Letting traffic build up until the highway is completely full, then closing the on-ramp abruptly (all cars stop) - **WRED:** Gradually reducing the number of cars allowed on the on-ramp as traffic increases, preventing complete gridlock ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TAIL DROP PROBLEM │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ TCP Global Synchronization: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Queue Size │ │ │ │ ▲ │ │ │ │ Max│███████████████████████████████████ │ │ │ │ │ ┌─────────────────┐ │ │ │ │ │ │ Queue fills │ │ │ │ │ │ │ Tail drop begins│ │ │ │ │ │──────────────────────────────┘ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 0 └────────────────────────────────────────────────► Time │ │ │ │ │ │ │ │ TCP Window Size │ │ │ │ ▲ │ │ │ │ │ ┌───┐ ┌───┐ ┌───┐ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ (All TCP flows slow down together) │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ └───┘ └───┘ └───┘ │ │ │ │ 0 └────────────────────────────────────────────────► Time │ │ │ │ │ │ │ │ Problem: │ │ │ │ • All TCP flows slow down simultaneously │ │ │ │ • Underutilization after congestion │ │ │ │ • Inefficient use of bandwidth │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. Tail Drop vs. WRED ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TAIL DROP vs. WRED │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ TAIL DROP: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Queue Depth │ │ │ │ ┌─────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ │ │ │ │ Max Queue ──────────────────────────────────────┐ │ │ │ │ │ │ │ │ │ │ │ │ │ ▼ │ │ │ │ │ │ Tail Drop Begins │ │ │ │ │ │ (All new packets dropped) │ │ │ │ │ │ │ │ │ │ │ │ 0 ────────────────────────────────────────────────────────│ │ │ │ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────┘ │ │ │ │ │ │ │ │ • Drops only when queue is full │ │ │ │ • All packets dropped until queue drains │ │ │ │ • Causes TCP global synchronization │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ WRED (Weighted Random Early Detection): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Drop Probability │ │ │ │ ▲ │ │ │ │ 100%│ ┌───────────────────────│ │ │ │ │ ┌────┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ 0% └───┘────────────────────────────────────────────────► Queue │ │ │ │ Min Max │ │ │ │ Threshold Threshold │ │ │ │ │ │ │ │ • Drops packets BEFORE queue fills │ │ │ │ • Drop probability increases as queue depth increases │ │ │ │ • Different drop profiles for different traffic classes │ │ │ │ • Prevents TCP global synchronization │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. WRED Parameters | Parameter | Description | Typical Values | |-----------|-------------|----------------| | **Min Threshold** | Queue depth where dropping begins | 20-50% of queue size | | **Max Threshold** | Queue depth where drop probability reaches 100% | 60-80% of queue size | | **Drop Probability** | Maximum drop rate at max threshold | 10% (1 in 10 packets) | | **Mark Probability Denominator** | 1 / denominator = drop probability | 10 (10% drop rate) | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WRED PARAMETERS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Drop Probability vs. Queue Depth: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Drop │ │ │ │ Prob │ │ │ │ 100%│ ┌───────────────────────│ │ │ │ │ ┌────┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ 0% └───┘────────────────────────────────────────────────► Queue │ │ │ │ ▲ ▲ │ │ │ │ │ │ │ │ │ │ Min Max │ │ │ │ Threshold Threshold │ │ │ │ │ │ │ │ • Min Threshold: No drops below this level │ │ │ │ • Between Min and Max: Linear increase in drop probability │ │ │ │ • Max Threshold: 100% drop rate │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 4. WRED vs. RED | Feature | RED | WRED | |---------|-----|------| | **Traffic Classes** | Single class | Multiple classes (based on IP precedence/DSCP) | | **Drop Profiles** | One profile | Different profiles per class | | **Configuration** | Simple | More complex | | **Use Case** | Best effort | Differentiated services (DiffServ) | | **Supported** | Legacy | Modern IOS | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WRED DROP PROFILES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Different Drop Profiles for Different Traffic Types: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Drop │ │ │ │ Prob │ │ │ │ 100%│ ┌───────────────────────│ │ │ │ │ ┌────┘ │ │ │ │ │ Voice ┌──┘ │ │ │ │ │ ┌───┘ (Very high drop threshold) │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ 0% └───┘────────────────────────────────────────────────► Queue │ │ │ │ │ │ │ │ │ │ │ │ Drop │ │ │ │ Prob │ │ │ │ 100%│ ┌───────────────────────│ │ │ │ │ ┌────┘ │ │ │ │ │ Data ┌─┘ │ │ │ │ │ ┌───┘ (Moderate drop threshold) │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ 0% └───┘────────────────────────────────────────────────► Queue │ │ │ │ │ │ │ │ │ │ │ │ Drop │ │ │ │ Prob │ │ │ │ 100%│ ┌───────────────────────│ │ │ │ │ ┌────┘ │ │ │ │ │ Scavenger ┌┘ │ │ │ │ │ ┌───┘ (Very low drop threshold) │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ │ ┌───┘ │ │ │ │ 0% └───┘────────────────────────────────────────────────► Queue │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 5. WRED Configuration **Basic WRED Configuration:** ```cisco ! Configure WRED on interface interface Serial0/0/0 random-detect ! WRED is now active with default parameters ! Configure WRED with custom thresholds interface Serial0/0/0 random-detect random-detect precedence 0 32 64 10 ! IPP 0: min 32, max 64, prob 1/10 random-detect precedence 1 30 60 10 random-detect precedence 2 28 56 10 random-detect precedence 3 26 52 10 random-detect precedence 4 24 48 10 random-detect precedence 5 22 44 10 random-detect precedence 6 20 40 10 random-detect precedence 7 18 36 10 ``` **WRED Parameters Explained:** ```cisco random-detect precedence [ipp] [min-threshold] [max-threshold] [mark-probability-denominator] Where: - ipp: IP Precedence value (0-7) - min-threshold: Minimum queue depth (packets) where dropping starts - max-threshold: Maximum queue depth where drop probability reaches 100% - mark-probability-denominator: 1/denominator = maximum drop probability (e.g., 10 = 10% drop rate at max threshold) ``` --- ### 6. DSCP-Based WRED **DSCP-Based WRED (on newer IOS):** ```cisco ! Configure DSCP-based WRED interface Serial0/0/0 random-detect dscp-based ! Configure per-DSCP thresholds random-detect dscp 46 30 60 10 ! EF (voice) - higher thresholds random-detect dscp 34 40 80 10 ! AF41 (video) random-detect dscp 26 20 40 10 ! AF31 (critical data) random-detect dscp 18 15 30 10 ! AF21 (business data) random-detect dscp 10 10 20 10 ! AF11 (best effort) ``` --- ### 7. WRED with CBWFQ/LLQ **WRED within CBWFQ Classes:** ```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 WRED policy-map QOS-POLICY class VOICE priority percent 30 random-detect dscp-based class VIDEO bandwidth percent 25 random-detect dscp-based class DATA bandwidth percent 20 random-detect dscp-based class class-default bandwidth percent 10 random-detect interface Serial0/0/0 service-policy output QOS-POLICY ``` --- ### 8. WRED Verification Commands | Command | Purpose | |---------|---------| | `show interfaces [int]` | Display drops (tail drop vs. WRED drops) | | `show queueing random-detect` | Display WRED configuration | | `show policy-map interface [int]` | Display QoS policy with WRED stats | | `show running-config | section random-detect` | Display WRED config | **Example Outputs:** ```cisco Router# show queueing random-detect Current random-detect configuration: Serial0/0/0 Queueing strategy: random early detection (WRED) WRED enabled exp-weight-constant: 9 (1/512) Precedence 0: min-threshold 32 (packets) max-threshold 64 mark-prob 1/10 Precedence 1: min-threshold 30 (packets) max-threshold 60 mark-prob 1/10 Precedence 2: min-threshold 28 (packets) max-threshold 56 mark-prob 1/10 Precedence 3: min-threshold 26 (packets) max-threshold 52 mark-prob 1/10 Precedence 4: min-threshold 24 (packets) max-threshold 48 mark-prob 1/10 Precedence 5: min-threshold 22 (packets) max-threshold 44 mark-prob 1/10 Precedence 6: min-threshold 20 (packets) max-threshold 40 mark-prob 1/10 Precedence 7: min-threshold 18 (packets) max-threshold 36 mark-prob 1/10 ``` ```cisco Router# show policy-map interface serial 0/0/0 Serial0/0/0 Service-policy output: QOS-POLICY Class-map: VOICE (match-all) 1234 packets, 123456 bytes 5 minute offered rate 1000 bps, drop rate 0 bps Match: dscp ef (46) Priority: 30% (300 kbps), burst 7500 bytes WRED: Exponential weight 9 DSCP 46 (EF) - min-threshold 30, max-threshold 60, mark-prob 1/10 (random-detect drops) 0/0 (total/bytes) Class-map: DATA (match-all) 5678 packets, 567890 bytes 5 minute offered rate 5000 bps, drop rate 0 bps Match: dscp af31 (26) Bandwidth 20% (200 kbps) WRED: Exponential weight 9 DSCP 26 (AF31) - min-threshold 20, max-threshold 40, mark-prob 1/10 (random-detect drops) 0/0 (total/bytes) ``` --- ### 9. WRED Drop Statistics ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WRED DROP STATISTICS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Interface Drops: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Router# show interfaces serial 0/0/0 │ │ │ │ │ │ │ │ Serial0/0/0 is up, line protocol is up │ │ │ │ ... │ │ │ │ Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1234│ │ │ ... │ │ │ │ │ │ │ │ Output drops: 1234 (including WRED drops) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ WRED Drops: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Class-map: VOICE │ │ │ │ (random-detect drops) 0/0 (total/bytes) │ │ │ │ │ │ │ │ Class-map: VIDEO │ │ │ │ (random-detect drops) 0/0 (total/bytes) │ │ │ │ │ │ │ │ Class-map: DATA │ │ │ │ (random-detect drops) 123/123456 (total/bytes) │ │ │ │ │ │ │ │ Total drops: 123 (WRED drops) │ │ │ │ Total drops: 0 (tail drops) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 10. WRED Design Best Practices ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WRED BEST PRACTICES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ 1. PROTECT VOICE TRAFFIC │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Set high thresholds for EF (voice) │ │ │ │ • Voice should rarely be dropped │ │ │ │ • Example: min 30, max 60 (vs. 20,40 for data) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 2. USE DIFFERENT PROFILES FOR DIFFERENT CLASSES │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Voice: Highest thresholds │ │ │ │ • Video: High thresholds │ │ │ │ • Critical Data: Medium thresholds │ │ │ │ • Best Effort: Lower thresholds │ │ │ │ • Scavenger: Lowest thresholds (drop aggressively) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 3. CONFIGURE WRED ON CONGESTED INTERFACES │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • WAN links (T1, DSL, cable) │ │ │ │ • Interfaces with queuing (CBWFQ/LLQ) │ │ │ │ • Not needed on under-utilized links │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 4. MONITOR DROPS │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Check WRED drops vs. tail drops │ │ │ │ • High tail drops indicate WRED thresholds too low │ │ │ │ • No drops indicate thresholds too high (ineffective) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 5. AVOID TCP GLOBAL SYNCHRONIZATION │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • WRED prevents all TCP flows from slowing simultaneously │ │ │ │ • Different drop probabilities for different flows │ │ │ │ • Maintains efficient link utilization │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 11. TCP Global Synchronization Explained ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TCP GLOBAL SYNCHRONIZATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Without WRED (Tail Drop): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Flow 1: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ Flow 2: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ Flow 3: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ Flow 4: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ │ │ │ │ All flows: ▲ ▼ ▲ ▼ ▲ ▼ │ │ │ │ (Sync) (Sync) (Sync) (Sync) │ │ │ │ │ │ │ │ • All TCP flows slow down simultaneously │ │ │ │ • Link underutilized after congestion │ │ │ │ • Inefficient │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ With WRED: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Flow 1: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ Flow 2: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ Flow 3: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ Flow 4: ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ │ │ │ │ │ Flows: ▲ ▲ ▼ ▲ ▼ ▲ ▼ ▲ ▼ │ │ │ │ (Random drops) │ │ │ │ │ │ │ │ • Random drops affect different flows at different times │ │ │ │ • Link remains efficiently utilized │ │ │ │ • Smoother throughput │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: Basic WRED Configuration ```cisco ! Basic WRED on interface interface Serial0/0/0 random-detect random-detect precedence 0 32 64 10 random-detect precedence 1 30 60 10 random-detect precedence 2 28 56 10 random-detect precedence 3 26 52 10 random-detect precedence 4 24 48 10 random-detect precedence 5 22 44 10 random-detect precedence 6 20 40 10 random-detect precedence 7 18 36 10 ``` --- ### Lab 2: DSCP-Based WRED ```cisco ! DSCP-based WRED interface Serial0/0/0 random-detect dscp-based random-detect dscp 46 30 60 10 ! Voice (EF) random-detect dscp 34 40 80 10 ! Video (AF41) random-detect dscp 26 20 40 10 ! Critical Data (AF31) random-detect dscp 18 15 30 10 ! Business Data (AF21) random-detect dscp 10 10 20 10 ! Best Effort (AF11) random-detect dscp 0 8 16 10 ! Default ``` --- ### Lab 3: WRED with CBWFQ ```cisco ! Class maps class-map VOICE match dscp ef class-map VIDEO match dscp af41 class-map CRITICAL match dscp af31 class-map BUSINESS match dscp af21 ! Policy map with WRED policy-map WAN-QOS class VOICE priority percent 30 random-detect dscp-based class VIDEO bandwidth percent 25 random-detect dscp-based class CRITICAL bandwidth percent 20 random-detect dscp-based class BUSINESS bandwidth percent 10 random-detect dscp-based class class-default bandwidth percent 5 random-detect ! Apply to interface interface Serial0/0/0 service-policy output WAN-QOS ``` --- ### Lab 4: WRED with LLQ (Voice Priority) ```cisco ! LLQ with WRED for non-voice classes policy-map LLQ-WRED class VOICE priority percent 30 random-detect dscp-based class VIDEO bandwidth percent 25 random-detect dscp-based class DATA bandwidth percent 20 random-detect dscp-based class class-default fair-queue random-detect interface Serial0/0/0 service-policy output LLQ-WRED ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **WRED Purpose** | Prevent TCP global synchronization | | **Tail Drop Problem** | Causes TCP global synchronization | | **WRED vs. RED** | WRED supports multiple traffic classes | | **WRED Parameters** | Min threshold, max threshold, drop probability | | **WRED Placement** | On congested interfaces with queuing | | **Voice Protection** | Higher WRED thresholds for voice traffic | ### Common Exam Scenarios: **Scenario 1:** "What problem does WRED solve that tail drop causes?" - **Answer:** TCP global synchronization **Scenario 2:** "Which traffic class should have the highest WRED thresholds?" - **Answer:** Voice (EF) - should be dropped last **Scenario 3:** "What happens when queue depth reaches the maximum WRED threshold?" - **Answer:** Drop probability reaches 100% (all new packets dropped) ### Mnemonics: **WRED Parameters:** **"M.M.P." - Min, Max, Probability** - **M**in Threshold: Where dropping starts - **M**ax Threshold: Where drop rate = 100% - **P**robability: Drop rate at max threshold **WRED Benefits:** **"P.E.S." - Prevents, Efficient, Smooth** - **P**revents TCP global synchronization - **E**fficient link utilization - **S**mooth throughput --- ## 📝 Summary (1-Minute Revision) ``` CONGESTION AVOIDANCE (RED/WRED): TAIL DROP PROBLEM: ├── Drops only when queue is full ├── Causes TCP global synchronization ├── All TCP flows slow down together └── Inefficient bandwidth utilization WRED (Weighted Random Early Detection): ├── Drops BEFORE queue fills ├── Random drop probability based on queue depth ├── Different thresholds per traffic class └── Prevents TCP global synchronization WRED PARAMETERS: ├── Min Threshold: No drops below this ├── Max Threshold: 100% drop rate at this level ├── Drop Probability: Maximum drop rate └── Mark Probability Denominator: 1/denom = max drop rate WRED CONFIGURATION: ├── random-detect (enable on interface) ├── random-detect precedence [ipp] [min] [max] [prob] ├── random-detect dscp-based (DSCP mode) └── random-detect dscp [value] [min] [max] [prob] WRED BEST PRACTICES: ├── Voice: High thresholds (30,60) ├── Video: High thresholds (40,80) ├── Critical Data: Medium thresholds (20,40) ├── Best Effort: Lower thresholds (10,20) └── Scavenger: Lowest thresholds (5,10) VERIFICATION: ├── show queueing random-detect ├── show policy-map interface └── show interfaces (output drops) ``` --- ## 🧪 Practice Questions **1. What problem does WRED solve that tail drop causes?** - A) High latency - B) TCP global synchronization - C) Packet reordering - D) Authentication issues <details> <summary>Answer</summary> <b>B) TCP global synchronization</b> - WRED prevents all TCP flows from slowing down simultaneously. </details> **2. What happens when queue depth reaches the minimum WRED threshold?** - A) All packets are dropped - B) Random dropping begins - C) No packets are dropped - D) Priority queuing starts <details> <summary>Answer</summary> <b>B) Random dropping begins</b> - Dropping starts at the minimum threshold. </details> **3. Which traffic class should have the highest WRED thresholds?** - A) Best Effort - B) Voice (EF) - C) Video (AF41) - D) Scavenger <details> <summary>Answer</summary> <b>B) Voice (EF)</b> - Voice should have highest thresholds to minimize drops. </details> **4. What is the purpose of WRED?** - A) Prioritize traffic - B) Prevent TCP global synchronization - C) Shape traffic - D) Encrypt traffic <details> <summary>Answer</summary> <b>B) Prevent TCP global synchronization</b> - WRED proactively drops packets to prevent synchronized TCP slowdowns. </details> **5. Which command enables WRED on an interface?** - A) `random-detect` - B) `wred enable` - C) `fair-queue` - D) `priority` <details> <summary>Answer</summary> <b>A) `random-detect`</b> - This command enables WRED on the interface. </details> **6. What does the mark probability denominator specify?** - A) Queue size - B) Maximum drop probability - C) Minimum threshold - D) Maximum threshold <details> <summary>Answer</summary> <b>B) Maximum drop probability</b> - 1/denominator = maximum drop rate. </details> **7. What is the difference between RED and WRED?** - A) WRED is for wireless - B) WRED supports multiple traffic classes - C) RED is faster - D) RED uses different algorithm <details> <summary>Answer</summary> <b>B) WRED supports multiple traffic classes</b> - Weighted RED uses IP precedence/DSCP for different drop profiles. </details> **8. What happens when queue depth reaches the maximum WRED threshold?** - A) No drops - B) 50% drop rate - C) 100% drop rate - D) Priority queuing starts <details> <summary>Answer</summary> <b>C) 100% drop rate</b> - All new packets are dropped at max threshold. </details> **9. Which command displays WRED configuration?** - A) `show wred` - B) `show queueing random-detect` - C) `show random-detect` - D) `show policy` <details> <summary>Answer</summary> <b>B) `show queueing random-detect`</b> - Displays WRED thresholds per precedence. </details> **10. What is TCP global synchronization?** - A) All TCP flows synchronized in time - B) All TCP flows slow down simultaneously after congestion - C) TCP flows speed up together - D) TCP connections are synchronized <details> <summary>Answer</summary> <b>B) All TCP flows slow down simultaneously after congestion</b> - Caused by tail drop. </details> **11. Which WRED parameter should be higher for voice traffic?** - A) Drop probability - B) Minimum threshold - C) Maximum threshold - D) Both B and C <details> <summary>Answer</summary> <b>D) Both B and C</b> - Voice should have higher min and max thresholds. </details> **12. Where should WRED typically be configured?** - A) On all interfaces - B) On congested interfaces with queuing - C) On management interfaces - D) On loopback interfaces <details> <summary>Answer</summary> <b>B) On congested interfaces with queuing</b> - WRED is effective where congestion occurs. </details> --- ## 📌 Next Steps After completing Video 43, you should be ready for: - **Video 44:** Traffic Policing and Shaping - **Video 45:** QoS Configuration Examples **Lab Practice:** 1. Configure basic WRED on serial interface 2. Configure DSCP-based WRED 3. Configure WRED with CBWFQ classes 4. Verify with `show queueing random-detect` 5. Monitor drops with `show policy-map interface` 6. Compare WRED drops vs. tail drops --- **Ready for Video 44?** Share the link or say "next" and I'll continue with Traffic Policing and Shaping. I'll continue with **Video 44: Traffic Policing and Shaping** based on the standard CCNA 200-301 curriculum. ---