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Module 26
Route Summarization
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# 📘 CCNA 200-301 - Video 26: Route Summarization ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - What route summarization is and why it's used - Benefits of summarization (smaller routing tables, stability) - How to calculate summary routes - Manual vs. automatic summarization - Summarization in different routing protocols (OSPF, EIGRP, RIP) - Configuration and verification --- ## 🧠 Core Concepts ### 1. What is Route Summarization? **Definition:** Route summarization (also called route aggregation or supernetting) is the process of combining multiple network prefixes into a single, less specific route that covers all the original networks. **Analogy:** Think of route summarization like a postal service consolidating mail for a neighborhood. Instead of sorting mail for 100 individual houses (100 routes), the postal service sorts mail for the entire neighborhood (1 route). The mail is still delivered correctly, but the sorting table (routing table) is much smaller. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WITHOUT SUMMARIZATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Routing Table (Core Router): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Destination Next Hop │ │ │ │ 192.168.1.0/24 Router A │ │ │ │ 192.168.2.0/24 Router A │ │ │ │ 192.168.3.0/24 Router A │ │ │ │ 192.168.4.0/24 Router A │ │ │ │ 192.168.5.0/24 Router A │ │ │ │ ... ... │ │ │ │ 192.168.100.0/24 Router A │ │ │ │ │ │ │ │ 100 routes for 100 networks │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ┌─────────────────────────────────────────────────────────────────────────────┐ │ WITH SUMMARIZATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Routing Table (Core Router): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Destination Next Hop │ │ │ │ 192.168.0.0/16 Router A │ │ │ │ │ │ │ │ 1 route for 100 networks! │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. Benefits of Route Summarization | Benefit | Description | |---------|-------------| | **Smaller Routing Tables** | Fewer routes to process, less memory | | **Faster Convergence** | Fewer route updates, quicker processing | | **Reduced CPU/Memory Usage** | Less overhead on routers | | **Improved Stability** | Flapping networks hidden from core | | **Easier Troubleshooting** | Simpler routing topology | | **Better Scaling** | Allows networks to grow without routing table explosion | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ SUMMARIZATION HIDES FLAPPING │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ WITHOUT SUMMARIZATION: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Branch Router (192.168.1.0/24) → Core Router │ │ │ │ │ (flapping) → "Network 192.168.1.0 is down" │ │ │ │ │ (up) → "Network 192.168.1.0 is up" │ │ │ │ │ (down) → "Network 192.168.1.0 is down" │ │ │ │ │ (up) → "Network 192.168.1.0 is up" │ │ │ │ │ │ │ │ Core router processes every flap → CPU spike │ │ │ │ Other networks affected │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ WITH SUMMARIZATION: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Branch Router (192.168.1.0/24) → Distribution Router │ │ │ │ │ (flapping) → "Network 192.168.1.0 is down" │ │ │ │ │ (up) → "Network 192.168.1.0 is up" │ │ │ │ → Distribution Router │ │ │ │ → (hides flapping) │ │ │ │ → Core Router │ │ │ │ → "Network 192.168.0.0/16 is up"│ │ │ │ │ │ │ │ Core router sees only one stable route → No flapping │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. How to Calculate Summary Routes **Method 1: Binary Method** ``` Step 1: Convert network addresses to binary Step 2: Find common bits (left to right) Step 3: Count common bits → prefix length Step 4: Summary address = common bits + zeros Example: Summarize 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24 192.168.1.0 = 11000000.10101000.00000001.00000000 192.168.2.0 = 11000000.10101000.00000010.00000000 192.168.3.0 = 11000000.10101000.00000011.00000000 Common bits: 11000000.10101000.000000 (22 bits) Summary: 192.168.0.0/22 ``` **Method 2: Common Prefix Method** ``` Step 1: List all network addresses Step 2: Find the smallest prefix that covers all Step 3: Use the formula: 2^(n) = number of networks covered Example: Summarize 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24 All have first two octets 10.1 Third octet ranges: 0,1,2,3 (4 networks) Need 2 bits to cover 4 networks (2^2 = 4) Summary: 10.1.0.0/22 (covers 10.1.0.0 - 10.1.3.255) ``` ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ SUMMARY CALCULATION EXAMPLES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Example 1: Summarize 192.168.0.0/24, 192.168.1.0/24 │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ 192.168.0.0 = 11000000.10101000.00000000.00000000 │ │ │ │ 192.168.1.0 = 11000000.10101000.00000001.00000000 │ │ │ │ Common bits: 23 bits │ │ │ │ Summary: 192.168.0.0/23 │ │ │ │ Covers: 192.168.0.0 - 192.168.1.255 │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Example 2: Summarize 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24 │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Common prefix: 10.0.0.0/22 │ │ │ │ Covers: 10.0.0.0 - 10.0.3.255 │ │ │ │ Hosts: 4 × 254 = 1016 usable addresses │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Example 3: Summarize 172.16.0.0/16, 172.17.0.0/16, 172.18.0.0/16, │ │ 172.19.0.0/16, 172.20.0.0/16 │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Need to cover 5 networks: 2^3 = 8 (3 bits) │ │ │ │ Summary: 172.16.0.0/13 │ │ │ │ Covers: 172.16.0.0 - 172.23.255.255 │ │ │ │ (May include extra networks - must ensure they don't exist) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 4. Summary Route Calculation Formula **Number of Networks:** 2^(bits borrowed) ``` If summary prefix length = /X Original prefix length = /Y Then: 2^(Y - X) = number of /Y networks covered Example: /24 networks summarized to /22 2^(24 - 22) = 2^2 = 4 networks covered /24 → /23 covers 2 networks /24 → /22 covers 4 networks /24 → /21 covers 8 networks /24 → /20 covers 16 networks ``` **Block Size:** 256 - (2^(8 - bits in octet)) ``` For /24 summarized to /22: - Bits used in 3rd octet: 22 - 16 = 6 bits - Bits remaining: 8 - 6 = 2 bits - Block size = 2^2 = 4 - Summary network: 192.168.0.0/22, next 192.168.4.0/22, etc. ``` --- ### 5. Manual vs. Automatic Summarization | Feature | Manual Summarization | Automatic Summarization | |---------|---------------------|------------------------| | **Control** | Administrator controls | Protocol determines | | **Location** | Configured on specific routers | Auto at classful boundaries | | **Routing Protocols** | OSPF, EIGRP, RIP, BGP | RIPv2, EIGRP (classful) | | **Precision** | Can be precise | Sometimes too broad | | **Recommendation** | Use manual | Disable auto (RIPv2, EIGRP) | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ AUTOMATIC SUMMARIZATION (Classful Boundaries) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Problem: Automatic summarization creates too broad routes │ │ │ │ Network: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ 10.1.1.0/24 ──┐ │ │ │ │ 10.1.2.0/24 ──┼── Router A ─── Router B │ │ │ │ 10.1.3.0/24 ──┘ │ │ │ │ │ │ │ │ 10.2.1.0/24 ──┐ │ │ │ │ 10.2.2.0/24 ──┼── Router C ─── Router B │ │ │ │ 10.2.3.0/24 ──┘ │ │ │ │ │ │ │ │ Automatic summarization at classful boundary (10.0.0.0/8): │ │ │ │ Router B sees: 10.0.0.0/8 via Router A │ │ │ │ 10.0.0.0/8 via Router C │ │ │ │ → Routing conflict! │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Solution: Disable auto-summarization │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Router(config-router)# no auto-summary │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 6. Summarization in EIGRP **EIGRP Manual Summarization:** ```cisco ! Configure manual summarization on interface Router(config)# interface gigabitEthernet 0/0 Router(config-if)# ip summary-address eigrp 100 192.168.0.0 255.255.252.0 Router(config-if)# exit ! Verify EIGRP summary routes Router# show ip route eigrp Router# show ip eigrp topology ``` **EIGRP Summary Example:** ```cisco ! Router A - Summarizing routes to Router B RouterA(config)# interface gigabitEthernet 0/0 RouterA(config-if)# ip summary-address eigrp 100 10.1.0.0 255.255.252.0 ! Summarizes 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24 into 10.1.0.0/22 ``` **EIGRP Auto-Summarization:** ```cisco ! Disable auto-summarization (recommended) Router(config)# router eigrp 100 Router(config-router)# no auto-summary ``` --- ### 7. Summarization in OSPF **OSPF Manual Summarization:** ```cisco ! OSPF Area Summarization (ABR) Router(config)# router ospf 1 Router(config-router)# area 1 range 192.168.0.0 255.255.252.0 ! Summarizes routes from Area 1 into Area 0 ! OSPF External Summarization (ASBR) Router(config)# router ospf 1 Router(config-router)# summary-address 192.168.0.0 255.255.252.0 ! Summarizes redistributed routes ``` **OSPF Summary Example:** ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ OSPF AREA SUMMARIZATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Area 1 (10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24) │ │ │ │ │ │ ABR (Area Border Router) │ │ │ │ │ Area 0 (Backbone) │ │ │ │ │ │ │ │ Rest of Network │ │ │ │ Without summarization: Area 0 sees 4 routes from Area 1 │ │ With summarization: Area 0 sees 1 route (10.1.0.0/22) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 8. Summarization in RIP **RIPv2 Manual Summarization:** ```cisco ! Configure interface summarization Router(config)# interface gigabitEthernet 0/0 Router(config-if)# ip summary-address rip 192.168.0.0 255.255.252.0 ! Disable auto-summarization (RIPv2) Router(config)# router rip Router(config-router)# version 2 Router(config-router)# no auto-summary ``` --- ### 9. Summary Route Verification | Command | Purpose | |---------|---------| | `show ip route` | Display routing table with summary routes | | `show ip route summary` | Summary of routing table statistics | | `show ip route [summary-address]` | Display specific summary route | | `show ip protocols` | Show auto-summary status | **Example Outputs:** ```cisco Router# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area D 10.1.0.0/22 [90/409600] via 192.168.1.2, 00:01:23, GigabitEthernet0/0 C 192.168.1.0/24 is directly connected, GigabitEthernet0/0 C 192.168.2.0/24 is directly connected, GigabitEthernet0/1 ``` ```cisco Router# show ip route summary IP routing table name is default (0x0) IP routing table maximum-paths is 32 Route Source Networks Subnets Replicates Overhead Memory (bytes) connected 0 2 0 176 320 static 0 0 0 0 0 eigrp 100 0 1 0 88 160 internal 1 136 Total 1 3 0 400 616 ``` --- ### 10. Best Practices for Summarization ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ SUMMARIZATION BEST PRACTICES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ 1. PLAN ADDRESSING FOR SUMMARIZATION │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Use contiguous address blocks │ │ │ │ • Assign subnets from larger blocks (e.g., /24 from /16) │ │ │ │ • Hierarchical addressing (site, building, floor) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 2. SUMMARIZE AT HIERARCHICAL BOUNDARIES │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Branch routers: Summarize to distribution │ │ │ │ • Distribution routers: Summarize to core │ │ │ │ • ASBR: Summarize external routes │ │ │ │ • ABR: Summarize between OSPF areas │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 3. AVOID OVER-SUMMARIZATION │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Don't include networks that don't exist │ │ │ │ • Ensure summary doesn't create routing loops │ │ │ │ • Example: Summarizing 10.1.0.0/16 when only 10.1.1.0/24 exists │ │ │ │ → Traffic may be sent to non-existent networks │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 4. DISABLE AUTO-SUMMARIZATION │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Use manual summarization for control │ │ │ │ • Disable in EIGRP and RIPv2 │ │ │ │ • OSPF doesn't auto-summarize │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 5. USE NULL ROUTE (BLACK HOLE) FOR SUMMARIES │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Add static route to null0 for summary to prevent loops │ │ │ │ • ip route 192.168.0.0 255.255.252.0 Null0 │ │ │ │ • EIGRP/OSPF automatically adds null routes for summaries │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 11. Null Route (Discard Route) **Purpose:** Prevents routing loops when a summary route exists but some subnets are not present. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ NULL ROUTE EXAMPLE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Problem: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Router A sends summary 192.168.0.0/22 to Router B │ │ │ │ Router B sends summary 192.168.0.0/22 to Router C │ │ │ │ │ │ │ │ Packet to 192.168.1.100 (which doesn't exist): │ │ │ │ Router C → Router B → Router A → (nowhere) │ │ │ │ Packet may loop! │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Solution: Null Route │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Router A(config)# ip route 192.168.0.0 255.255.252.0 Null0 │ │ │ │ │ │ │ │ Packets to non-existent subnets are dropped immediately │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 12. Summary Route Calculation Table | Networks to Summarize | Summary Prefix | Number of /24 Networks | |----------------------|----------------|------------------------| | 192.168.0.0 - 192.168.1.0 | 192.168.0.0/23 | 2 | | 192.168.0.0 - 192.168.3.0 | 192.168.0.0/22 | 4 | | 192.168.0.0 - 192.168.7.0 | 192.168.0.0/21 | 8 | | 192.168.0.0 - 192.168.15.0 | 192.168.0.0/20 | 16 | | 192.168.0.0 - 192.168.31.0 | 192.168.0.0/19 | 32 | | 192.168.0.0 - 192.168.63.0 | 192.168.0.0/18 | 64 | | 192.168.0.0 - 192.168.127.0 | 192.168.0.0/17 | 128 | | 192.168.0.0 - 192.168.255.0 | 192.168.0.0/16 | 256 | --- ## 🔧 Complete Configuration Examples ### Lab 1: EIGRP Manual Summarization **Topology:** ``` Branch A (10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24, 10.1.4.0/24) │ │ (EIGRP 100) ▼ Distribution Router │ │ (EIGRP 100) ▼ Core Router ``` **Distribution Router Configuration:** ```cisco hostname Distribution ! ! Enable EIGRP router eigrp 100 network 10.0.0.0 no auto-summary ! ! Summarize routes from Branch A interface GigabitEthernet0/0 description Link to Core ip address 192.168.1.2 255.255.255.0 ip summary-address eigrp 100 10.1.0.0 255.255.252.0 ! interface GigabitEthernet0/1 description Link to Branch A ip address 10.1.0.1 255.255.255.0 ! end ``` **Verification:** ```cisco Distribution# show ip route eigrp D 10.1.0.0/22 [90/409600] via 192.168.1.1, 00:00:05, GigabitEthernet0/0 ! Only one summary route advertised to Core ``` --- ### Lab 2: OSPF Area Summarization **Topology:** ``` Area 1 (10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24) │ │ (ABR) ▼ Area 0 (Backbone) ``` **ABR Configuration:** ```cisco hostname ABR ! ! OSPF configuration router ospf 1 network 10.1.0.0 0.0.3.255 area 1 network 192.168.1.0 0.0.0.255 area 0 ! ! Summarize routes from Area 1 into Area 0 area 1 range 10.1.0.0 255.255.252.0 ! end ``` **Verification:** ```cisco CoreRouter# show ip route ospf O IA 10.1.0.0/22 [110/2] via 192.168.1.2, 00:00:05, GigabitEthernet0/0 ! Only one summary route from Area 1 ``` --- ### Lab 3: RIP Summarization with Null Route **Topology:** ``` Branch (192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24, 192.168.4.0/24) │ │ (RIPv2) ▼ Headend Router ``` **Headend Router Configuration:** ```cisco hostname Headend ! ! RIP configuration router rip version 2 network 192.168.0.0 no auto-summary ! ! Manual summarization on interface to core interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.0 ip summary-address rip 192.168.0.0 255.255.248.0 ! ! Null route to prevent loops ip route 192.168.0.0 255.255.248.0 Null0 ! end ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **Summarization Benefits** | Smaller routing tables, faster convergence, stability | | **Calculation** | Find common bits, determine summary prefix | | **EIGRP Summarization** | `ip summary-address eigrp` on interface | | **OSPF Summarization** | `area [id] range` on ABR, `summary-address` on ASBR | | **Auto-Summarization** | Disable with `no auto-summary` in EIGRP/RIP | | **Null Route** | Prevents routing loops for summaries | ### Common Exam Scenarios: **Scenario 1:** "Which summary route covers 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24?" - **Answer:** 10.1.0.0/22 **Scenario 2:** "Why should auto-summarization be disabled in EIGRP?" - **Answer:** Classful summarization can create suboptimal routes or routing loops **Scenario 3:** "What is the purpose of a null route for a summary address?" - **Answer:** Prevents routing loops by dropping packets to non-existent subnets ### Mnemonics: **Summary Calculation:** **"Find Common Bits"** - Convert to binary - Find matching bits left to right - Count common bits = prefix length **Summary Benefits:** **"S.C.S." - Smaller, Calmer, Stable** - **S**maller routing tables - **C**almer (faster convergence) - **S**table (hides flapping) --- ## 📝 Summary (1-Minute Revision) ``` ROUTE SUMMARIZATION: DEFINITION: ├── Combine multiple routes into one ├── More specific → less specific prefix └── Also called route aggregation or supernetting BENEFITS: ├── Smaller routing tables (less memory) ├── Faster convergence (fewer updates) ├── Reduced CPU usage ├── Hides network instability (flapping) └── Better scaling CALCULATION: ├── Convert networks to binary ├── Find common bits ├── Summary address = common bits + zeros ├── Prefix length = number of common bits EIGRP SUMMARIZATION: ├── Manual: ip summary-address eigrp [as] [net] [mask] ├── Auto: no auto-summary (disable) └── Configure on interface OSPF SUMMARIZATION: ├── ABR: area [id] range [net] [mask] ├── ASBR: summary-address [net] [mask] └── No auto-summarization RIP SUMMARIZATION: ├── Manual: ip summary-address rip [net] [mask] ├── Auto: no auto-summary (RIPv2) └── Configure on interface NULL ROUTE: ├── ip route [net] [mask] Null0 ├── Prevents routing loops └── Automatic in EIGRP/OSPF for summaries VERIFICATION: ├── show ip route ├── show ip route summary └── show ip protocols ``` --- ## 🧪 Practice Questions **1. What is the summary route for 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24?** - A) 192.168.0.0/23 - B) 192.168.0.0/22 - C) 192.168.0.0/21 - D) 192.168.0.0/20 <details> <summary>Answer</summary> <b>B) 192.168.0.0/22</b> - Covers 4 networks: 192.168.0.0 - 192.168.3.255. </details> **2. Which command configures EIGRP manual summarization?** - A) `summary-address eigrp 100 192.168.0.0 255.255.252.0` - B) `ip summary-address eigrp 100 192.168.0.0 255.255.252.0` - C) `eigrp summary 192.168.0.0 255.255.252.0` - D) `ip eigrp summary 192.168.0.0 255.255.252.0` <details> <summary>Answer</summary> <b>B) `ip summary-address eigrp 100 192.168.0.0 255.255.252.0`</b> - Correct syntax for EIGRP manual summarization on interface. </details> **3. What is the primary benefit of route summarization?** - A) Faster packet forwarding - B) Smaller routing tables - C) More specific routes - D) Better security <details> <summary>Answer</summary> <b>B) Smaller routing tables</b> - Summarization reduces the number of routes in the routing table. </details> **4. Which command disables auto-summarization in EIGRP?** - A) `no auto-summary` - B) `disable auto-summary` - C) `auto-summary off` - D) `no summary` <details> <summary>Answer</summary> <b>A) `no auto-summary`</b> - This command is used in router configuration mode to disable auto-summarization. </details> **5. What is the purpose of a null route for a summary address?** - A) Increase bandwidth - B) Prevent routing loops - C) Reduce latency - D) Improve security <details> <summary>Answer</summary> <b>B) Prevent routing loops</b> - Null route drops packets to non-existent subnets, preventing loops. </details> **6. Which OSPF command summarizes routes between areas?** - A) `summary-address` - B) `area range` - C) `ip summary-address` - D) `area summary` <details> <summary>Answer</summary> <b>B) `area range`</b> - `area [id] range [net] [mask]` summarizes routes on an ABR. </details> **7. Which summary route covers 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24, and 10.1.4.0/24?** - A) 10.1.0.0/22 - B) 10.1.0.0/21 - C) 10.1.0.0/20 - D) 10.1.0.0/19 <details> <summary>Answer</summary> <b>B) 10.1.0.0/21</b> - Covers 8 networks (0-7), enough for 5 networks. </details> **8. How many /24 networks can a /22 summary cover?** - A) 2 - B) 4 - C) 8 - D) 16 <details> <summary>Answer</summary> <b>B) 4</b> - 2^(24-22) = 2^2 = 4 networks. </details> **9. Which routing protocol automatically summarizes at classful boundaries by default?** - A) OSPF - B) EIGRP - C) IS-IS - D) BGP <details> <summary>Answer</summary> <b>B) EIGRP</b> - EIGRP auto-summarizes at classful boundaries unless disabled with `no auto-summary`. </details> **10. What is the summary route for 172.16.0.0/16, 172.17.0.0/16, and 172.18.0.0/16?** - A) 172.16.0.0/14 - B) 172.16.0.0/15 - C) 172.16.0.0/13 - D) 172.16.0.0/12 <details> <summary>Answer</summary> <b>A) 172.16.0.0/14</b> - Covers 172.16.0.0 - 172.19.255.255 (4 networks, enough for 3). </details> **11. Which command displays the routing table summary statistics?** - A) `show ip route statistics` - B) `show ip route summary` - C) `show ip summary` - D) `show ip protocols` <details> <summary>Answer</summary> <b>B) `show ip route summary`</b> - Shows counts of routes from each source. </details> **12. Why should auto-summarization be disabled in modern networks?** - A) It uses too much bandwidth - B) It can create suboptimal routes and loops - C) It doesn't work with IPv6 - D) It's not supported on new routers <details> <summary>Answer</summary> <b>B) It can create suboptimal routes and loops</b> - Auto-summarization at classful boundaries can cause routing issues in discontiguous networks. </details> --- ## 📌 Next Steps After completing Video 26, you should be ready for: - **Video 27:** OSPF Network Types - **Video 28:** OSPF LSA Types and Areas **Lab Practice:** 1. Calculate summary routes for various network ranges 2. Configure EIGRP manual summarization 3. Configure OSPF area summarization 4. Verify routing tables before and after summarization 5. Configure null route to prevent loops 6. Compare auto vs. manual summarization --- **Ready for Video 27?** Share the link or say "next" and I'll continue with OSPF Network Types. I'll continue with **Video 27: OSPF Network Types** based on the standard CCNA 200-301 curriculum. ---