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Module 27
OSPF Network Types
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# 📘 CCNA 200-301 - Video 27: OSPF Network Types ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - OSPF network types and their characteristics - Broadcast vs. non-broadcast networks - Point-to-point and point-to-multipoint networks - DR/BDR election (when and why) - OSPF over different media (Ethernet, Frame Relay, MPLS) - Configuration and verification --- ## 🧠 Core Concepts ### 1. What are OSPF Network Types? **Definition:** OSPF network types define how OSPF operates on a specific interface, determining neighbor discovery, DR/BDR election, and how LSAs are flooded. **Analogy:** Think of OSPF network types like different types of conversations: - **Broadcast:** A lecture hall (one speaker, many listeners - DR/BDR) - **Point-to-Point:** A private conversation (two people talking directly) - **Point-to-Multipoint:** A conference call (one person talking to many, but everyone hears each other) - **Non-Broadcast:** A mailing list (no direct communication, need to explicitly address messages) ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ OSPF NETWORK TYPES OVERVIEW │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ NETWORK TYPE | DR/BDR | Neighbor Discovery | Hello Interval │ │ -----------------------|--------|-------------------|-------------------│ │ Broadcast | Yes | Multicast | 10 sec │ │ Non-Broadcast (NBMA) | Yes | Manual (neighbor) | 30 sec │ │ Point-to-Point | No | Multicast | 10 sec │ │ Point-to-Multipoint | No | Multicast | 30 sec │ │ Point-to-Multipoint NBMA| No | Manual (neighbor) | 30 sec │ │ Loopback | N/A | N/A | N/A │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. OSPF Network Types Summary | Network Type | Default Media | DR/BDR Election | Hello Timer | Neighbor Discovery | OSPF Packet Type | |--------------|---------------|-----------------|-------------|--------------------|------------------| | **Broadcast** | Ethernet, Token Ring | Yes | 10 sec | Multicast (224.0.0.5/6) | Multicast | | **Non-Broadcast (NBMA)** | Frame Relay, ATM | Yes | 30 sec | Manual configuration | Unicast | | **Point-to-Point** | PPP, HDLC, T1/E1 | No | 10 sec | Multicast | Multicast | | **Point-to-Multipoint** | Frame Relay (partial mesh) | No | 30 sec | Multicast | Multicast | | **Point-to-Multipoint NBMA** | Frame Relay (partial mesh) | No | 30 sec | Manual configuration | Unicast | | **Loopback** | Loopback interface | N/A | N/A | N/A | N/A | --- ### 3. Broadcast Network Type **Characteristics:** - Default on Ethernet interfaces - Supports multiple routers (more than 2) - DR/BDR elected - Uses multicast for OSPF packets - Hello interval: 10 seconds, Dead interval: 40 seconds ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ BROADCAST NETWORK (Ethernet) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ All routers on same segment: │ │ │ │ ┌─────────┐ │ │ │ Router A│ │ │ │ DR │ │ │ └────┬────┘ │ │ │ │ │ ┌─────────┼─────────┐ │ │ │ │ │ │ │ ▼ ▼ ▼ │ │ ┌─────┐ ┌─────┐ ┌─────┐ │ │ │Router│ │Router│ │Router│ │ │ │ B │ │ C │ │ D │ │ │ │ BDR │ │DROther│ │DROther│ │ │ └─────┘ └─────┘ └─────┘ │ │ │ │ DR = Designated Router (elected) │ │ BDR = Backup Designated Router │ │ DROther = Other routers (form adjacencies only with DR/BDR) │ │ │ │ Multicast Addresses: │ │ • 224.0.0.5: All OSPF routers │ │ • 224.0.0.6: DR/BDR only │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Broadcast Configuration:** ```cisco ! Default on Ethernet - no configuration needed Router(config)# interface gigabitEthernet 0/0 Router(config-if)# ip ospf network broadcast ! Default behavior, command optional ``` --- ### 4. Point-to-Point Network Type **Characteristics:** - Default on PPP, HDLC, and T1/E1 interfaces - Exactly 2 routers on the link - No DR/BDR election - Uses multicast (224.0.0.5) - Hello interval: 10 seconds, Dead interval: 40 seconds ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ POINT-TO-POINT NETWORK │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Two routers directly connected: │ │ │ │ ┌─────────┐ ┌─────────┐ │ │ │ Router A│═════════════════════════│ Router B│ │ │ │ │ Point-to-Point │ │ │ │ └─────────┘ Link └─────────┘ │ │ │ │ Features: │ │ • No DR/BDR election (only 2 routers) │ │ • Direct adjacency between both routers │ │ • Uses multicast (224.0.0.5) │ │ • Faster convergence than broadcast │ │ • Ideal for WAN links (T1, E1, serial) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Point-to-Point Configuration:** ```cisco ! Configure serial interface as point-to-point Router(config)# interface serial 0/0/0 Router(config-if)# encapsulation ppp Router(config-if)# ip address 10.1.1.1 255.255.255.252 Router(config-if)# ip ospf network point-to-point ! Or default on PPP/HDLC ``` --- ### 5. Non-Broadcast (NBMA) Network Type **Characteristics:** - Default on Frame Relay, ATM (non-broadcast media) - Multiple routers possible - DR/BDR elected - Requires manual neighbor configuration - Uses unicast for OSPF packets - Hello interval: 30 seconds, Dead interval: 120 seconds ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ NBMA NETWORK (Frame Relay) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Frame Relay Cloud (Non-Broadcast) │ │ │ │ ┌─────────┐ │ │ │ Router A│ (Hub) │ │ │ DR │ │ │ └────┬────┘ │ │ │ │ │ PVC1 │ PVC2 │ │ │ │ │ ┌─────────┼─────────┐ │ │ │ │ │ │ │ ▼ ▼ ▼ │ │ ┌─────┐ ┌─────┐ ┌─────┐ │ │ │Router│ │Router│ │Router│ │ │ │ B │ │ C │ │ D │ │ │ │ BDR │ │DROther│ │DROther│ │ │ └─────┘ └─────┘ └─────┘ │ │ │ │ Challenges: │ │ • No broadcast capability │ │ • Must manually specify neighbors │ │ • DR/BDR must have full connectivity to all routers │ │ • Hello interval 30 seconds (slower convergence) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **NBMA Configuration:** ```cisco ! Configure OSPF for NBMA Router(config)# router ospf 1 Router(config-router)# network 10.1.1.0 0.0.0.255 area 0 Router(config-router)# neighbor 10.1.1.2 Router(config-router)# neighbor 10.1.1.3 Router(config-router)# exit ! Configure interface Router(config)# interface serial 0/0/0 Router(config-if)# encapsulation frame-relay Router(config-if)# ip address 10.1.1.1 255.255.255.0 Router(config-if)# ip ospf network non-broadcast ``` --- ### 6. Point-to-Multipoint Network Type **Characteristics:** - No DR/BDR election - Treats NBMA network as collection of point-to-point links - Uses multicast (224.0.0.5) - Hello interval: 30 seconds, Dead interval: 120 seconds - Better for partial mesh networks ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ POINT-TO-MULTIPOINT NETWORK │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Frame Relay Cloud (Treated as point-to-point links) │ │ │ │ ┌─────────┐ │ │ │ Router A│ │ │ └────┬────┘ │ │ │ │ │ ┌───────┼───────┐ │ │ │ │ │ │ │ ▼ ▼ ▼ │ │ ┌─────┐ ┌─────┐ ┌─────┐ │ │ │Router│ │Router│ │Router│ │ │ │ B │ │ C │ │ D │ │ │ └─────┘ └─────┘ └─────┘ │ │ │ │ Features: │ │ • Each link treated as point-to-point │ │ • No DR/BDR election (less overhead) │ │ • Uses multicast (if broadcast capability) │ │ • Each router forms adjacency with all others │ │ • Better for partial mesh (no full connectivity required) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Point-to-Multipoint Configuration:** ```cisco ! Configure OSPF point-to-multipoint Router(config)# interface serial 0/0/0 Router(config-if)# encapsulation frame-relay Router(config-if)# ip address 10.1.1.1 255.255.255.0 Router(config-if)# ip ospf network point-to-multipoint ! No neighbor statements needed (uses multicast) ``` --- ### 7. Point-to-Multipoint Non-Broadcast Network Type **Characteristics:** - Same as point-to-multipoint but uses unicast - Requires manual neighbor configuration - No DR/BDR election - Hello interval: 30 seconds, Dead interval: 120 seconds - Used when multicast not available **Point-to-Multipoint Non-Broadcast Configuration:** ```cisco ! Configure OSPF point-to-multipoint non-broadcast Router(config)# router ospf 1 Router(config-router)# neighbor 10.1.1.2 Router(config-router)# neighbor 10.1.1.3 Router(config)# interface serial 0/0/0 Router(config-if)# encapsulation frame-relay Router(config-if)# ip address 10.1.1.1 255.255.255.0 Router(config-if)# ip ospf network point-to-multipoint non-broadcast ``` --- ### 8. DR/BDR Election **When is DR/BDR elected?** - Broadcast networks - NBMA networks **When is DR/BDR NOT elected?** - Point-to-point - Point-to-multipoint - Point-to-multipoint non-broadcast ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ DR/BDR ELECTION PROCESS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Election Criteria (in order): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ 1. Highest OSPF priority (0-255, default 1) │ │ │ │ 2. Highest router ID (if priorities equal) │ │ │ │ │ │ │ │ Note: Priority 0 means router cannot become DR/BDR │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Election Flow: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Router A (Priority 100) │ │ │ │ │ │ │ │ │ Router B (Priority 50) │ │ │ │ │ │ │ │ │ Router C (Priority 1) │ │ │ │ │ │ │ │ │ All routers send Hello packets │ │ │ │ │ │ │ │ │ Election: Highest priority wins │ │ │ │ │ │ │ │ │ Result: Router A = DR, Router B = BDR, Router C = DROther │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ DR/BDR Responsibilities: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ DR: │ │ │ │ • Generates network LSA for multi-access segment │ │ │ │ • Maintains full adjacency with all routers │ │ │ │ • Floods LSAs to all routers │ │ │ │ │ │ │ │ BDR: │ │ │ │ • Backs up DR │ │ │ │ • Maintains full adjacency with all routers │ │ │ │ • Takes over if DR fails │ │ │ │ │ │ │ │ DROther: │ │ │ │ • Form adjacencies only with DR/BDR │ │ │ │ • Do NOT form adjacencies with other DROthers │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **DR/BDR Configuration:** ```cisco ! Set OSPF priority on interface Router(config)# interface gigabitEthernet 0/0 Router(config-if)# ip ospf priority 100 ! Higher priority = more likely to be DR ! Prevent router from becoming DR/BDR Router(config-if)# ip ospf priority 0 ``` --- ### 9. OSPF over Different Media Types | Media | Default Network Type | Recommended Network Type | |-------|---------------------|-------------------------| | **Ethernet** | Broadcast | Broadcast | | **PPP/HDLC** | Point-to-Point | Point-to-Point | | **Frame Relay (Full Mesh)** | Non-Broadcast | Broadcast or Point-to-Multipoint | | **Frame Relay (Partial Mesh)** | Non-Broadcast | Point-to-Multipoint | | **Frame Relay (Hub-and-Spoke)** | Non-Broadcast | Point-to-Multipoint | | **MPLS L3VPN** | Broadcast | Broadcast | | **GRE Tunnel** | Point-to-Point | Point-to-Point | --- ### 10. OSPF Hello and Dead Timers | Network Type | Hello Interval | Dead Interval | Default | |--------------|----------------|---------------|---------| | Broadcast | 10 sec | 40 sec | Default on Ethernet | | Point-to-Point | 10 sec | 40 sec | Default on serial | | NBMA | 30 sec | 120 sec | Default on Frame Relay | | Point-to-Multipoint | 30 sec | 120 sec | Configured | | Point-to-Multipoint NBMA | 30 sec | 120 sec | Configured | **Timer Configuration:** ```cisco ! Configure hello and dead timers Router(config)# interface gigabitEthernet 0/0 Router(config-if)# ip ospf hello-interval 5 Router(config-if)# ip ospf dead-interval 20 ! Note: Dead interval should be 4x hello interval ``` --- ### 11. OSPF Network Type Verification | Command | Purpose | |---------|---------| | `show ip ospf interface [int]` | Display OSPF network type, timers, DR/BDR | | `show ip ospf neighbor` | Display OSPF neighbors and roles | | `show ip ospf interface brief` | Quick interface summary | **Example Outputs:** ```cisco Router# show ip ospf interface gigabitEthernet 0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 192.168.1.1/24, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1 Transmit Delay is 1 sec, State DR, Priority 1 Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1 Backup Designated router (ID) 2.2.2.2, Interface address 192.168.1.2 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:03 Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor 2.2.2.2 (Backup Designated Router) Adjacent with neighbor 3.3.3.3 (Designated Router) Suppress hello for 0 neighbor(s) ``` ```cisco Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/BDR 00:00:34 192.168.1.2 GigabitEthernet0/0 3.3.3.3 1 FULL/DROTHER 00:00:35 192.168.1.3 GigabitEthernet0/0 ``` --- ### 12. OSPF Network Type Troubleshooting | Problem | Symptom | Solution | |---------|---------|----------| | **Neighbors Not Forming** | `show ip ospf neighbor` empty | Check network type matches, hello timers match | | **DR/BDR Election Issue** | Wrong router is DR | Adjust priorities, use `ip ospf priority` | | **NBMA Neighbors Not Forming** | No adjacency | Verify `neighbor` statements, reachability | | **Timer Mismatch** | Neighbor stuck in EXSTART/EXCHANGE | Ensure hello/dead timers match | | **Partial Mesh Issues** | Some neighbors not forming | Use point-to-multipoint instead of broadcast | --- ## 🔧 Complete Configuration Examples ### Lab 1: Broadcast Network with DR/BDR Control **Topology:** ``` Router A (Priority 100) Router B (Priority 50) Router C (Priority 1) │ │ │ └────────────────────────────┼───────────────────────────┘ │ Ethernet Switch ``` **Configuration:** ```cisco ! Router A - Desired DR hostname RouterA ! interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 ip ospf priority 100 ! router ospf 1 router-id 1.1.1.1 network 192.168.1.0 0.0.0.255 area 0 ! end ! Router B - Desired BDR hostname RouterB ! interface GigabitEthernet0/0 ip address 192.168.1.2 255.255.255.0 ip ospf priority 50 ! router ospf 1 router-id 2.2.2.2 network 192.168.1.0 0.0.0.255 area 0 ! end ! Router C - DROther (never becomes DR/BDR) hostname RouterC ! interface GigabitEthernet0/0 ip address 192.168.1.3 255.255.255.0 ip ospf priority 0 ! router ospf 1 router-id 3.3.3.3 network 192.168.1.0 0.0.0.255 area 0 ! end ``` --- ### Lab 2: Point-to-Point Serial Link **Topology:** ``` Router A ───────────────────────── Router B (Serial Link - PPP) ``` **Configuration:** ```cisco ! Router A hostname RouterA ! interface Serial0/0/0 encapsulation ppp ip address 10.1.1.1 255.255.255.252 ip ospf network point-to-point ! router ospf 1 router-id 1.1.1.1 network 10.1.1.0 0.0.0.3 area 0 ! end ! Router B hostname RouterB ! interface Serial0/0/0 encapsulation ppp ip address 10.1.1.2 255.255.255.252 ip ospf network point-to-point ! router ospf 1 router-id 2.2.2.2 network 10.1.1.0 0.0.0.3 area 0 ! end ``` --- ### Lab 3: Frame Relay Point-to-Multipoint **Topology:** ``` ┌─────────────────┐ │ Router A │ │ (Hub) │ └────────┬────────┘ │ ┌──────────────┼──────────────┐ │ │ │ ▼ ▼ ▼ ┌─────────┐ ┌─────────┐ ┌─────────┐ │ Router B│ │ Router C│ │ Router D│ │ (Spoke) │ │ (Spoke) │ │ (Spoke) │ └─────────┘ └─────────┘ └─────────┘ ``` **Configuration:** ```cisco ! Router A (Hub) hostname RouterA ! interface Serial0/0/0 encapsulation frame-relay ip address 10.1.1.1 255.255.255.0 ip ospf network point-to-multipoint frame-relay map ip 10.1.1.2 102 broadcast frame-relay map ip 10.1.1.3 103 broadcast frame-relay map ip 10.1.1.4 104 broadcast ! router ospf 1 router-id 1.1.1.1 network 10.1.1.0 0.0.0.255 area 0 ! end ! Router B (Spoke) hostname RouterB ! interface Serial0/0/0 encapsulation frame-relay ip address 10.1.1.2 255.255.255.0 ip ospf network point-to-multipoint frame-relay map ip 10.1.1.1 201 broadcast ! router ospf 1 router-id 2.2.2.2 network 10.1.1.0 0.0.0.255 area 0 ! end ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **Broadcast Network** | Ethernet, DR/BDR elected, hello 10 sec | | **Point-to-Point** | Serial links, no DR/BDR, hello 10 sec | | **NBMA** | Frame Relay, DR/BDR elected, manual neighbors, hello 30 sec | | **Point-to-Multipoint** | No DR/BDR, multicast, hello 30 sec | | **DR/BDR Election** | Highest priority (1-255), then highest router ID | | **Priority 0** | Router cannot become DR/BDR | ### Common Exam Scenarios: **Scenario 1:** "An Ethernet segment has 5 OSPF routers. How many adjacencies will be formed?" - **Answer:** DR forms adjacencies with all 4 others; BDR forms adjacencies with all 4; DROthers only form adjacencies with DR/BDR (not each other). Total = (n-1) for DR + (n-2) for BDR = 4 + 3 = 7 adjacencies. **Scenario 2:** "Which OSPF network type should be used for a partial mesh Frame Relay network?" - **Answer:** Point-to-Multipoint (no DR/BDR requirement, each link treated as point-to-point) **Scenario 3:** "A router with OSPF priority 0 is added to a broadcast network. What role will it have?" - **Answer:** DROther (cannot become DR or BDR) ### Mnemonics: **OSPF Network Types:** **"BP-PM" - Broadcast, Point-to-Point, Point-to-Multipoint** - **B**roadcast: Ethernet, DR/BDR - **P**oint-to-Point: Serial links, no DR/BDR - **P**oint-to-**M**ultipoint: Frame Relay, no DR/BDR **DR/BDR Election:** **"Higher is Better"** - Higher priority = more likely to be DR - Priority 0 = never DR/BDR - If priorities equal, higher router ID wins --- ## 📝 Summary (1-Minute Revision) ``` OSPF NETWORK TYPES: BROADCAST: ├── Default: Ethernet ├── DR/BDR: Yes ├── Hello: 10 sec, Dead: 40 sec ├── Multicast: 224.0.0.5 (all), 224.0.0.6 (DR/BDR) └── Command: ip ospf network broadcast POINT-TO-POINT: ├── Default: PPP, HDLC, Serial ├── DR/BDR: No ├── Hello: 10 sec, Dead: 40 sec ├── Multicast: 224.0.0.5 └── Command: ip ospf network point-to-point NON-BROADCAST (NBMA): ├── Default: Frame Relay, ATM ├── DR/BDR: Yes ├── Hello: 30 sec, Dead: 120 sec ├── Neighbors: Manual configuration ├── Unicast OSPF packets └── Command: ip ospf network non-broadcast POINT-TO-MULTIPOINT: ├── DR/BDR: No ├── Hello: 30 sec, Dead: 120 sec ├── Multicast (if supported) ├── Best for partial mesh └── Command: ip ospf network point-to-multipoint DR/BDR ELECTION: ├── Priority (1-255, default 1) ├── Highest priority = DR ├── Second highest = BDR └── Priority 0 = DROther (cannot become DR/BDR) VERIFICATION: ├── show ip ospf interface ├── show ip ospf neighbor └── show ip ospf interface brief ``` --- ## 🧪 Practice Questions **1. Which OSPF network type is the default on Ethernet interfaces?** - A) Point-to-Point - B) Broadcast - C) Non-Broadcast - D) Point-to-Multipoint <details> <summary>Answer</summary> <b>B) Broadcast</b> - Ethernet interfaces default to OSPF broadcast network type. </details> **2. Which OSPF network type does NOT perform DR/BDR election?** - A) Broadcast - B) Non-Broadcast - C) Point-to-Point - D) All of the above <details> <summary>Answer</summary> <b>C) Point-to-Point</b> - Point-to-point networks have only 2 routers, so no DR/BDR election is needed. </details> **3. What is the default hello interval for OSPF broadcast networks?** - A) 5 seconds - B) 10 seconds - C) 30 seconds - D) 40 seconds <details> <summary>Answer</summary> <b>B) 10 seconds</b> - Hello interval is 10 seconds on broadcast and point-to-point networks. </details> **4. What is the default hello interval for OSPF NBMA networks?** - A) 10 seconds - B) 30 seconds - C) 40 seconds - D) 60 seconds <details> <summary>Answer</summary> <b>B) 30 seconds</b> - NBMA networks have a 30-second hello interval. </details> **5. Which OSPF network type requires manual neighbor configuration?** - A) Broadcast - B) Point-to-Point - C) Non-Broadcast (NBMA) - D) Point-to-Multipoint <details> <summary>Answer</summary> <b>C) Non-Broadcast (NBMA)</b> - NBMA requires manual neighbor statements because it cannot use multicast. </details> **6. What OSPF priority prevents a router from becoming DR or BDR?** - A) 0 - B) 1 - C) 255 - D) -1 <details> <summary>Answer</summary> <b>A) 0</b> - Priority 0 means the router cannot become DR or BDR. </details> **7. Which command sets OSPF priority on an interface?** - A) `ip ospf priority 100` - B) `ospf priority 100` - C) `router ospf priority 100` - D) `priority 100` <details> <summary>Answer</summary> <b>A) `ip ospf priority 100`</b> - This sets the OSPF priority on the interface. </details> **8. Which multicast address do all OSPF routers listen on?** - A) 224.0.0.1 - B) 224.0.0.5 - C) 224.0.0.6 - D) 224.0.0.9 <details> <summary>Answer</summary> <b>B) 224.0.0.5</b> - All OSPF routers listen on 224.0.0.5. </details> **9. Which multicast address do DR and BDR only listen on?** - A) 224.0.0.1 - B) 224.0.0.5 - C) 224.0.0.6 - D) 224.0.0.9 <details> <summary>Answer</summary> <b>C) 224.0.0.6</b> - DR and BDR listen on 224.0.0.6 for updates from DROthers. </details> **10. Which OSPF network type is recommended for partial mesh Frame Relay networks?** - A) Broadcast - B) Non-Broadcast - C) Point-to-Point - D) Point-to-Multipoint <details> <summary>Answer</summary> <b>D) Point-to-Multipoint</b> - Point-to-multipoint works best with partial mesh because it doesn't require full connectivity. </details> **11. What command displays OSPF network type and DR/BDR information?** - A) `show ip ospf` - B) `show ip ospf interface` - C) `show ip ospf neighbor` - D) `show ip route ospf` <details> <summary>Answer</summary> <b>B) `show ip ospf interface`</b> - Shows network type, timers, DR/BDR information. </details> **12. How many adjacencies are formed on a broadcast network with 6 OSPF routers?** - A) 6 - B) 7 - C) 10 - D) 15 <details> <summary>Answer</summary> <b>B) 7</b> - DR forms 5 adjacencies (with all others), BDR forms 1 adjacency with DR (already counted), total = 7. </details> --- ## 📌 Next Steps After completing Video 27, you should be ready for: - **Video 28:** OSPF LSA Types and Areas - **Video 29:** EIGRP Advanced Features **Lab Practice:** 1. Configure OSPF broadcast network with 3 routers 2. Set priorities to control DR/BDR election 3. Configure point-to-point OSPF on serial links 4. Configure point-to-multipoint OSPF for Frame Relay 5. Verify network types with `show ip ospf interface` 6. Observe neighbor states and DR/BDR roles --- **Ready for Video 28?** Share the link or say "next" and I'll continue with OSPF LSA Types and Areas. I'll continue with **Video 28: OSPF LSA Types and Areas** based on the standard CCNA 200-301 curriculum. ---