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Module 28
OSPF LSA Types and Areas
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# 📘 CCNA 200-301 - Video 28: OSPF LSA Types and Areas ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - OSPF LSA (Link State Advertisement) types - OSPF area types (Backbone, Standard, Stub, Totally Stubby, NSSA) - How LSAs are flooded across areas - OSPF router types (Internal, Backbone, ABR, ASBR) - Configuration and verification of different area types --- ## 🧠 Core Concepts ### 1. What are LSAs? **Definition:** Link State Advertisements (LSAs) are OSPF messages that describe the state of links and networks. Each LSA type carries different information and is flooded within specific areas or across the entire OSPF domain. **Analogy:** Think of LSAs like different types of newspapers: - **Type 1 (Router LSA):** Local news (router's own links) - **Type 2 (Network LSA):** City news (multi-access segment info) - **Type 3 (Summary LSA):** Regional news (routes from other areas) - **Type 4 (ASBR Summary):** International news (how to reach ASBR) - **Type 5 (External LSA):** World news (routes from other routing domains) ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ LSA TYPES OVERVIEW │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ LSA Type | Name | Originated By | Flooded Within │ │ ---------|-------------------------|---------------|---------------------│ │ 1 | Router LSA | All routers | Area only │ │ 2 | Network LSA | DR | Area only │ │ 3 | Summary LSA | ABR | Area (to other areas)│ │ 4 | ASBR Summary LSA | ABR | Area (to other areas)│ │ 5 | External LSA | ASBR | Entire OSPF domain │ │ 6 | Group Membership LSA | (Multicast) | N/A │ │ 7 | NSSA External LSA | ASBR in NSSA | NSSA only │ │ 8-11 | (Various) | | │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. LSA Type 1: Router LSA **Description:** Every OSPF router generates a Type 1 LSA for each area it belongs to. It describes the router's links and their states. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TYPE 1 - ROUTER LSA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Contents: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Router ID of the originating router │ │ │ │ • List of all router's interfaces in the area │ │ │ │ • Link type (point-to-point, transit, stub, virtual link) │ │ │ │ • Link ID (neighbor router ID or network address) │ │ │ │ • Link data (interface IP or mask) │ │ │ │ • Metric (cost) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Router A Type 1 LSA (Area 0): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Router ID: 1.1.1.1 │ │ │ │ Links: │ │ │ │ - Link to network 192.168.1.0/24 (cost 1) │ │ │ │ - Link to Router B (point-to-point, cost 10) │ │ │ │ - Stub network 10.1.1.0/24 (cost 1) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. LSA Type 2: Network LSA **Description:** The Designated Router (DR) generates a Type 2 LSA for each multi-access network. It describes all routers attached to that network. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TYPE 2 - NETWORK LSA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Contents: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • DR's IP address (as Link ID) │ │ │ │ • Subnet mask of the network │ │ │ │ • List of all routers (Router IDs) attached to the segment │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Example: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Network: 192.168.1.0/24 │ │ │ │ DR: 1.1.1.1 │ │ │ │ Attached Routers: 1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4 │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 4. LSA Type 3: Summary LSA **Description:** Area Border Routers (ABRs) generate Type 3 LSAs to advertise networks from one area to another. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TYPE 3 - SUMMARY LSA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Function: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • ABR takes Type 1/2 LSAs from one area │ │ │ │ • Summarizes them into Type 3 LSAs │ │ │ │ • Floods Type 3 LSAs into other areas │ │ │ │ • Inter-area routes appear as Type 3 LSAs in routing table │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Example: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Area 1 Area 0 │ │ │ │ ┌─────────┐ ┌─────────┐ │ │ │ │ │ Router A│ │ Router B│ │ │ │ │ │ (ABR) │─────────────────────│ │ │ │ │ │ └─────────┘ └─────────┘ │ │ │ │ │ │ │ │ │ Networks: │ │ │ │ 10.1.1.0/24 │ │ │ │ 10.1.2.0/24 │ │ │ │ │ │ │ │ Router A sends Type 3 LSA for 10.1.1.0/24 and 10.1.2.0/24 │ │ │ │ Router B sees inter-area routes (O IA) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 5. LSA Type 4: ASBR Summary LSA **Description:** ABRs generate Type 4 LSAs to advertise the location of an ASBR (Autonomous System Border Router) to other areas. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TYPE 4 - ASBR SUMMARY LSA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Function: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • ASBR originates Type 5 LSAs (external routes) │ │ │ │ • ABR creates Type 4 LSA to tell other areas how to reach ASBR │ │ │ │ • Type 4 LSA contains Router ID of the ASBR │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Example: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Area 1 Area 0 │ │ │ │ ┌─────────┐ ┌─────────┐ │ │ │ │ │ ASBR │ │ Router B│ │ │ │ │ │ Router A│─────────────────────│ (ABR) │ │ │ │ │ │(Redist) │ └─────────┘ │ │ │ │ └─────────┘ │ │ │ │ │ │ │ │ │ │ │ External │ │ │ │ │ Routes ▼ │ │ │ │ Area 2 │ │ │ │ │ │ │ │ • Router A (ASBR) sends Type 5 LSAs for external routes │ │ │ │ • Router B creates Type 4 LSA to advertise ASBR (Router A) │ │ │ │ • Area 2 routers get Type 5 LSAs (external routes) + Type 4 LSA │ │ │ │ (path to ASBR) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 6. LSA Type 5: External LSA **Description:** ASBRs generate Type 5 LSAs to advertise routes redistributed from other routing protocols (EIGRP, RIP, BGP, static) into OSPF. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TYPE 5 - EXTERNAL LSA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Characteristics: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Originated by ASBR │ │ │ │ • Flooded throughout entire OSPF domain (all areas) │ │ │ │ • Two types: │ │ │ │ - E1 (Type 1 External): Cost = external cost + internal cost │ │ │ │ - E2 (Type 2 External): Cost = external cost only (default) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ E1 vs E2 Comparison: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ ASBR (External Cost = 20) │ │ │ │ │ │ │ │ │ Router A (Cost 10) │ │ │ │ │ │ │ │ │ Router B (Cost 10) │ │ │ │ │ │ │ │ E2 (Default): Cost = 20 (external cost only) │ │ │ │ E1: Cost = 20 + 10 + 10 = 40 (cumulative) │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 7. LSA Type 7: NSSA External LSA **Description:** ASBRs in Not-So-Stubby Areas (NSSA) generate Type 7 LSAs instead of Type 5. ABR converts Type 7 to Type 5 when flooding to other areas. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TYPE 7 - NSSA EXTERNAL LSA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Function: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Used only in NSSA (Not-So-Stubby Areas) │ │ │ │ • Allows NSSA to have external routes while maintaining stub │ │ │ │ • ABR converts Type 7 to Type 5 when flooding to other areas │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Example: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ NSSA Area 1 Area 0 │ │ │ │ ┌─────────┐ ┌─────────┐ │ │ │ │ │ ASBR │ │ Router B│ │ │ │ │ │ Router A│─────────────────────│ (ABR) │ │ │ │ │ │(Redist) │ └─────────┘ │ │ │ │ └─────────┘ │ │ │ │ │ │ │ │ │ │ │ Type 7 LSA Type 5 LSA (converted) │ │ │ │ │ │ │ │ External routes stay within NSSA as Type 7 │ │ │ │ ABR converts to Type 5 for rest of OSPF domain │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 8. OSPF Area Types ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ OSPF AREA TYPES COMPARISON │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ AREA TYPE | Type 3 | Type 4/5 | Type 7 | Default Route │ │ -------------------|--------|----------|--------|-----------------------│ │ Backbone (Area 0) | Yes | Yes | No | No │ │ Standard (Non-0) | Yes | Yes | No | No │ │ Stub | Yes | No | No | Yes (0.0.0.0) │ │ Totally Stubby | No* | No | No | Yes (0.0.0.0) │ │ NSSA | Yes | No (Type5)| Yes | No │ │ Totally NSSA | No* | No (Type5)| Yes | Yes (0.0.0.0) │ │ │ │ *Totally Stubby and Totally NSSA block Type 3 LSAs except default route │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 9. Area Type Details **Backbone Area (Area 0):** - All non-backbone areas must connect to Area 0 - All inter-area traffic goes through Area 0 - All LSA types allowed **Standard Area:** - Any area other than Area 0 - All LSA types allowed - Routes from other areas appear as Type 3 **Stub Area:** - Blocks Type 4/5 LSAs (no external routes) - ABR injects default route (0.0.0.0) - No ASBR allowed inside stub area ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ STUB AREA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Area 1 (Stub) Area 0 │ │ ┌─────────┐ ┌─────────┐ │ │ │ Router A│──────────────────────│ Router B│ │ │ │ │ │ (ABR) │ │ │ └─────────┘ └─────────┘ │ │ │ │ │ │ • No Type 4/5 LSAs │ │ │ • Default route from ABR ▼ │ │ • Cannot have ASBR External Networks │ │ │ │ Routing Table on Router A: │ │ O*IA 0.0.0.0/0 [110/2] via 192.168.1.2 │ │ O IA 192.168.2.0/24 [110/2] via 192.168.1.2 │ │ C 192.168.1.0/24 directly connected │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Totally Stubby Area (Cisco Proprietary):** - Blocks Type 3, 4, 5 LSAs - ABR injects default route (0.0.0.0) - Inter-area routes also summarized by default route ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ TOTALLY STUBBY AREA │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Area 1 (Totally Stubby) Area 0 │ │ ┌─────────┐ ┌─────────┐ │ │ │ Router A│──────────────────────│ Router B│ │ │ │ │ │ (ABR) │ │ │ └─────────┘ └─────────┘ │ │ │ │ │ │ • No Type 3/4/5 LSAs │ │ │ • Default route only ▼ │ │ • Cannot have ASBR All External/Inter-area routes │ │ │ │ Routing Table on Router A: │ │ O*IA 0.0.0.0/0 [110/2] via 192.168.1.2 │ │ C 192.168.1.0/24 directly connected │ │ (No other routes) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **NSSA (Not-So-Stubby Area):** - Blocks Type 4/5 LSAs but allows Type 7 - Can have ASBR (redistribution within area) - ABR converts Type 7 to Type 5 ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ NSSA (Not-So-Stubby Area) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ NSSA Area 1 Area 0 │ │ ┌─────────┐ ┌─────────┐ │ │ │ ASBR │ │ Router B│ │ │ │ Router A│──────────────────────│ (ABR) │ │ │ │(Redist) │ └─────────┘ │ │ └─────────┘ │ │ │ │ │ │ │ Type 7 LSA Type 5 LSA (converted) │ │ │ │ │ │ External routes External routes │ │ (within NSSA) (to other areas) │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Totally NSSA (Cisco Proprietary):** - Combines Totally Stubby and NSSA - Blocks Type 3, 4, 5 LSAs - Allows Type 7 LSAs - ABR injects default route --- ### 10. OSPF Router Types | Router Type | Description | Characteristics | |-------------|-------------|-----------------| | **Internal Router** | All interfaces in same area | Only Type 1 LSAs | | **Backbone Router** | At least one interface in Area 0 | Can be internal or ABR | | **ABR (Area Border Router)** | Connects Area 0 to another area | Generates Type 3 LSAs | | **ASBR (Autonomous System Border Router)** | Redistributes external routes | Generates Type 5 or Type 7 LSAs | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ OSPF ROUTER TYPES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ ┌─────────────────┐ │ │ │ Area 0 │ │ │ │ (Backbone) │ │ │ │ │ │ │ ┌───────│ Router B │ │ │ │ │ (Backbone) │ │ │ │ └────────┬────────┘ │ │ │ │ │ │ Area 1 │ │ Area 2 │ │ ┌─────────┐ │ ┌────────┴────────┐ │ │ │ Router A│ │ │ Router C │ │ │ │(Internal│ │ │ (ABR) │ │ │ │ Router) │ │ └────────┬────────┘ │ │ └─────────┘ │ │ │ │ │ │ │ │ Area 1 │ │ Area 2 │ │ ┌─────────┐ │ ┌────────┴────────┐ │ │ │ Router D│ │ │ Router E │ │ │ │ (ABR) │────────┘ │ (Internal) │ │ │ └─────────┘ └─────────────────┘ │ │ │ │ Router F (ASBR) - Redistributes external routes │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 11. Area Configuration **Standard Area (Default):** ```cisco ! No special configuration needed Router(config)# router ospf 1 Router(config-router)# network 192.168.1.0 0.0.0.255 area 1 ``` **Stub Area:** ```cisco ! On ABR and all routers in stub area Router(config)# router ospf 1 Router(config-router)# network 192.168.1.0 0.0.0.255 area 1 Router(config-router)# area 1 stub ``` **Totally Stubby Area (Cisco):** ```cisco ! On ABR only Router(config)# router ospf 1 Router(config-router)# area 1 stub no-summary ! On other routers in area (standard stub) Router(config-router)# area 1 stub ``` **NSSA (Not-So-Stubby Area):** ```cisco ! On ABR and all routers in NSSA Router(config)# router ospf 1 Router(config-router)# network 192.168.1.0 0.0.0.255 area 1 Router(config-router)# area 1 nssa ``` **Totally NSSA (Cisco):** ```cisco ! On ABR only Router(config)# router ospf 1 Router(config-router)# area 1 nssa no-summary ! On other routers in area (standard NSSA) Router(config-router)# area 1 nssa ``` --- ### 12. LSA Verification Commands | Command | Purpose | |---------|---------| | `show ip ospf database` | Display entire OSPF database | | `show ip ospf database router` | Display Type 1 LSAs | | `show ip ospf database network` | Display Type 2 LSAs | | `show ip ospf database summary` | Display Type 3 LSAs | | `show ip ospf database asbr-summary` | Display Type 4 LSAs | | `show ip ospf database external` | Display Type 5 LSAs | | `show ip ospf database nssa-external` | Display Type 7 LSAs | | `show ip ospf border-routers` | Display ABR/ASBR information | **Example Outputs:** ```cisco Router# show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 123 0x80000002 0x1234 3 2.2.2.2 2.2.2.2 110 0x80000003 0x5678 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 192.168.1.2 2.2.2.2 115 0x80000001 0x9ABC Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 10.1.1.0 1.1.1.1 120 0x80000001 0x1234 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 0.0.0.0 3.3.3.3 100 0x80000001 0xABCD 1 ``` ```cisco Router# show ip ospf border-routers OSPF Process 1 internal Routing Table Codes: i - Intra-area route, I - Inter-area route i 192.168.1.2 [1] area 0, ABR, ASBR via 192.168.1.2, GigabitEthernet0/0, cost 1 i 192.168.2.1 [2] area 1, ABR via 192.168.2.1, GigabitEthernet0/1, cost 2 ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: Stub Area Configuration **Topology:** ``` Area 1 (Stub) Area 0 Router A (Internal) Router B (ABR) Router C (Backbone) │ │ │ │ OSPF │ OSPF │ └───────────────────────┼────────────────────┘ │ External Routes ``` **Router A (Internal in Area 1):** ```cisco hostname RouterA ! router ospf 1 router-id 1.1.1.1 network 192.168.1.0 0.0.0.255 area 1 network 10.1.1.0 0.0.0.255 area 1 area 1 stub ! interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 ! interface Loopback0 ip address 10.1.1.1 255.255.255.0 ! end ``` **Router B (ABR):** ```cisco hostname RouterB ! router ospf 1 router-id 2.2.2.2 network 192.168.1.0 0.0.0.255 area 1 network 192.168.2.0 0.0.0.255 area 0 area 1 stub ! interface GigabitEthernet0/0 ip address 192.168.1.2 255.255.255.0 ! interface GigabitEthernet0/1 ip address 192.168.2.1 255.255.255.0 ! end ``` --- ### Lab 2: Totally Stubby Area Configuration **Router A (Internal in Area 1):** ```cisco hostname RouterA ! router ospf 1 router-id 1.1.1.1 network 192.168.1.0 0.0.0.255 area 1 area 1 stub ! Same as standard stub for internal routers ! end ``` **Router B (ABR - Totally Stubby):** ```cisco hostname RouterB ! router ospf 1 router-id 2.2.2.2 network 192.168.1.0 0.0.0.255 area 1 network 192.168.2.0 0.0.0.255 area 0 area 1 stub no-summary ! Totally stubby ! end ``` --- ### Lab 3: NSSA Configuration **Topology:** ``` NSSA Area 1 Area 0 Router A (ASBR) Router B (ABR) Router C (Redistribute Static) │ │ │ │ │ │ OSPF │ OSPF │ └─────────────────────┼────────────────────┘ (Type 7) │ │ External Routes ``` **Router A (ASBR in NSSA):** ```cisco hostname RouterA ! ! Redistribute static routes router ospf 1 router-id 1.1.1.1 network 192.168.1.0 0.0.0.255 area 1 area 1 nssa redistribute static subnets ! ! Static route to external network ip route 172.16.1.0 255.255.255.0 10.10.10.1 ! interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 ! end ``` **Router B (ABR):** ```cisco hostname RouterB ! router ospf 1 router-id 2.2.2.2 network 192.168.1.0 0.0.0.255 area 1 network 192.168.2.0 0.0.0.255 area 0 area 1 nssa ! end ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **LSA Types** | Type 1 (Router), Type 2 (Network), Type 3 (Summary), Type 5 (External) | | **Area Types** | Stub (blocks Type 5), Totally Stubby (blocks Type 3/5), NSSA (allows Type 7) | | **ABR** | Connects Area 0 to another area, generates Type 3 | | **ASBR** | Redistributes external routes, generates Type 5 | | **Stub Area Rules** | No ASBR, no Type 5 LSAs, ABR injects default route | ### Common Exam Scenarios: **Scenario 1:** "A router has interfaces in Area 0 and Area 1. What type of router is this?" - **Answer:** ABR (Area Border Router) **Scenario 2:** "Which LSA type is used by ASBR to advertise external routes?" - **Answer:** Type 5 (External LSA) or Type 7 (NSSA External LSA) **Scenario 3:** "An area needs to have external routes from redistribution but cannot receive Type 5 LSAs from other areas. Which area type should be used?" - **Answer:** NSSA (Not-So-Stubby Area) ### Mnemonics: **LSA Types:** **"1 Router, 2 Network, 3 Summary, 4 ASBR, 5 External, 7 NSSA"** **Area Types:** **"S.T.N." - Stub, Totally Stubby, NSSA** - **S**tub: Blocks Type 5, default route from ABR - **T**otally Stubby: Blocks Type 3/5, only default route - **N**SSA: Allows Type 7, blocks Type 5 **Router Types:** **"I.B.A." - Internal, Backbone, ABR, ASBR** --- ## 📝 Summary (1-Minute Revision) ``` OSPF LSA TYPES: TYPE 1 - ROUTER LSA: ├── Originated by: All routers ├── Contains: Router's links in area └── Flooded within area only TYPE 2 - NETWORK LSA: ├── Originated by: DR ├── Contains: All routers on multi-access segment └── Flooded within area only TYPE 3 - SUMMARY LSA: ├── Originated by: ABR ├── Contains: Inter-area routes └── Flooded between areas TYPE 4 - ASBR SUMMARY LSA: ├── Originated by: ABR ├── Contains: Location of ASBR └── Flooded between areas TYPE 5 - EXTERNAL LSA: ├── Originated by: ASBR ├── Contains: Redistributed routes (E1/E2) └── Flooded throughout OSPF domain TYPE 7 - NSSA EXTERNAL LSA: ├── Originated by: ASBR in NSSA ├── Contains: Redistributed routes in NSSA └── Flooded within NSSA only OSPF AREA TYPES: STUB: ├── Blocks: Type 4/5 ├── Has: Default route from ABR └── Cannot have ASBR TOTALLY STUBBY (Cisco): ├── Blocks: Type 3/4/5 ├── Has: Only default route └── Cannot have ASBR NSSA: ├── Blocks: Type 4/5 ├── Allows: Type 7 (external within area) └── Can have ASBR VERIFICATION: ├── show ip ospf database ├── show ip ospf border-routers └── show ip ospf interface ``` --- ## 🧪 Practice Questions **1. Which LSA type is generated by the DR on a multi-access network?** - A) Type 1 (Router LSA) - B) Type 2 (Network LSA) - C) Type 3 (Summary LSA) - D) Type 5 (External LSA) <details> <summary>Answer</summary> <b>B) Type 2 (Network LSA)</b> - The Designated Router generates Type 2 LSAs for multi-access networks. </details> **2. Which LSA type is used by ABRs to advertise routes from one area to another?** - A) Type 1 (Router LSA) - B) Type 2 (Network LSA) - C) Type 3 (Summary LSA) - D) Type 5 (External LSA) <details> <summary>Answer</summary> <b>C) Type 3 (Summary LSA)</b> - ABRs generate Type 3 LSAs to advertise inter-area routes. </details> **3. Which area type blocks Type 4 and Type 5 LSAs but allows Type 3 LSAs?** - A) Stub Area - B) Totally Stubby Area - C) NSSA - D) Backbone Area <details> <summary>Answer</summary> <b>A) Stub Area</b> - Stub areas block Type 4 and Type 5 LSAs but allow Type 3 LSAs. </details> **4. Which LSA type is used to advertise the location of an ASBR to other areas?** - A) Type 3 (Summary LSA) - B) Type 4 (ASBR Summary LSA) - C) Type 5 (External LSA) - D) Type 7 (NSSA External LSA) <details> <summary>Answer</summary> <b>B) Type 4 (ASBR Summary LSA)</b> - ABRs generate Type 4 LSAs to advertise ASBR location. </details> **5. Which area type allows external routes to be redistributed within the area without accepting Type 5 LSAs from other areas?** - A) Stub Area - B) Totally Stubby Area - C) NSSA - D) Standard Area <details> <summary>Answer</summary> <b>C) NSSA (Not-So-Stubby Area)</b> - NSSA allows Type 7 LSAs for local redistribution. </details> **6. What is the default metric type for redistributed routes in OSPF?** - A) E1 (Type 1 External) - B) E2 (Type 2 External) - C) O (Intra-area) - D) IA (Inter-area) <details> <summary>Answer</summary> <b>B) E2 (Type 2 External)</b> - Redistributed routes default to E2 (external cost only). </details> **7. Which router type has interfaces in multiple areas?** - A) Internal Router - B) Backbone Router - C) ABR (Area Border Router) - D) ASBR (Autonomous System Border Router) <details> <summary>Answer</summary> <b>C) ABR (Area Border Router)</b> - ABRs connect multiple areas, including Area 0. </details> **8. Which command displays the OSPF database including all LSA types?** - A) `show ip ospf` - B) `show ip ospf database` - C) `show ip route ospf` - D) `show ip ospf neighbor` <details> <summary>Answer</summary> <b>B) `show ip ospf database`</b> - Displays the entire OSPF link-state database. </details> **9. Which area type uses the `no-summary` keyword on the ABR?** - A) Stub Area - B) Totally Stubby Area - C) NSSA - D) Standard Area <details> <summary>Answer</summary> <b>B) Totally Stubby Area</b> - `area [id] stub no-summary` creates a totally stubby area. </details> **10. Which LSA type is converted from Type 7 to Type 5 at the ABR?** - A) Type 1 - B) Type 2 - C) Type 3 - D) Type 7 <details> <summary>Answer</summary> <b>D) Type 7</b> - NSSA Type 7 LSAs are converted to Type 5 by the ABR. </details> **11. What is the purpose of a Type 4 LSA?** - A) Describe router's links - B) Advertise inter-area routes - C) Advertise the location of an ASBR - D) Describe multi-access network <details> <summary>Answer</summary> <b>C) Advertise the location of an ASBR</b> - Type 4 LSAs tell other areas how to reach the ASBR. </details> **12. Which statement about Stub areas is true?** - A) They can contain ASBRs - B) They receive Type 5 LSAs - C) They receive a default route from the ABR - D) They block Type 3 LSAs <details> <summary>Answer</summary> <b>C) They receive a default route from the ABR</b> - Stub areas get a default route to reach external networks. </details> --- ## 📌 Next Steps After completing Video 28, you should be ready for: - **Video 29:** EIGRP Advanced Features - **Video 30:** Route Redistribution **Lab Practice:** 1. Configure standard OSPF with multiple areas 2. Verify LSA types with `show ip ospf database` 3. Configure stub area and verify default route 4. Configure totally stubby area 5. Configure NSSA with redistribution 6. Verify router types with `show ip ospf border-routers` --- **Ready for Video 29?** Share the link or say "next" and I'll continue with EIGRP Advanced Features. I'll continue with **Video 29: EIGRP Advanced Features** based on the standard CCNA 200-301 curriculum. ---