2020 Paper PDF - OSPF Routing

2020 Paper Question 3 (a) Using figure 2; (i) Identify the four router types found in a multi-area OSPF hierarchical network. (4 marks). (A) Based on the OSPF hierarchical network shown in the figure, the four router typ...

2020 Paper Question 3 (a) Using figure 2; (i) Identify the four router types found in a multi-area OSPF hierarchical network. (4 marks). (A) Based on the OSPF hierarchical network shown in the figure, the four router types are: 1. Backbone Router (BR) - Located in Area 0 (backbone area) 2. Area Border Router (ABR) - Connects multiple areas to the backbone area 3. Internal Router (IR) - Located within a single area 4. Autonomous System Boundary Router (ASBR) - Connects to external networks or different routing domains (ii) Based on the identification of the router types, explain how each router type utilises Link State Advertisement (LSA) types 1-5 to achieve network convergence. (10 marks) (A) Each router type utilises LSAs to achieve network convergence: 2020 Paper 1  Internal Routers: Use Types 1 and 2 for intra-area communication. ABRs: Summarise and propagate inter-area routes using Types 1, 2, and 3. ASBRs: Advertise external routes via Types 4 and 5. Backbone Routers: Ensure inter-area and external connectivity by processing all LSA types. (iv) Describe the effect of making Area 51 a Totally Stubby Area. (3 marks). (A) Effect of Making Area 51 a Totally Stubby Area 1. Blocks External and Inter-Area Routes: Both external routes (Type 5 LSAs) and inter-area routes (Type 3 LSAs) are blocked. The ABR injects a single default route ( 0.0.0.0 ) into Area 51 for all external and inter-area traffic. 2. Minimal Routing Table: Routers in Area 51 only store: Intra-area routes (Type 1 LSAs). A single default route for all other traffic. This drastically reduces the size of the routing table. 3. Improved Scalability and Performance: Reduces the number of LSAs processed and exchanged within Area 51, saving memory and CPU resources. Ideal for small, simple areas with no need for detailed inter-area or external routes. (B) IPv6 implementations introduce two new LSA types: Types 8 and 9. Briefly describe each of their functions. 1. Type 8: Link-LSA 2020 Paper 2  Function: Advertises IPv6 link-local addresses and associated network information to neighboring routers on the same link. Key Use: Ensures that all routers on a link share the same IPv6 prefix and link-local addresses. Scope: Link-local (does not propagate beyond the link). 2. Type 9: Intra-Area-Prefix LSA Function: Advertises IPv6 prefixes for a specific OSPF area or associated to a router or network within the area. Key Use: Provides the IPv6 routing information for OSPFv3, replacing Type 3, 4, and 5 LSAs used in IPv4 OSPF. Scope: Remains within the OSPF area. These new LSAs are critical for handling IPv6-specific routing in OSPFv3. Explain the concept of an OSPF Virtual Link. Concept: An OSPF Virtual Link is a logical connection used to establish OSPF adjacency between two routers when direct connectivity to the OSPF backbone area (Area 0) is not possible. Purpose: 1. Connect Disjointed Area 0 Segments: Ensures that all OSPF areas remain connected to the backbone. 2. Provide Backbone Connectivity to Non-Backbone Areas: Allows an area to function as a transit area for connecting other areas to Area 0. Configuration Example: Scenario: Routers R1 and R3 need an OSPF virtual link through Area 1. On R1: 2020 Paper 3  Router(config)# router ospf 1 Router(config-router)# area 1 virtual-link 3.3.3.3 On R3: Router(config)# router ospf 1 Router(config-router)# area 1 virtual-link 1.1.1.1 1.1.1.1 and 3.3.3.3 are the router IDs of R1 and R3, respectively. Question 4 2020 Paper 4