Lab Simulation 24 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access

Question

Exhibits

Explanation

The correct interaction involves identifying R3 as the Area Border Router (ABR) for Area 20 and applying an OSPF area filter-list with the 'out' keyword to prevent advertising R20's loopback route out of Area 20, while recognizing that R2's configuration for filtering R1's loopback route into Area 10 is already complete.

Approach. The core of this question lies in correctly identifying the Area Border Routers (ABRs) responsible for inter-area route filtering and applying OSPF area filter-lists in the correct direction. The phrase 'Answer: R2 R3' indicates these are the routers where configuration might be needed or verified.

1. Analyze Task 1: 'The route for R1 Loopback 0 should not be advertised into Area 10.'

   • ABR Identification: R2 is the ABR connecting Area 0 (where R1 resides) and Area 10. Filtering needs to occur on R2.
   • Direction: The route is coming from Area 0 into Area 10. Therefore, on R2, the filter-list needs to be applied to area 10 with the in keyword.
   • Verification from Exhibit: The R2 console output clearly shows area 10 filter-list prefix deny_R1_Lo0 in and the corresponding ip prefix-list deny_R1_Lo0 which denies 10.0.1.1/32 (presumably R1's loopback). This confirms that Task 1 is already completed on R2.

2. Analyze Task 2: 'The route for R20 Loopback 0 should not be advertised out of area 20.'

   • ABR Identification: R3 is the ABR connecting Area 20 (where R20 resides) and Area 0. Filtering needs to occur on R3.
   • Direction: The route is originating in Area 20 and should be prevented from being advertised out of Area 20 (i.e., into Area 0). Therefore, on R3, the filter-list needs to be applied to area 20 with the out keyword.
   • Missing Configuration: Based on the completion of Task 1 and the general nature of these lab questions, Task 2 is the one requiring user interaction. The problem states 'use the partially configured prefix list'. While the exact name for R20's prefix list isn't shown, it can be inferred to be similar to deny_R1_Lo0, perhaps deny_R20_Lo0. The OSPF process ID can be assumed to be 10, consistent with R2.

   The correct interaction is to access the console of R3 and apply the following commands:

   Router>enable
   Router#configure terminal
   Router(config)#router ospf 10
   Router(config-router)#area 20 filter-list prefix deny_R20_Lo0 out
   Router(config-router)#end
   Router#write memory
   

   (Note: deny_R20_Lo0 is an assumed name based on typical lab conventions and the problem stating 'partially configured prefix list'. The actual name might be specified elsewhere in a real lab environment or implicitly defined in the background.)

Common mistakes.

• common_mistake. 1. Incorrect ABR Selection: Attempting to configure R1, R10, or R20 for inter-area filtering. These are internal routers, not ABRs, and cannot perform inter-area filtering.

1. Incorrect Filter Direction: Applying the filter with the in keyword on R3 for Area 20 (e.g., area 20 filter-list prefix deny_R20_Lo0 in). This would filter routes into Area 20 from Area 0, which is not the requirement. Task 2 requires filtering routes out of Area 20.
2. Attempting to Configure Task 1 on R2: Since the show run on R2 clearly indicates Task 1 is already configured, attempting to reconfigure it or apply a different filter would be redundant or incorrect.
3. Using area 0 command: The task explicitly forbids using the area 0 filter-list command. Using area 0 filter-list prefix <name> out on R3 would filter routes leaving Area 0 into Area 20, which is the opposite of the requirement for Task 2. Using area 0 filter-list prefix <name> in on R2 would filter routes into Area 0 from Area 10, which is also not the requirement for Task 1.
4. Not using a Prefix List: The task explicitly states to use a 'partially configured prefix list'. Using other filtering mechanisms like access-lists would be incorrect.
5. Incorrect Prefix List Name: If the specific prefix list name for R20's loopback route (e.g., deny_R20_Lo0) is not correctly identified and used, the filter will not apply.

Concept tested. The core technical concepts being tested are:

1. OSPF Area Border Router (ABR) Functionality: Understanding that ABRs are responsible for inter-area routing and filtering. R2 is an ABR between Area 0 and Area 10, and R3 is an ABR between Area 0 and Area 20.
2. OSPF Area Filtering (filter-list command): The ability to use the area <area-id> filter-list prefix <prefix-list-name> {in | out} command to control the advertisement of Type 3 LSAs (summary LSAs) between OSPF areas.
3. Prefix-list Configuration and Application: Understanding how to define a prefix-list to match specific network prefixes and apply it within OSPF filtering.
4. Directional Filtering (in vs out): Correctly determining whether a filter needs to be applied for routes coming in to an area or going out of an area from the perspective of the ABR and the specific area being configured.
5. Interpreting Network Topology and Configuration Output: Analyzing a network diagram to identify router roles and understanding existing router configurations to determine what tasks are already complete and what needs to be done.

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