Packet Tracer Lab: OSPF Configuration and Default Route Advertisement

Lab Objectives

1. Configure hostnames and IP addresses on each router and enable the interfaces.
2. Create loopback interfaces on each router to simulate host addresses.
3. Configure OSPF on all routers, enabling it on all interfaces (except the ISP link) and marking loopback interfaces as passive.
4. Configure R1 as an ASBR (Autonomous System Boundary Router) to advertise a default route into the OSPF domain.
5. Verify the default routes in the routing tables of R2, R3, and R4.

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Lab Topology Overview

• R1, R2, R3, and R4 are the core routers.
• SW1 is connected to R4 and provides connectivity to PC1.
• R1 connects to the ISP router (ISPR1), which is outside the OSPF domain.
• All routers are connected via point-to-point serial or Ethernet links with OSPF running in Area 0.

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Task 1: Configure Hostnames and IP Addresses

1. Assign the following hostnames to your routers:

   • R1, R2, R3, R4

2. Configure IP addresses on the routers as shown in the topology:

   • R1:
     • G0/0 -> 10.0.12.1/30
     • F1/0 -> 10.0.13.1/30
   • R2:
     • G0/0 -> 10.0.12.2/30
     • F1/0 -> 10.0.24.1/30
   • R3:
     • F1/0 -> 10.0.13.2/30
     • F2/0 -> 10.0.34.1/30
   • R4:
     • F1/0 -> 10.0.24.2/30
     • F2/0 -> 10.0.34.2/30
     • G0/0 -> 192.168.4.254/24
   • PC1:
     • IP -> 192.168.4.1/24

3. Enable router interfaces by using the no shutdown command on each.

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Task 2: Configure Loopback Interfaces

For each router, configure a loopback interface to simulate a host address as follows:

• R1: 1.1.1.1/32
• R2: 2.2.2.2/32
• R3: 3.3.3.3/32
• R4: 4.4.4.4/32

Commands Example for R1:

R1(config)# interface loopback 0
R1(config-if)# ip address 1.1.1.1 255.255.255.255

Repeat this for the other routers with their respective IP addresses.

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Task 3: OSPF Configuration

Step 1: Configure OSPF on Each Router

• Use OSPF process ID 1 on all routers.
• Enable OSPF on all router interfaces and ensure that the loopback interfaces are also included in the OSPF domain.

Example OSPF configuration for R1:

R1(config)# router ospf 1
R1(config-router)# network 10.0.12.0 0.0.0.3 area 0
R1(config-router)# network 10.0.13.0 0.0.0.3 area 0
R1(config-router)# network 1.1.1.1 0.0.0.0 area 0

Step 2: Set Passive Interfaces

• Set the loopback interfaces and any interfaces not connected to OSPF routers as passive.

Example for R1:

R1(config-router)# passive-interface loopback 0

• Do not enable OSPF on the link between R1 and ISPR1.

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Task 4: Configure R1 as an ASBR

On R1, configure the advertisement of a default route into the OSPF domain. This will allow the other routers (R2, R3, R4) to use R1 as a gateway to reach the internet.

1. First, configure a static default route pointing towards the ISP router (ISPR1):

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2

2. Next, advertise the default route into the OSPF domain:

R1(config-router)# default-information originate

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Task 5: Verify the Routing Tables

On R2, R3, and R4, verify the presence of the default route learned via OSPF.

Command:

R2# show ip route

Check that a default route (0.0.0.0/0) has been added via R1.

• The routing tables should show the default route pointing to R1's internal address (e.g., 10.0.12.1 for R2).

Expected Output:

You should see the default route propagated to each router's routing table, allowing them to forward packets to R1 for destinations outside the OSPF domain.

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Conclusion

This lab demonstrates how to configure OSPF across multiple routers, set up loopback interfaces, and advertise a default route using an ASBR. By the end of this lab, all internal routers (R2, R3, and R4) should have a default route pointing to R1, which provides a path to the internet.

Acknowledgements

Special thanks to Jeremy's IT Lab for providing valuable resources and tutorials that greatly contributed to the completion of this exercise. His in-depth explanations and practical demonstrations have been instrumental in enhancing my understanding of Cisco networking concepts and the effective use of Packet Tracer.

For more information and additional resources, visit Jeremy's IT Lab and check out his YouTube for the full course, Jeremy's IT Lab Free CCNA 200-301 | Complete Course