Chapter 5: OSPF Virtual Links

Key Features Implemented

• Configuring OSPF Virtual Links
• The following OSPF design rules must be adhered to:
  1. All areas that are not Area 0 must be directly connected to Area 0 (Backbone).
  2. Area 0 (Backbone) must be contiguous.
• Establishing End-to-End Connectivity Across Non-Backbone Areas
• Validating Virtual Link Establishment and Stability

Topology

The lab utilizes the OSPF multi-area topology as shown in the diagram from the "Chapter 5: OSPF Virtual Links" PDF. (A simplified version from page 3 of the PDF is also referenced for clarity).

Configuration Tasks

• Configure IP addresses and OSPF routing as shown in the topology:
  • Assign IP addresses to all interfaces as per the topology.
  • Configure OSPF on router interfaces and place them into the appropriate areas.
• Verify Initial Connectivity:
  • Ensure all routers have established OSPF neighbor adjacencies within their designated areas.
  • Confirm that all networks are not reachable and not present by inspecting the OSPF database and routing table. (Note: This implies some initial disconnectivity is expected before virtual links).
• Configure OSPF Virtual Links:
  • Establish a virtual link between R2 and R3 through Area 1 to extend Area 0 (maintaining Area 0 continuity).
  • Establish a virtual link between R4 and R5 through Area 2 to extend Area 0. This will ensure all non-backbone areas connect directly to Area 0.
  • Ensure the virtual links are up and operational by verifying their OSPF states.
• Validate End-to-End Connectivity:
  • Ensure that loopback interfaces of all routers are reachable across the OSPF domain.
  • Verify that the virtual links allow proper propagation of OSPF LSAs across the network.

Base Configurations

Ready to get started? Here, you'll find the initial configurations for each device to begin this lab.

hostname R1

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
interface Ethernet0/0
 ip address 12.1.1.1 255.255.255.252
 ip ospf 1 area 0

router ospf 1

hostname R2

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 ip ospf 2 area 1
interface Ethernet0/0
 ip address 12.1.1.2 255.255.255.252
 ip ospf 2 area 0
interface Ethernet0/1
 ip address 23.1.1.1 255.255.255.252
 ip ospf 2 area 1

router ospf 2

hostname R3

interface Loopback0
 ip address 3.3.3.3 255.255.255.255
 ip ospf 3 area 0
interface Ethernet0/0
 ip address 23.1.1.2 255.255.255.252
 ip ospf 3 area 1
interface Ethernet0/1
 ip address 34.1.1.1 255.255.255.252
 ip ospf 3 area 0

router ospf 3

hostname R4

interface Loopback0
 ip address 4.4.4.4 255.255.255.255
 ip ospf 4 area 2
interface Ethernet0/0
 ip address 34.1.1.2 255.255.255.252
 ip ospf 4 area 0
interface Ethernet0/1
 ip address 45.1.1.1 255.255.255.252
 ip ospf 4 area 2

router ospf 4

hostname R5

interface Loopback0
 ip address 5.5.5.5 255.255.255.255
 ip ospf 5 area 3
interface Ethernet0/0
 ip address 45.1.1.2 255.255.255.252
 ip ospf 5 area 2
interface Ethernet0/1
 ip address 56.1.1.1 255.255.255.252
 ip ospf 5 area 3

router ospf 5

hostname R6

interface Loopback0
 ip address 6.6.6.6 255.255.255.255
 ip ospf 6 area 3
interface Ethernet0/0
 ip address 56.1.1.2 255.255.255.252
 ip ospf 6 area 3

router ospf 6

Verification Commands

• show ip ospf
  • Verify general OSPF process details, including summarization settings.
• show ip ospf interface
  • Confirm which interfaces are participating in OSPF.
• show ip ospf neighbor
  • Check adjacency states.
• show ip ospf database
  • Verify LSA Type-3 and Type-5 summary LSAs.
• show ip route ospf
  • Confirm that summarized routes are installed in the routing table.
• ping
  • Test end-to-end connectivity between loopback interfaces of different routers.

Expected Outcomes

• OSPF adjacencies should form successfully over virtual links.
• Virtual links should be stable and ensure continuity of Area 0.
• All routers should have full connectivity to loopback networks across different areas.

Final Configurations

Want to take a look for yourself? Here, you will find the final configurations of each device required for this lab.

hostname R1

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
interface Ethernet0/0
 ip address 12.1.1.1 255.255.255.252
 ip ospf 1 area 0

router ospf 1

hostname R2

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 ip ospf 2 area 1
interface Ethernet0/0
 ip address 12.1.1.2 255.255.255.252
 ip ospf 2 area 0
interface Ethernet0/1
 ip address 23.1.1.1 255.255.255.252
 ip ospf 2 area 1

router ospf 2
 area 1 virtual-link 3.3.3.3

hostname R3

interface Loopback0
 ip address 3.3.3.3 255.255.255.255
 ip ospf 3 area 0
interface Ethernet0/0
 ip address 23.1.1.2 255.255.255.252
 ip ospf 3 area 1
interface Ethernet0/1
 ip address 34.1.1.1 255.255.255.252
 ip ospf 3 area 0

router ospf 3
 area 1 virtual-link 2.2.2.2

hostname R4

interface Loopback0
 ip address 4.4.4.4 255.255.255.255
 ip ospf 4 area 2
interface Ethernet0/0
 ip address 34.1.1.2 255.255.255.252
 ip ospf 4 area 0
interface Ethernet0/1
 ip address 45.1.1.1 255.255.255.252
 ip ospf 4 area 2

router ospf 4
 area 2 virtual-link 5.5.5.5

hostname R5

interface Loopback0
 ip address 5.5.5.5 255.255.255.255
 ip ospf 5 area 3
interface Ethernet0/0
 ip address 45.1.1.2 255.255.255.252
 ip ospf 5 area 2
interface Ethernet0/1
 ip address 56.1.1.1 255.255.255.252
 ip ospf 5 area 3

router ospf 5
 area 2 virtual-link 4.4.4.4

hostname R6

interface Loopback0
 ip address 6.6.6.6 255.255.255.255
 ip ospf 6 area 3
interface Ethernet0/0
 ip address 56.1.1.2 255.255.255.252
 ip ospf 6 area 3

router ospf 6