Large networks present unique challenges, such as excessive routing table sizes, slower convergence times, and increased CPU/memory load on routers. Open Shortest Path First (OSPF), a dynamic link-state routing protocol, addresses these challenges through its multi-area architecture. This LAB will demonstrate how to configure a multi-area OSPF topology on MikroTik RouterOS, step by step, with detailed explanations, practical insights, and troubleshooting tips.

 

Why Multi-Area OSPF?

OSPF uses the Shortest Path First (SPF) algorithm to compute the best routes. In large networks, running SPF calculations for all routers and networks within a single area can become resource-intensive. Multi-area OSPF mitigates this by dividing the topology into smaller areas:

• Backbone Area (Area 0): All other areas must connect to this backbone.
• Non-Backbone Areas (e.g., Area 1): These connect to Area 0 via an Area Border Router (ABR). Routing updates are summarized between areas, reducing overhead and resource consumption.

 

Key Benefits of Multi-Area OSPF:

Scalability: By summarizing routes between areas, the number of entries in routing tables is reduced.

Faster Convergence: SPF recalculations are limited to individual areas.

Improved Performance: ABRs handle inter-area route summarization, reducing CPU/memory usage on other routers.

 

Lab Topology Overview

Our topology consists of three routers:

Router 1: Belongs to Area 0 (Backbone). It represents the core of the OSPF topology.

Router 2: The Area Border Router (ABR), connecting Area 0 and Area 1. This router plays a crucial role in summarizing and redistributing routes between the two areas.

Router 3: Belongs to Area 1 and has a loopback interface to simulate a virtual endpoint.

 

Objectives:

1. Configure IP addressing and OSPF on all routers.
2. Ensure that Router 1 can reach Router 3’s loopback (3.3.3.3).
3. Confirm that Router 3 can reach Router 1’s loopback (1.1.1.1).
4. Verify OSPF neighbor relationships, routing tables, and connectivity across areas.

 

Step 1: Configuring IP Addressing on Router 2

Router 2 connects to Router 3 in Area 1 via its ether2 interface. First, we assign an IP address to ether2.

Explanation:

Assigning 192.168.23.2/24 to ether2 ensures Layer 3 connectivity between Router 2 and Router 3. Without this IP addressing, OSPF cannot form neighbor relationships or exchange routing information.

Pro Tip:

Ensure the subnet mask is consistent across connected routers to prevent mismatched network issues.

 

Step 2: Adding Router 2’s Interface to OSPF Area 1

Next, we configure OSPF on Router 2, assigning its ether2 interface to Area 1.

 

Explanation:

The first command creates Area 1 with an ID of 0.0.0.1.
The second command adds the network 192.168.23.0/24 to OSPF and assigns it to Area 1.
The print command confirms that the area has been successfully configured.

 

Key Concept:

Router 2 now acts as the Area Border Router (ABR). It processes SPF calculations for both Area 0 and Area 1, summarizing routes to reduce overhead.

 

Step 3: Configuring IP Addressing and Loopback on Router 3

Router 3 requires:

An IP address on its ether2 interface for direct communication with Router 2.
A loopback interface (3.3.3.3/32) to simulate a virtual endpoint.