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CCNA Project 03 – Multi-Segment OSPF Routing Design with DR/BDR Election and Default Route Origination

Overview

This project demonstrates a multi-router OSPF design built in Cisco Packet Tracer to study how OSPF behaves across different interface types and network segments.

The topology was intentionally redesigned beyond a basic 3-router branch-to-HQ lab so I could observe and document:

  • OSPF over serial point-to-point links
  • OSPF over Ethernet broadcast multiaccess segments
  • DR/BDR election behavior
  • OSPF interface priority effects
  • OSPF simple authentication
  • loopback advertisement
  • passive-interface behavior
  • default route origination from a WAN-edge router

The project simulates an HQ site, a Branch site, an internal transit segment, and a WAN-edge router connected to a simulated ISP.

Objectives

  • Build a multi-segment routed topology in Cisco Packet Tracer
  • Configure single-area OSPF (Area 0) across serial and Ethernet links
  • Compare point-to-point serial OSPF behavior versus Ethernet broadcast OSPF behavior
  • Validate OSPF neighbor formation and route learning
  • Observe DR/BDR election on a shared Ethernet segment
  • Validate OSPF authentication on active transit links
  • Originate a default route from the WAN edge into OSPF
  • Document baseline, failure, and recovery behavior in a portfolio-ready format

Physical Topology

The image below shows the actual Cisco Packet Tracer topology used in this project, including the HQ LAN, Branch LAN, shared Ethernet OSPF transit segment, WAN-edge router, and simulated ISP connection.

Project 03 Physical Topology

Logical Topology

The diagram below shows the logical routed design used in this project.

flowchart LR

    subgraph HQ[HQ LAN]
        direction LR
        PCHQ[PC-HQ\n192.168.10.10/24\nGW: 192.168.10.1]
        SWHQ[SW1-HQ\nVLAN 1: 192.168.10.2/24]
        R1[R1-HQ\nFa0/0: 192.168.10.1/24\nS0/3/0: 10.0.1.1/30\nLo0: 1.1.1.1/32]
        PCHQ --> SWHQ --> R1
    end

    subgraph AREA0[AREA 0]
        direction TB

        subgraph CORE_TOP[ ]
            direction LR
            R2[R2-HQ\nS0/3/0: 10.0.1.2/30\nFa0/0: 10.0.3.1/29\nLo0: 2.2.2.2/32\nPriority: 20]
            SWT[SW0\nBroadcast OSPF Segment\n10.0.3.0/29]
        end

        subgraph CORE_BOTTOM[ ]
            direction LR
            R4[R4-BRANCH\nFa0/0: 10.0.3.2/29\nS0/3/0: 10.0.2.2/30\nLo0: 4.4.4.4/32\nPriority: 0]
            R5[R5-WAN\nFa0/0: 10.0.3.3/29\nGi0/2/0: 203.0.113.1/30\nLo0: 5.5.5.5/32\nPriority: 30\nASBR]
        end

        R2 --> SWT
        R4 --> SWT
        R5 --> SWT
    end

    subgraph BRANCH[BRANCH LAN]
        direction LR
        R3[R3-BRANCH\nS0/3/0: 10.0.2.1/30\nFa0/0: 192.168.20.1/24\nLo0: 3.3.3.3/32]
        SWBR[SW2-BRANCH\nVLAN 1: 192.168.20.2/24]
        PCBR[PC-BRANCH\n192.168.20.10/24\nGW: 192.168.20.1]
        R3 --> SWBR --> PCBR
    end

    ISP[ISPR1\nGi0/0: 203.0.113.2/30]

    R1 -->|Serial P2P\n10.0.1.0/30| R2
    R4 -->|Serial P2P\n10.0.2.0/30| R3
    R5 -->|203.0.113.0/30| ISP
Loading

Device Roles

  • R1-HQ – default gateway for the HQ LAN
  • R2-HQ – HQ-side transit router connected to the serial WAN link and shared Ethernet OSPF segment
  • R3-BRANCH – default gateway for the Branch LAN
  • R4-BRANCH – branch-side transit router connected to the branch serial WAN link and shared Ethernet OSPF segment
  • R5-WAN – WAN-edge router connected to the internal Ethernet OSPF segment and simulated ISP
  • ISPR1 – simulated upstream ISP router
  • SW1-HQ – Layer 2 switch for HQ LAN
  • SW2-BRANCH – Layer 2 switch for Branch LAN
  • SW0 – shared Ethernet transit switch for the OSPF broadcast segment

IP Addressing Plan

HQ LAN IP Address Branch LAN IP Address
Network: 192.168.10.0/24 Network: 192.168.20.0/24
R1-HQ Fa0/0: 192.168.10.1 R3-BRANCH Fa0/0: 192.168.20.1
SW1-HQ VLAN 1: 192.168.10.2 SW2-BRANCH VLAN 1: 192.168.20.2
PC-HQ: 192.168.10.10 PC-BRANCH: 192.168.20.10
Default gateway: 192.168.10.1 Default gateway: 192.168.20.1

Serial Point-to-Point Links

R1-HQ to R2-HQ IP Address R4-BRANCH to R3-BRANCH IP Address
Network: 10.0.1.0/30 Network: 10.0.2.0/30
R1-HQ S0/3/0: 10.0.1.1 R3-BRANCH S0/3/0: 10.0.2.1
R2-HQ S0/3/0: 10.0.1.2 R4-BRANCH S0/3/0: 10.0.2.2
Ethernet OSPF Transit Segment
Network: 10.0.3.0/29
R2-HQ Fa0/0: 10.0.3.1
R4-BRANCH Fa0/0: 10.0.3.2
R5-WAN Fa0/0: 10.0.3.3
ISP Link IP Address Loopbacks IP Address
Network: 203.0.113.0/30 R1-HQ Lo0: 1.1.1.1/32
R5-WAN Gi0/2/0: 203.0.113.1 R2-HQ Lo0: 2.2.2.2/32
ISPR1 Gi0/0: 203.0.113.2 R3-BRANCH Lo0: 3.3.3.3/32
R4-BRANCH Lo0: 4.4.4.4/32
R5-WAN Lo0: 5.5.5.5/32

Routing Design

  • Routing protocol: OSPF
  • Process ID: 1
  • Area: 0
  • Design type: Single-area OSPF
  • Internal routing domain: Area 0 only
  • External connectivity: default route originated by R5-WAN

OSPF Design Notes

  • LAN-facing interfaces were configured as passive where appropriate
  • Loopback interfaces were advertised and used as stable router IDs
  • OSPF authentication was enabled on active transit links
  • Serial links were used to validate point-to-point OSPF behavior
  • Ethernet transit was used to validate broadcast multiaccess OSPF behavior
  • R5-WAN acted as an ASBR and originated the default route into OSPF

OSPF Behavior Observed

1. OSPF over Serial Point-to-Point

The following links behaved as point-to-point OSPF adjacencies:

  • R1-HQ ↔ R2-HQ
  • R4-BRANCH ↔ R3-BRANCH

Observed behavior:

  • direct FULL adjacency formation
  • no DR/BDR election
  • expected point-to-point route propagation

2. OSPF over Ethernet Broadcast Multiaccess

The shared Ethernet segment 10.0.3.0/29 connected:

  • R2-HQ
  • R4-BRANCH
  • R5-WAN

Observed behavior:

  • DR/BDR election occurred
  • multiple neighbors formed on the shared segment
  • route exchange happened through the broadcast network type

3. DR/BDR Election Results

Configured priorities:

  • R5-WAN = 30
  • R2-HQ = 20
  • R4-BRANCH = 0

Validated results:

  • R5-WAN = DR
  • R2-HQ = BDR
  • R4-BRANCH = DROTHER

This confirmed that:

  • higher OSPF priority influences election
  • an interface with priority 0 cannot become DR or BDR

4. Default Route Origination

R5-WAN was configured with:

  • a static default route toward ISPR1
  • default-information originate under OSPF

Result:

  • internal routers installed O*E2 0.0.0.0/0
  • the default route was successfully propagated across the OSPF domain

5. OSPF Authentication

OSPF simple authentication was configured on active transit interfaces.

Result:

  • adjacencies formed successfully only when authentication settings matched
  • this reinforced the dependency between OSPF control-plane formation and matching interface security settings

Baseline Validation Summary

Interface Status

Validated that all required active interfaces were operational:

  • HQ LAN interfaces up/up
  • Branch LAN interfaces up/up
  • serial transit links up/up
  • Ethernet OSPF segment interfaces up/up
  • WAN-edge ISP-facing interface up/up

Neighbor Status

Validated:

  • R1-HQ formed FULL adjacency with R2-HQ
  • R3-BRANCH formed FULL adjacency with R4-BRANCH
  • R2-HQ, R4-BRANCH, and R5-WAN formed expected adjacencies on the Ethernet segment

Route Learning

Validated:

  • HQ LAN and Branch LAN were learned dynamically through OSPF
  • loopbacks were learned across the topology
  • internal routers learned the external default route from R5-WAN

End-to-End Connectivity

Validated:

  • PC-to-default-gateway reachability
  • HQ-to-Branch host reachability
  • Branch-to-HQ host reachability
  • default route installation on internal routers

Failure Testing Scope

This project includes the following failure and recovery scenarios:

  • Failure Test 1 – R1-HQ to R2-HQ serial link down
  • Recovery Test 1 – restore R1-HQ to R2-HQ serial link
  • Failure Test 2 – R4-BRANCH to R3-BRANCH serial link down
  • Recovery Test 2 – restore R4-BRANCH to R3-BRANCH serial link
  • Failure Test 3 – R5-WAN Ethernet uplink to internal OSPF segment down
  • Recovery Test 3 – restore R5-WAN Ethernet uplink

These tests allow observation of:

  • site isolation due to point-to-point serial failure
  • route withdrawal and neighbor loss
  • loss and restoration of default-route origination from the WAN edge
  • internal versus external reachability differences

What I Learned

  • How OSPF behaves differently on serial point-to-point links versus Ethernet multiaccess segments
  • That point-to-point serial OSPF links form direct adjacencies without DR/BDR election
  • That Ethernet broadcast OSPF segments elect a Designated Router (DR) and Backup Designated Router (BDR)
  • How ip ospf priority affects DR/BDR election outcomes
  • That an interface with ip ospf priority 0 can still form adjacency but will never be elected DR or BDR
  • How loopback interfaces can be used for stable router IDs and additional route validation
  • How passive interfaces allow route advertisement without forming unnecessary OSPF adjacencies on user-facing LANs
  • How OSPF simple authentication must match on both ends of participating interfaces for adjacencies to form
  • How a static default route can be injected into OSPF with default-information originate
  • How to validate OSPF using interface state, neighbor state, routing tables, and default-route entries
  • How to distinguish internal routing continuity from WAN-edge default-route dependency

Interview-Ready Summary

I built a multi-router single-area OSPF lab in Cisco Packet Tracer to compare OSPF behavior across serial point-to-point links and Ethernet broadcast segments. I validated direct adjacencies on serial links, DR/BDR election on a shared Ethernet segment using interface priorities, configured OSPF authentication, advertised loopbacks, and originated a default route from a WAN-edge router into Area 0. I then documented baseline, failure, and recovery behavior to show how routing changed during link loss and restoration.

Tools Used

  • Cisco Packet Tracer
  • Cisco IOS CLI
  • Markdown documentation
  • Screenshot-based validation
  • Mermaid.js logical topology diagram

Project Status

  • Topology redesigned and built
  • OSPF configured across serial and Ethernet segments
  • OSPF authentication configured
  • DR/BDR election validated
  • Default route origination validated
  • Baseline validation completed
  • Failure Test 1 completed
  • Recovery Test 1 completed
  • Failure Test 2 completed
  • Recovery Test 2 completed
  • Failure Test 3 completed
  • Recovery Test 3 completed
  • Final screenshot set completed

About

Multi-segment OSPF lab with DR/BDR election, authentication, default route origination, and failure/recovery testing.

Topics

ospf ppp ospf-configuration ospf-protocol

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