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OSPF and NAT multiarea

Network Architect2024

Simulating a redundant, multi-area enterprise network routing structure. Configured secure NAT borders, routing filters, and OSPF area transitions using GNS3 emulator.

Network Topology

networkOSPF Area 1 (Internal)
networkABR Router (R3)
networkOSPF Area 0 (Backbone)
networkNAT Gateway Router (R5)
externalExternal ISP / Internet

Network Routes & Connections

OSPF Area 1 (Internal)ABR Router (R3)Intra-area Routing
ABR Router (R3)OSPF Area 0 (Backbone)Inter-area Routing
OSPF Area 0 (Backbone)NAT Gateway Router (R5)Default Route
NAT Gateway Router (R5)External ISP / InternetPAT Translation

CLI Command Simulation

Select a scenario to trigger and view simulated Cisco IOS CLI outputs.

cisco-ios-cli.log
R3# show ip ospf neighbor
Neighbor ID Pri State Address Interface
192.168.1.1 1 FULL/BDR 192.168.1.1 GigabitEthernet0/0
10.0.0.1 1 FULL/DR 10.0.0.1 GigabitEthernet0/1
OSPF Area 0 and Area 1 synchronized successfully.
R3# show ip route ospf
O IA 10.0.2.0/24 [110/3] via 10.0.0.1, 00:04:12, GigabitEthernet0/1

Network & Routing Principles

OSPF, Cisco IOS

Multi-area Routing

Configured OSPF routing protocols separating Area 1 and Area 2, using Area Border Routers (ABR) to summarize route advertisements.

NAT, PAT

Network Address Translation

Implemented static and dynamic NAT (PAT) translating internal private addresses to access outside public ISP zones.

GNS3

Network Emulation

Designed, wired, and simulated Cisco routers, switches, and hosts using virtualized GNS3 environments.

Get Cisco Configurations

Clone the repository to access the GNS3 topology files and startup configurations.

# Clone GNS3 network topology files
git clone https://github.com/everleepham/ospf_multiarea.git

Engineering Challenges & Design Decisions

Route Loop Prevention

Problem

Connecting Area 1 and Area 2 directly without passing through Backbone Area 0 caused infinite routing loops.

Engineering Solution

Restructured the physical topology to route all transit traffic through Area 0, and configured virtual links on Cisco routers to bridge disconnected areas.

Key Achievements & Metrics

  • Achieved full end-to-end network reachability across different routing domains.
  • Secured internal IP addresses using Port Address Translation (PAT).
  • Gained deep hands-on expertise in Cisco IOS commands and GNS3 network mapping.