GNS3 Lab: OSPF Multi-Area + NAT

Objective

Simulate an enterprise network with two separate OSPF domains connected via a central router (R3). One side of the network uses NAT on R5 to provide external (internet-like) access to devices within the OSPF domain.

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Network Topology

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Network Structure

OSPF 1 (Process ID 1)

• Area 0: R1 ↔ R2 ↔ R3 ↔ R4
• Area 1: R1 loopbacks (192.168.1.0/24)
• Area 2: R3 loopbacks (192.168.1.3, .1.4)

OSPF 2 (Process ID 2)

• Area 0: R3 ↔ R5
• R6 simulates an external network or "internet"

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Routing

• R1–R4 run OSPF 1 in a multi-area configuration
• R3 acts as an ABR between Area 0 and Area 2
• R3 and R5 run a separate OSPF 2 process
• Redistribution or static routing may be required to share routes between OSPF 1 & 2

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NAT on R5

• Goal: Allow internal devices (e.g. R1) to ping external device R6
• R5 performs NAT (PAT style)
• Uses NAT overload via external interface

Sample NAT config on R5:

! Access list to define internal traffic
access-list 1 permit 192.168.10.0 0.0.0.255

! NAT overload config
ip nat inside source list 1 interface g1/0 overload

! Interface roles
interface g2/0
 ip nat inside
!
interface g1/0
 ip nat outside