Build a Three-Router OSPF Network

Configure and troubleshoot a three-router OSPF Area 0 network in Packet Tracer.

Introduction

30 Second Summary

A router knows how to reach networks connected directly to it. Remote sites stay out of reach until routers share a map of the wider network.

In this project, you will build a three-site network in Cisco Packet Tracer where three routers use OSPF to learn remote LANs through Area 0. You will restore remote connectivity before steering traffic through R2 with path cost.

What You'll Build

Picture showing someone a live three-site network where a trace from PC1 to PC3 follows the route you deliberately selected with OSPF cost.

By the end of this project, you'll have:

  • Working remote connectivity between PC1, PC2, and PC3 across routes learned through Area 0.
  • A collection of OSPF evidence that connects Hello packets to FULL neighbors. It also connects synchronized topology data to SPF results and installed routes.
  • A controlled forwarding path that sends PC1-to-PC3 traffic through R2. Passive LAN interfaces keep those networks advertised without unnecessary Hello traffic.
  • Secret Mission: Create a Hello-timer mismatch between R2 and R3. Diagnose the missing adjacency before restoring the neighbor relationship to FULL.

Are there any prerequisites?

You need prior experience with basic Cisco IOS interface configuration. This project assumes Cisco Packet Tracer version 9.0.1 is already installed on your Mac.

Before We Start

Before any devices are placed, this checkpoint locks in what your three-site network must achieve. It gives you a clear reason for using OSPF topology knowledge plus path cost once the hands-on work begins.

Build and Address the Triangle

A routing protocol can only describe links that are correctly cabled. OSPF cannot repair incorrect IPv4 addresses or disabled interfaces.

You will build the physical network in Cisco Packet Tracer. This step stops with every router interface showing up/up plus every PC ready on its local LAN.

In this step, get ready to:
  • Build a labeled triangle with three routers plus three attached PCs.
  • Configure the router names plus all nine router interfaces.
  • Assign the PC addresses plus their default gateways.
Assemble and cable the triangle

A routed triangle gives each router two transit links. This creates a direct path plus an alternate path for the later cost experiment.

  • Drag three Cisco 2911 routers from the device palette into the workspace.
  • Label the routers R1, R2, and R3.
  • Arrange the three routers as a physical triangle.
  • Drag three PCs from the device palette into the workspace.
  • Label the PCs PC1, PC2, and PC3.

Why Use a Triangle?

A chain provides only one route between the outer sites. The triangle gives OSPF two possible routes between R1 plus R3.

That alternate route makes path selection visible when you tune OSPF cost later.

  • Connect PC1's Ethernet port to R1 GigabitEthernet0/0.
  • Connect PC2's Ethernet port to R2 GigabitEthernet0/0.
  • Connect PC3's Ethernet port to R3 GigabitEthernet0/0.
  • Connect R1 GigabitEthernet0/1 to R2 GigabitEthernet0/1.
  • Connect R1 GigabitEthernet0/2 to R3 GigabitEthernet0/1.
  • Connect R2 GigabitEthernet0/2 to R3 GigabitEthernet0/2.

What Should the Topology Show?

Each PC has one link to its local router. Each router has two links to the other routers.

The six devices now form three LAN edges around one routed triangle.

Configure the router interfaces

The Cisco IOS command-line interface applies the hostname plus interface settings to each router. The LAN interfaces use /24 masks while the point-to-point transit links use /30 masks.

R1 connects the first LAN to both transit networks. Its three addresses end in .1.

  • Select R1 in the workspace.
  • Open the CLI tab.
  • Decline the initial setup dialog if the router offers one.
  • Configure R1 by entering these commands:
enable
configure terminal
hostname R1
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 10.0.12.1 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/2
ip address 10.0.13.1 255.255.255.252
no shutdown
exit
end

What Does This Configuration Do?

  • The hostname command identifies this router as R1.
  • The 192.168.1.1 address makes GigabitEthernet0/0 the gateway for PC1's LAN.
  • The two 10.0 addresses place R1 on the R1-R2 plus R1-R3 transit links.
  • Each no shutdown command activates its interface.
  • Inspect R1's interface summary by running this command:
show ip interface brief

What Should You See on R1?

You should see GigabitEthernet0/0 with 192.168.1.1 in an up/up state.

You should also see 10.0.12.1 plus 10.0.13.1 on the two transit interfaces. Both should show up/up.

Is an R1 Interface Down?

  • Check that the cable reaches the interface named in the connection plan.
  • Confirm that the affected interface contains the correct address plus no shutdown.

Help me diagnose my R1 interface state.

R2 connects the second LAN to the R1-R2 plus R2-R3 transit links. Its address plan keeps every router-to-router link in a separate /30 subnet.

  • Select R2 in the workspace.
  • Open the CLI tab.
  • Decline the initial setup dialog if the router offers one.
  • Configure R2 by entering these commands:
enable
configure terminal
hostname R2
interface GigabitEthernet0/0
ip address 192.168.2.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 10.0.12.2 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/2
ip address 10.0.23.1 255.255.255.252
no shutdown
exit
end

How Does R2 Fit the Triangle?

  • The 192.168.2.1 address makes R2 the gateway for PC2's LAN.
  • The 10.0.12.2 address shares the R1-R2 subnet with R1.
  • The 10.0.23.1 address starts the R2-R3 transit subnet.
  • The activation commands allow all three links to carry traffic.
  • Inspect R2's interface summary by running this command:
show ip interface brief

What Should You See on R2?

You should see 192.168.2.1, 10.0.12.2, and 10.0.23.1 assigned to the three Gigabit Ethernet interfaces.

Every configured interface should show up/up.

Is an R2 Interface Down?

  • Check that GigabitEthernet0/1 reaches R1.
  • Check that GigabitEthernet0/2 reaches R3.
  • Confirm that every configured interface contains no shutdown.

Help me diagnose my R2 interface state.

R3 completes both the third LAN plus the two remaining transit links. Its interface numbers follow the physical cable plan you built.

  • Select R3 in the workspace.
  • Open the CLI tab.
  • Decline the initial setup dialog if the router offers one.
  • Configure R3 by entering these commands:
enable
configure terminal
hostname R3
interface GigabitEthernet0/0
ip address 192.168.3.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 10.0.13.2 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/2
ip address 10.0.23.2 255.255.255.252
no shutdown
exit
end

How Does R3 Complete the Design?

  • The 192.168.3.1 address makes R3 the gateway for PC3's LAN.
  • The 10.0.13.2 address completes the R1-R3 subnet.
  • The 10.0.23.2 address completes the R2-R3 subnet.
  • The three active interfaces complete all physical sides of the lab.
  • Inspect R3's interface summary by running this command:
show ip interface brief

What Should You See on R3?

You should see 192.168.3.1, 10.0.13.2, and 10.0.23.2 assigned to the three Gigabit Ethernet interfaces.

Every configured interface should show up/up.

Is an R3 Interface Down?

  • Check that GigabitEthernet0/1 reaches R1 GigabitEthernet0/2.
  • Check that GigabitEthernet0/2 reaches R2 GigabitEthernet0/2.
  • Confirm that every configured interface contains no shutdown.

Help me diagnose my R3 interface state.

The router foundation is now complete. All three LAN gateways plus all three transit subnets are active.

Address the PCs and verify the activity

Each PC needs an address from its local LAN. Its gateway points to the router interface that carries traffic beyond that LAN.

  • Select PC1 in the workspace.
  • Open the Desktop tab.
  • Select IP Configuration.
  • Enter 192.168.1.10 as the IPv4 Address.
  • Enter 255.255.255.0 as the Subnet Mask.
  • Enter 192.168.1.1 as the Default Gateway.

PC1 now belongs to the 192.168.1.0/24 LAN. Its gateway is R1.

  • Select PC2 in the workspace.
  • Open the Desktop tab.
  • Select IP Configuration.
  • Enter 192.168.2.10 as the IPv4 Address.
  • Enter 255.255.255.0 as the Subnet Mask.
  • Enter 192.168.2.1 as the Default Gateway.

PC2 now belongs to the 192.168.2.0/24 LAN. Its gateway is R2.

  • Select PC3 in the workspace.
  • Open the Desktop tab.
  • Select IP Configuration.
  • Enter 192.168.3.10 as the IPv4 Address.
  • Enter 255.255.255.0 as the Subnet Mask.
  • Enter 192.168.3.1 as the Default Gateway.

PC3 now belongs to the 192.168.3.0/24 LAN. Its gateway is R3.

Why Does Each PC Need a Gateway?

A PC sends local traffic directly to another device on its LAN. It sends remote traffic to its default gateway.

The three router LAN addresses provide that first hop for the three PCs.

  • Open the File menu.
  • Select Save As.
  • Enter OSPF-Triangle.pkt as the activity name.
  • Click Save.

You should see OSPF-Triangle.pkt as the saved activity name. The local file now preserves the topology plus every device setting.

✔️ Awesome, I've got everything!

Great. Your saved activity contains the complete addressed triangle plus three configured PCs.

ⓧ I'd like to double check the full code

Compare each router with the complete configuration commands below.

enable
configure terminal
hostname R1
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 10.0.12.1 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/2
ip address 10.0.13.1 255.255.255.252
no shutdown
exit
end

This is the complete R1 configuration for this step.

enable
configure terminal
hostname R2
interface GigabitEthernet0/0
ip address 192.168.2.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 10.0.12.2 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/2
ip address 10.0.23.1 255.255.255.252
no shutdown
exit
end

This is the complete R2 configuration for this step.

enable
configure terminal
hostname R3
interface GigabitEthernet0/0
ip address 192.168.3.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 10.0.13.2 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/2
ip address 10.0.23.2 255.255.255.252
no shutdown
exit
end

This is the complete R3 configuration for this step.

  • Confirm PC1 uses 192.168.1.10/24 with gateway 192.168.1.1.
  • Confirm PC2 uses 192.168.2.10/24 with gateway 192.168.2.1.
  • Confirm PC3 uses 192.168.3.10/24 with gateway 192.168.3.1.
  • Confirm the R1-R2 link joins both GigabitEthernet0/1 interfaces.
  • Confirm the R1-R3 link joins R1 GigabitEthernet0/2 to R3 GigabitEthernet0/1.
  • Confirm the R2-R3 link joins both GigabitEthernet0/2 interfaces.

The activity should contain no OSPF configuration at this point.

Before you check, do you expect all nine configured router interfaces to report the same operational state?

  • Select R1 in the workspace.
  • Open the CLI tab.
  • Check R1's interface addresses plus states by running:
show ip interface brief

What Does This Command Check?

The command summarizes each interface's assigned address. It also reports the physical plus line-protocol states.

Together, those values reveal address mistakes plus inactive links.

  • Repeat the same interface check on R2.
  • Repeat the same interface check on R3.

What Should All Three Routers Show?

  • R1 should show 192.168.1.1, 10.0.12.1, and 10.0.13.1 with up/up states.
  • R2 should show 192.168.2.1, 10.0.12.2, and 10.0.23.1 with up/up states.
  • R3 should show 192.168.3.1, 10.0.13.2, and 10.0.23.2 with up/up states.

Does the Final Check Show a Problem?

  • Compare the affected interface address with the saved configuration reference.
  • Trace the affected cable to the exact interface named in the topology plan.
  • Return to interface configuration mode if the affected port still needs no shutdown.

Help me diagnose my final interface check.

That foundation is locked in. Your saved triangle now has nine active router interfaces plus three addressed PCs.

Next, you will test the local LANs before exposing the missing remote routing knowledge.

Test the Connected-Route Limit

Your addressed triangle in Cisco Packet Tracer now has working physical links. Every configured router interface is up.

Now you need to separate link readiness from end-to-end routing. Local links can work while a router's routing table still lacks knowledge of remote LANs.

In this step, get ready to:
  • Verify that each PC can reach its local default gateway.
  • Preserve the PC1-to-PC3 result as evidence of the routing limit.
  • Connect the result to the routes installed on R1.
Prove the local paths work

A local gateway ping checks the PC address plus the first router interface. A reply proves that traffic can cross the local LAN.

  • Access PC1's command prompt.
  • Test PC1's local gateway by running this command:
ping 192.168.1.1

What Does This Command Test?

  • The ping diagnostic sends reachability probes from PC1.
  • The destination 192.168.1.1 is the router interface serving PC1's LAN.
  • Replies prove that PC1 can reach its configured first hop.

You should see replies from PC1's gateway. This confirms that the first local path works.

  • Access PC2's command prompt.
  • Use the same ping diagnostic with 192.168.2.1 as the destination.

You should see replies from PC2's gateway. PC2 can reach its first hop.

  • Access PC3's command prompt.
  • Use the same ping diagnostic with 192.168.3.1 as the destination.

You should see replies from PC3's gateway. Good work. All three LANs now have proven local connectivity.

Does a Local Gateway Test Fail?

Confirm that you are testing the gateway assigned to the selected PC. Check that the PC address belongs to the same local subnet.

Confirm that the attached router's GigabitEthernet0/0 interface is still up.

Help me diagnose a failed local gateway ping.

Trigger the remote failure

A ping to a different LAN asks R1 to look beyond its directly attached networks. This check exposes whether R1 has learned a next hop for PC3.

Before you run this, do you expect local connectivity alone to carry the request across the triangle?

  • Return to PC1's command prompt.
  • Test PC3 from PC1 by running this command:
ping 192.168.3.10

What Does This Test Ask?

  • The destination 192.168.3.10 belongs to PC3's remote LAN.
  • PC1 sends the traffic to its default gateway because the destination is outside PC1's subnet.
  • R1 must find a matching remote route before it can forward the traffic toward PC3.

The remote test fails. The requests time out or report that the destination cannot be reached.

That outcome is intentional. The links are healthy while the routers still lack remote LAN routes.

Does the Remote Ping Succeed?

A successful result means the topology already has another source of remote routing information. Check R1 for a static route or a previously configured dynamic routing protocol.

Remove that extra routing configuration before repeating the test. This checkpoint depends on the routers having connected routes only.

Help me find why the remote ping succeeds.

Find the missing route

The failed ping shows the forwarding problem. R1's routing table reveals the exact piece of knowledge that is missing.

Before you run this, which networks do you expect R1 to list?

  • Access R1's command-line interface.
  • Display R1's routing table by running this command:
show ip route

How Do You Read This Table?

  • The show ip route command displays the routes R1 can currently use.
  • A route marked C represents a network connected directly to R1.
  • A route marked L represents an address assigned to one of R1's interfaces.

R1 lists 192.168.1.0/24, 10.0.12.0/30, and 10.0.13.0/30 because those networks touch R1 directly.

You should find no entry for 192.168.3.0/24. That missing route explains why R1 cannot forward PC1's traffic to PC3.

Does R1 Show Unexpected Routes?

Confirm that you accessed R1 before running the command. R2 and R3 have different directly connected networks.

If R1 lists the remote LAN, check for routing configuration left over from an earlier attempt. The intended table contains connected plus local routes only.

Help me interpret R1's routing table.

You have proven that healthy interfaces do not supply remote topology knowledge. Next, you will configure OSPF so each router can learn the other LANs.

Form Area 0 Neighbors

Your addressed triangle still has one blind spot: each router knows only the networks attached to itself. The failed PC1-to-PC3 ping exposed that missing topology knowledge.

You will use OSPF to exchange topology information across the transit links. Explicit Router IDs will make every neighbor easy to identify inside Area 0.

In this step, get ready to:
  • Translate the LAN plus transit masks into OSPF wildcard masks.
  • Configure OSPF process 1 with an explicit Router ID on every router.
  • Form two FULL Area 0 adjacencies on every router.
Derive the wildcard masks

A wildcard mask selects interface addresses for an OSPF network statement. Each wildcard mask is the bitwise inverse of its subnet mask.

  • Pair each /24 LAN with wildcard mask 0.0.0.255.
  • Pair each /30 transit network with wildcard mask 0.0.0.3.

How Does a Wildcard Mask Work?

A zero bit requires the corresponding address bit to match. A one bit allows that address bit to vary.

The network statement uses this match to activate OSPF on an interface. It also assigns that interface to the specified area.

Configure explicit Router IDs

The process ID selects an OSPF process within one router. The Router ID gives that router a unique identity in OSPF messages plus neighbor output.

  • Switch back to the Cisco Packet Tracer window from earlier.
  • Select R1 in the topology workspace.
  • Select the CLI tab.
  • Create OSPF process 1 with Router ID 1.1.1.1 by entering these commands:
enable
configure terminal
router ospf 1
router-id 1.1.1.1
end
show ip ospf 1

What Does This Configuration Do?

  • The router ospf 1 command creates local OSPF process 1 on R1.
  • The router-id 1.1.1.1 command gives R1 a fixed OSPF identity.
  • The end command returns to privileged EXEC mode for verification.
  • Confirm the output identifies Router ID 1.1.1.1 for process 1.

Does R1 Show a Different Router ID?

Confirm that you entered router-id 1.1.1.1 inside OSPF router configuration mode. The prompt should contain config-router when you enter the Router ID.

Help me check R1's OSPF process and Router ID.

  • Return to the topology workspace.
  • Select R2.
  • Select the CLI tab.
  • Create OSPF process 1 with Router ID 2.2.2.2 by entering these commands:
enable
configure terminal
router ospf 1
router-id 2.2.2.2
end
show ip ospf 1

Why Give R2 a Different Router ID?

R2 uses the same local process ID as R1. Its unique Router ID allows OSPF to distinguish R2 from every other router in the area.

  • Confirm the output identifies Router ID 2.2.2.2 for process 1.

Does R2 Show the Wrong Identity?

Check that the Router ID contains four 2 values. Confirm that the command appears under OSPF process 1.

Help me check R2's OSPF identity.

  • Return to the topology workspace.
  • Select R3.
  • Select the CLI tab.
  • Create OSPF process 1 with Router ID 3.3.3.3 by entering these commands:
enable
configure terminal
router ospf 1
router-id 3.3.3.3
end
show ip ospf 1

What Does R3 Complete?

R3 completes the identity plan with Router ID 3.3.3.3. All three routers now have predictable identities for the neighbor checks ahead.

  • Confirm the output identifies Router ID 3.3.3.3 for process 1.

Does R3 Show the Wrong Router ID?

Confirm that you configured router-id 3.3.3.3 on R3. Check that you entered the command under router ospf 1.

Help me check R3's OSPF identity.

Good. Every router now has a stable OSPF identity that you can recognize in neighbor output.

Advertise the six subnets in Area 0

Each network statement matches one or more interface addresses. The matching interfaces begin sending Hello packets on their connected segments.

  • Return to the topology workspace.
  • Select R1.
  • Select the CLI tab.
  • Add R1's LAN plus two transit networks to Area 0 by entering these commands:
enable
configure terminal
router ospf 1
network 192.168.1.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.3 area 0
network 10.0.13.0 0.0.0.3 area 0
end
show ip ospf neighbor

What Do R1's Network Statements Match?

  • The first statement matches R1's 192.168.1.1/24 LAN interface.
  • The second statement matches R1's 10.0.12.1/30 interface toward R2.
  • The third statement matches R1's 10.0.13.1/30 interface toward R3.

R1's neighbor table is empty at this point. This confirms that R1 is participating before either neighboring router has joined the area.

Does R1 Reject a Network Statement?

Check the subnet address plus wildcard mask on the rejected line. Confirm that every statement ends with area 0.

Help me check R1's OSPF network statements.

  • Return to the topology workspace.
  • Select R2.
  • Select the CLI tab.
  • Add R2's LAN plus two transit networks to Area 0 by entering these commands:
enable
configure terminal
router ospf 1
network 192.168.2.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.3 area 0
network 10.0.23.0 0.0.0.3 area 0
end
show ip ospf neighbor

What Do R2's Network Statements Match?

  • The LAN statement matches R2's 192.168.2.1/24 interface.
  • The 10.0.12.0 statement activates OSPF toward R1.
  • The 10.0.23.0 statement activates OSPF toward R3.
  • Confirm that R2 lists Router ID 1.1.1.1 in state FULL after the R1-R2 adjacency forms.

Does R2 Fail to Reach FULL?

Compare the R1-R2 statements on both routers. Both statements must match the 10.0.12.0/30 transit network with wildcard mask 0.0.0.3.

Help me diagnose the R1-R2 adjacency.

  • Return to the topology workspace.
  • Select R3.
  • Select the CLI tab.
  • Add R3's LAN plus two transit networks to Area 0 by entering these commands:
enable
configure terminal
router ospf 1
network 192.168.3.0 0.0.0.255 area 0
network 10.0.13.0 0.0.0.3 area 0
network 10.0.23.0 0.0.0.3 area 0
end
show ip ospf neighbor

What Does R3 Add to the Area?

  • The LAN statement adds R3's 192.168.3.0/24 network to the OSPF topology.
  • The 10.0.13.0 statement allows R3 to discover R1.
  • The 10.0.23.0 statement allows R3 to discover R2.
  • Confirm that R3 lists Router IDs 1.1.1.1 plus 2.2.2.2 in state FULL.

Is One R3 Neighbor Missing?

Check the transit statement for the missing neighbor. Use 10.0.13.0 0.0.0.3 area 0 for R1 or 10.0.23.0 0.0.0.3 area 0 for R2.

Help me find R3's missing OSPF neighbor.

✔️ Awesome, I've got everything!

Great. All three routers now have the required OSPF process plus their Area 0 network statements.

ⓧ I'd like to double check the full code

R1 should contain this complete OSPF configuration.

router ospf 1
 router-id 1.1.1.1
 network 192.168.1.0 0.0.0.255 area 0
 network 10.0.12.0 0.0.0.3 area 0
 network 10.0.13.0 0.0.0.3 area 0

R2 should contain this complete OSPF configuration.

router ospf 1
 router-id 2.2.2.2
 network 192.168.2.0 0.0.0.255 area 0
 network 10.0.12.0 0.0.0.3 area 0
 network 10.0.23.0 0.0.0.3 area 0

R3 should contain this complete OSPF configuration.

router ospf 1
 router-id 3.3.3.3
 network 192.168.3.0 0.0.0.255 area 0
 network 10.0.13.0 0.0.0.3 area 0
 network 10.0.23.0 0.0.0.3 area 0
  • Before you check all three routers, predict which two Router IDs each one will report.
  • Check R1's neighbors with the command below:
  • Check R2's neighbors with the command below:
  • Check R3's neighbors with the command below:
show ip ospf neighbor

What Should You See?

R1 shows Router IDs 2.2.2.2 plus 3.3.3.3 in state FULL.

R2 shows Router IDs 1.1.1.1 plus 3.3.3.3 in state FULL.

R3 shows Router IDs 1.1.1.1 plus 2.2.2.2 in state FULL.

Process ID Versus Router ID

Process ID 1 selects the local OSPF process on each router. Neighboring routers do not need matching process IDs.

The Router ID identifies each OSPF router. The neighbor tables display those identities as 1.1.1.1, 2.2.2.2, plus 3.3.3.3.

  • Click File in the Packet Tracer menu bar.
  • Click Save to update OSPF-Triangle.pkt.

That is the neighbor foundation complete. Your saved triangle now has two FULL OSPF neighbors on every router.

Does a Router Show Fewer Than Two FULL Neighbors?

Match the missing Router ID to its transit subnet. Check the network statement on both ends of that link.

Confirm that the transit interfaces remain up/up from the earlier addressing step. Confirm that both ends belong to Area 0.

Help me diagnose the incomplete OSPF triangle.

Your routers now exchange topology information across the triangle. Next, you will trace how OSPF turns that shared view into routes plus successful remote forwarding.

Verify OSPF Decisions

Your OSPF adjacencies are already FULL. R1, R2, and R3 can now exchange topology information.

A successful ping only proves that a destination replied. In this step, you will inspect the control-plane evidence that explains the reply.

In this step, get ready to:
  • Confirm process 1 plus SPF activity in Area 0.
  • Trace neighbor formation from interface timers to FULL state.
  • Prove remote routing from PC1.
Inspect OSPF process evidence

The process output connects each router's identity to its area. It also shows whether the router has run the shortest-path calculation.

  • Return to R1's CLI from the previous step.
  • Display R1's OSPF process information by running this command:
show ip ospf

What Does This Output Show?

  • The process section identifies the local OSPF process number.
  • The identity section displays the fixed Router ID.
  • The area section summarizes Area 0.
  • The calculation details provide evidence that SPF has executed.

On R1, you should see process 1 with Router ID 1.1.1.1. You should also see Area 0 plus information about SPF calculations.

  • Repeat the process check on R2.
  • Repeat the process check on R3.

R2 should report Router ID 2.2.2.2. R3 should report Router ID 3.3.3.3.

Missing the OSPF Process Details?

Confirm that you are using the router's privileged EXEC mode. The process details are unavailable from interface configuration mode.

If the output lacks the expected Router ID, compare that router with the saved configuration from the previous step.

Help me inspect the OSPF process output.

Trace Hello timers to FULL state

OSPF uses periodic Hello packets to discover neighbors. The Dead interval controls how long a router waits before removing a silent neighbor.

  • Return to R1's CLI.
  • Collect R1's adjacency evidence by running these commands:
show ip ospf neighbor
show ip ospf interface GigabitEthernet0/1

How Do These Checks Connect?

  • The neighbor table shows which Router IDs have reached FULL state with R1.
  • The interface output shows OSPF timers for the R1-R2 transit link.
  • The interface output also reports the neighbor count on that link.

R1 should show Router IDs 2.2.2.2 plus 3.3.3.3 in FULL state. Its GigabitEthernet0/1 output should show a 10-second Hello interval plus a 40-second Dead interval.

  • Return to R2's CLI.
  • Inspect R2's side of the R1-R2 link by running this command:
show ip ospf interface GigabitEthernet0/1

Why Compare Both Ends?

Neighbors on the same link need compatible timer settings. Matching interface output explains why the adjacency can remain FULL.

R2 should show the same 10-second Hello interval. It should also show the same 40-second Dead interval.

Do the Neighbor Results Look Incomplete?

Confirm that both R1 and R2 still show their transit interfaces as active. A down interface removes the corresponding adjacency.

Check that you inspected GigabitEthernet0/1 on both routers. That interface carries the R1-R2 link in this topology.

Help me diagnose the neighbor output.

Connect the database to forwarding

A synchronized link-state database gives every router the topology information needed for SPF. Link-state advertisements describe that topology before the best routes reach the routing table.

  • Return to R1's CLI.
  • Compare R1's database with its routing table by running these commands:
show ip ospf database
show ip route

How Does Topology Become a Route?

  • The database output shows the topology information collected for Area 0.
  • SPF evaluates that shared information to find the lowest-cost paths.
  • The routing table marks installed OSPF routes with O.

R1 should contain an OSPF route for 192.168.3.0/24. That entry supplies the missing route from the earlier failed test.

  • Repeat the database comparison on R2.
  • Repeat the database comparison on R3.

Each router should represent the same Area 0 topology in its database. Each routing table should contain OSPF entries for the remote LANs.

Before you test from PC1, do you expect the earlier remote failure or successful replies?

  • Return to PC1's command prompt.
  • Test the current OSPF path by running these commands:
ping 192.168.3.10
tracert 192.168.3.10

What Do These Tests Prove?

  • The ping proves that PC1 can reach PC3 through the installed OSPF route.
  • The trace exposes the routers that forward traffic along OSPF's current lowest-cost path.

You should see successful replies from 192.168.3.10. The trace should pass through 192.168.1.1 plus R3 at 10.0.13.2 before reaching PC3.

Does the Remote Test Still Fail?

Return to R1's routing table. Confirm that an O entry exists for 192.168.3.0/24.

Confirm that PC1 still uses 192.168.1.1 as its default gateway. Confirm that PC3 still uses 192.168.3.1.

Help me diagnose the failed remote test.

Strong work. Your successful ping now has control-plane evidence behind it.

You have followed OSPF from Hello packets to the forwarding path. Next, you will shape where OSPF talks plus which route it prefers.

Tune OSPF Behavior

Your OSPF Area 0 network in Cisco Packet Tracer now reaches all three LANs. The routing process still sends Hello packets toward PCs that can never become OSPF neighbors.

This step shapes where OSPF talks. You will also raise the direct R1-R3 link cost so traffic follows R2.

In this step, get ready to:
  • Inspect OSPF behavior on each PC-facing interface.
  • Stop unnecessary Hello packets while keeping each LAN advertised.
  • Tune OSPF cost so PC1 reaches PC3 through R2.
Inspect the PC-facing interfaces

The LAN network statement activates OSPF on GigabitEthernet0/0. A connected PC cannot form an OSPF adjacency with the router.

  • Switch to R1's CLI tab from earlier.
  • Inspect R1's PC-facing OSPF interface by running these commands:
enable
show ip ospf interface GigabitEthernet0/0

What Does This Check?

  • The first command enters privileged EXEC mode.
  • The second command displays OSPF settings for the LAN interface.
  • The output connects the advertised LAN to an interface with no possible OSPF neighbor.
  • Repeat the same inspection on R2.
  • Repeat the same inspection on R3.

Each router reports OSPF information for GigabitEthernet0/0. Each interface has no OSPF neighbor because it faces a PC.

Missing OSPF Interface Information?

Confirm that the router's LAN network statement still matches its GigabitEthernet0/0 address. Confirm that the interface remains up/up.

Run the check from privileged EXEC mode.

Help me diagnose the missing OSPF interface information.

Make each LAN interface passive

A passive interface stops OSPF Hello packets on one interface. The connected LAN remains available for OSPF to advertise.

  • Switch back to R1's CLI tab.
  • Make R1's GigabitEthernet0/0 passive by entering these commands:
enable
configure terminal
router ospf 1
passive-interface GigabitEthernet0/0
end

What Does This Configuration Do?

  • The OSPF router command selects local process 1.
  • The passive interface command suppresses Hello packets on the PC-facing interface.
  • The network statement remains in place, so the LAN continues to appear in OSPF advertisements.
  • The final command returns the router to privileged EXEC mode.
  • Repeat the same configuration on R2.
  • Repeat the same configuration on R3.

Before you check the result, consider whether the remote LAN routes stay installed after the interfaces stop sending Hellos.

  • Return to R1's CLI tab.
  • Verify the passive interface plus the routing state by running these commands:
show ip protocols
show ip ospf neighbor
show ip route

What Do These Checks Prove?

  • The protocol output confirms that GigabitEthernet0/0 is passive.
  • The neighbor output confirms that both transit adjacencies remain FULL.
  • The routing table confirms that remote LANs still have O routes.
  • Repeat the same verification on R2.
  • Repeat the same verification on R3.

Each router identifies GigabitEthernet0/0 as passive. Each router still reports two FULL neighbors plus remote OSPF routes.

  • Switch back to PC1's Command Prompt from earlier.
  • Confirm that the advertised PC3 LAN remains reachable by running:
ping 192.168.3.10

What Does This Ping Prove?

The ping crosses the OSPF network to PC3. Successful replies prove that a passive LAN interface remains advertised.

You should receive replies from 192.168.3.10. The LAN stays reachable even though its router no longer sends OSPF Hellos toward the PC.

Does the Remote Ping Fail?

Confirm that only GigabitEthernet0/0 is passive on each router. A passive transit interface can remove the adjacency needed for remote routes.

Check that all three routers still report two FULL neighbors.

Help me restore connectivity after configuring passive interfaces.

Steer traffic through R2

An OSPF cost contributes to the total metric for a path. OSPF selects the route with the lowest total cost.

The direct R1-R3 link currently wins. Raising its cost at both ends makes the R1-R2-R3 path preferable in both directions.

Before you capture the baseline, predict which transit router the current trace reaches after R1.

  • Return to PC1's Command Prompt.
  • Capture the current direct path by running:
tracert 192.168.3.10

What Does the Baseline Show?

The trace lists the Layer 3 hops used to reach PC3. This baseline gives you a path to compare after cost tuning.

The trace follows the direct R1-R3 link. You should see R3's direct-link address 10.0.13.2 before PC3.

  • Switch to R1's CLI tab.
  • Set the R1-R3 interface cost to 50 by running these commands:
enable
configure terminal
interface GigabitEthernet0/2
ip ospf cost 50
end
show ip ospf interface GigabitEthernet0/2

How Does R1's Cost Change Work?

  • The interface command selects R1's link toward R3.
  • The cost command assigns an explicit OSPF metric of 50 to that interface.
  • The final display confirms the cost applied to the OSPF interface.

R1 should now report an OSPF interface cost of 50 on GigabitEthernet0/2.

  • Switch to R3's CLI tab.
  • Set the R3-R1 interface cost to 50 by running these commands:
enable
configure terminal
interface GigabitEthernet0/1
ip ospf cost 50
end
show ip ospf interface GigabitEthernet0/1

Why Tune R3 as Well?

R3 now assigns the same higher cost to its direction of the direct link. Matching the policy at both ends keeps the preferred path symmetric.

The final display confirms that the intended interface has a cost of 50.

R3 should now report an OSPF interface cost of 50 on GigabitEthernet0/1.

Does the Cost Stay Unchanged?

Confirm that R1 uses GigabitEthernet0/2. Confirm that R3 uses GigabitEthernet0/1.

Check that the cost command was entered in interface configuration mode.

Help me diagnose my OSPF cost configuration.

Before you run the final checks, predict which transit address the new PC1-to-PC3 trace should include.

  • Switch back to R1's CLI tab.
  • Confirm R1's route plus its neighbor state by running:
show ip route
show ip ospf neighbor

What Should R1 Report?

  • The routing table identifies the OSPF route toward the PC3 LAN.
  • The next hop 10.0.12.2 proves that R1 now prefers R2.
  • The neighbor table confirms that both transit adjacencies remain FULL.
  • Repeat the neighbor check on R2.
  • Repeat the neighbor check on R3.

R1 identifies 10.0.12.2 as the next hop for the PC3 LAN. Every router continues to report two FULL neighbors.

  • Return to PC1's Command Prompt.
  • Verify reachability plus the preferred path by running these commands:
ping 192.168.3.10
tracert 192.168.3.10

What Do the Final Tests Prove?

  • The ping proves that PC3 remains reachable after the metric change.
  • The trace proves that forwarding now follows the lower-cost path through R2.
  • The address 10.0.12.2 identifies R2 on the R1-R2 transit link.

That is the routing policy in action. PC1 still receives replies from PC3.

The trace now includes R2 at 10.0.12.2 before continuing toward R3.

Does the Trace Still Use the Direct Link?

Confirm that R1 GigabitEthernet0/2 has cost 50. Confirm that R3 GigabitEthernet0/1 has the same cost.

Check R1's routing table for next hop 10.0.12.2 before running the trace again.

Help me diagnose why the trace still uses the direct R1-R3 link.

  • Save the updated OSPF-Triangle.pkt activity in Cisco Packet Tracer.

✔️ Awesome, I've got everything!

Strong finish. Your saved lab now suppresses LAN Hellos while steering PC1-to-PC3 traffic through R2.

ⓧ I'd like to double check the full code

Compare your saved router settings plus the final verification results with this checklist.

  • Confirm R1 has passive GigabitEthernet0/0 under OSPF process 1.
  • Confirm R1 GigabitEthernet0/2 has OSPF cost 50.
  • Confirm R2 has passive GigabitEthernet0/0 under OSPF process 1.
  • Confirm R3 has passive GigabitEthernet0/0 under OSPF process 1.
  • Confirm R3 GigabitEthernet0/1 has OSPF cost 50.
  • Confirm every router has two FULL neighbors plus remote OSPF routes.
  • Confirm PC1 can ping 192.168.3.10.
  • Confirm PC1's preferred trace includes R2 at 10.0.12.2.
  • Confirm the final topology is saved in OSPF-Triangle.pkt.

Secret mission

Break and Repair a Neighbor

Create a Hello-timer mismatch on the R2-R3 link. Diagnose the missing adjacency from router evidence before restoring compatible timers and proving the repair.

Clean Up Your Resources

Clean Up Your Resources

Your Cisco Packet Tracer lab runs entirely on your Mac, so it creates no ongoing costs. Choose whether to keep the activity ready, close it for later, or move it to the Trash.

Resources you used:

  • The OSPF-Triangle.pkt activity stores your complete three-router OSPF Area 0 lab.

Keep everything running

This option keeps the complete lab ready for more route selection tests. The activity remains available with no ongoing cost.

  • Save OSPF-Triangle.pkt after your final checks.
  • Keep Cisco Packet Tracer open while you continue testing the topology.

Pause - I'll come back to this later

Closing Cisco Packet Tracer stops the active simulation while preserving every saved configuration. The OSPF-Triangle.pkt activity remains ready for your next session.

  • Save OSPF-Triangle.pkt.
  • Close Cisco Packet Tracer.
  • Double-click OSPF-Triangle.pkt in Finder when you return.

You'll see R1, R2, R3, plus all three PCs with the saved OSPF configuration intact.

Delete - I don't want to use this again

Moving a finished lab to the Trash can feel drastic. Cisco Packet Tracer stays installed on your Mac.

  • Close Cisco Packet Tracer.
  • Locate OSPF-Triangle.pkt in Finder at the location where you saved it.
  • Drag OSPF-Triangle.pkt to the Trash in the Dock.
  • Confirm OSPF-Triangle.pkt appears in the Trash.

This completes cleanup for the only resource created by the project.

Nice Work!

Nice Work!

You did it! Your saved OSPF-Triangle.pkt lab in Cisco Packet Tracer now runs a working three-router OSPF network in Area 0. You can now demonstrate how topology knowledge becomes real forwarding decisions.

You've learned how to:

  • Turn a planned remote ping failure into end-to-end connectivity across three OSPF-learned LAN routes.
  • Verify two FULL neighbors on every router. Connect synchronized link-state databases to SPF-selected routes.
  • Suppress unnecessary Hello packets with passive interfaces. Steer PC1 traffic through R2 with symmetric cost tuning on the direct R1-R3 link.
  • Secret Mission: Diagnose the R2-R3 Hello-timer mismatch from neighbor output. Restore Router ID 3.3.3.3 to FULL without removing OSPF.

Ready to quiz yourself?