Build a Mini ISP IPv4 Network

Build and troubleshoot a routed IPv4 network with Cisco Packet Tracer.

Introduction

30 Second Summary

Home internet feels simple until a connection fails somewhere between two locations. Finding the fault means tracing where traffic stops.

In this project, you will build a mini ISP IPv4 network in Cisco Packet Tracer that connects two customer LANs through a provider core. You will diagnose a missing route before repairing a provider-side outage.

What You'll Build

You'll watch a successful test travel from PC-A through ISP-CORE to PC-B, then use the same lab to pinpoint a broken path.

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

  • A working customer-to-customer connection that carries traffic through three routers from PC-A to PC-B.
  • An IPv4 address plan you can explain, with /24 customer networks plus /30 transit links.
  • A repeatable troubleshooting workflow that uses routing tables, ping, plus path tracing to isolate a missing route.
  • Secret Mission: Disable the provider interface toward CPE-B, locate the fault using interface state plus packet-path evidence, then restore service.

Are there any prerequisites?

You need basic familiarity with IPv4 addresses. You also need a Cisco account with Cisco Packet Tracer installed on a Windows PC.

Before We Start

Your mini ISP lab lives inside Cisco Packet Tracer. The simulator needs to be ready before you can place routers or test traffic.

This readiness check confirms that your Cisco account can access the simulator. It also confirms that the prescribed router and switch models are available for the topology.

In this step, get ready to:
  • Install Cisco Packet Tracer on Windows.
  • Access the Logical workspace with your Cisco account.
  • Confirm that the 4321 router model and 2960 switch model are available.
Install or open Cisco Packet Tracer

Packet Tracer provides the virtual routers and switches used throughout this project. Start by checking whether the application is already installed on your Windows PC.

  • Press the Windows key to open Windows search.
  • Type Cisco Packet Tracer into the search field.
  • Choose the tab below that matches your search result.

✔️ Packet Tracer opens

  • Select Cisco Packet Tracer from the search results.

You'll see Packet Tracer open on your PC. Keep the application running for the sign-in check.

ⓧ Packet Tracer is not installed

  • Open the official Cisco Resource Hub in your browser.
  • Choose the Packet Tracer download for Windows.
  • Save the installation file in your Downloads folder.
  • Double-click the Packet Tracer installation file in your browser's downloads list.
  • Select I accept the agreement in the License Agreement window.
  • Click Next.

The remaining setup screens control the installation location and shortcuts. The default choices work for this project.

  • Keep the default destination in Select Destination Location.
  • Keep the default folder in Select Start Menu Folder.
  • Keep the default shortcuts in Select Additional Tasks.
  • Review the installation choices in Ready to Install.
  • Click Install.
  • Leave Launch Cisco Packet Tracer selected after the installation completes.
  • Click Finish to launch Packet Tracer.

You'll see Packet Tracer open on your PC. The installer has completed successfully.

Your simulator is now running. That gives the project a safe workspace where you can build the network without physical equipment.

Packet Tracer Won't Open?

  • Confirm that the installer reached its final screen before searching for Packet Tracer again.
  • Download a fresh installation file from the official Cisco Resource Hub if the original file does not launch.
  • Restart Windows if the installation completed but Packet Tracer does not appear in search.

Help me troubleshoot my Packet Tracer installation. You can also search the Cisco Community for Windows-specific help.

Sign in and reach the Logical workspace

Packet Tracer uses your Cisco account to authenticate the application. A browser handles the sign-in before returning you to the simulator.

Your password stays inside Cisco's sign-in flow. You do not enter it anywhere in this project.

  • Click No if the first launch asks Would you like to run multi-user when the application starts?.
  • Choose the firewall access allowed by your network policy if Windows displays a security prompt.
  • Click LOGIN in the Cisco Packet Tracer Login window.
  • Complete the Cisco account sign-in in the browser window.
  • Close the browser tab after it shows You have successfully logged in to Cisco Packet Tracer. You may close this tab..
  • Return to Packet Tracer.
  • Select Logical above the main workspace.

You'll see the Logical workspace as the main canvas. This is where the mini ISP topology takes shape in the next step.

Sign-In Isn't Completing?

  • Confirm that your browser has finished signing in to the same Cisco account you use for Packet Tracer.
  • Return to the Packet Tracer login window after closing the browser tab.
  • Repeat the login flow if the Logical workspace remains unavailable.

Help me troubleshoot Packet Tracer authentication. You can also search the Cisco Community for account access help.

Confirm the required device models

The project uses a specific router model plus a specific switch model. Confirming both models now prevents a device mismatch when you begin cabling the ISP path.

Before you check, do you expect both prescribed models to appear in the device palette?

  • Confirm that Logical remains selected above the workspace.
  • Point to the top-row icons in the Device-Type Selection box until the label area identifies the category for routers and switches.
  • Click that device category.
  • Point to the lower-row icons until the Device-Specific Selection box displays router models.
  • Confirm that 4321 appears in the router list.
  • Point to the lower-row icons until the label area shows Switches.
  • Click Switches.
  • Confirm that 2960 appears in the switch list.

You'll see the 4321 router model in the router palette. You'll also see the 2960 switch model in the switch palette.

  • Leave both models in the device palette so the Logical workspace stays empty.

Can't Find One of the Models?

  • Confirm that you are checking the Device-Specific Selection box at the bottom of Packet Tracer.
  • Return to the router category before searching for the 4321 model.
  • Return to the Switches category before searching for the 2960 model.

Help me locate the required Packet Tracer devices.

That's the readiness check complete. The exact router and switch models are available for the build.

Cisco Packet Tracer is ready for the lab. Next, you'll assemble the mini ISP topology without setup getting in the way.

Assemble the ISP Topology

Every later network decision depends on a layout you can follow from PC-A to PC-B. A clear topology makes each hop easy to trace.

In this step, you'll assemble that topology in Cisco Packet Tracer. The saved lab stays free of IPv4 configuration for now.

In this step, get ready to:
  • Create a saved lab in the Logical workspace.
  • Place seven devices with the project names.
  • Connect the ISP path with the required router interfaces.
Launch Packet Tracer and save the lab

The Logical workspace is the canvas where you place devices. Saving an empty lab first gives every later change a known home.

  • Press the Windows key to open the search bar.
  • Type Cisco Packet Tracer in the search bar.
  • Press Enter to launch Cisco Packet Tracer.
  • Select Logical in the workspace selector.

Why Cisco Packet Tracer?

Cisco Packet Tracer keeps this project focused on IPv4 forwarding. GNS3 would add virtual machine setup before you could practise the routing logic.

  • Click File in the Packet Tracer menu bar.
  • Select Save As from the menu.
  • Choose your Windows Desktop in the Save File window.
  • Enter mini-isp-ipv4.pkt as the file name.
  • Click Save to create the lab file.

Good start. Your empty mini-isp-ipv4.pkt file now gives the topology a safe home.

Can't find the saved file?

  • Return to File in the Packet Tracer menu bar.
  • Select Save As again.
  • Confirm that the destination is your Windows Desktop.
  • Confirm that the file name is mini-isp-ipv4.pkt.

Help me save my Packet Tracer lab correctly.

Place and name the devices

The layout should read from left to right like a customer path through a provider. Device position now makes later fault isolation much easier.

  • Use the Device-Type Selection box to choose the router category.
  • Use the Device-Specific Selection box to choose the 4321 router.
  • Drag three 4321 routers into the Logical workspace.
  • Arrange the routers in one horizontal row with space between them.

You should see three router icons forming the centre of the provider path.

  • Use the Device-Type Selection box to choose the switch category.
  • Use the Device-Specific Selection box to choose the 2960 switch.
  • Drag two 2960 switches into the Logical workspace.
  • Place one switch to the left of the router row.
  • Place the other switch to the right of the router row.
  • Use the Device-Type Selection box to choose the end-device category.
  • Use the Device-Specific Selection box to choose PC-PT.
  • Drag two PC-PT devices into the Logical workspace.
  • Place one PC at each end of the row.

Your workspace should now show seven devices arranged from the left-side PC to the right-side PC.

Use This Naming Map

  • The left router is CPE-A.
  • The centre router is ISP-CORE.
  • The right router is CPE-B.
  • The left switch is SW-A.
  • The right switch is SW-B.
  • The left PC is PC-A.
  • The right PC is PC-B.
  • Click the left router to open its device configuration window.
  • Select the Config tab.
  • Select Global Settings in the left pane.
  • Replace the Display Name value with CPE-A.
  • Close the CPE-A configuration window.
  • Repeat the same naming process for every remaining device using the naming map.

You should now see the labels PC-A, SW-A, CPE-A, ISP-CORE, CPE-B, SW-B, and PC-B below the devices.

Device Count or Name Off?

  • Remove any extra device from the Logical workspace.
  • Place any missing model from the device selection boxes.
  • Reopen a misnamed device through its Config tab.
  • Correct the value under Display Name.

Help me fix my Packet Tracer device layout.

Connect and verify the path

The automatic connection tool chooses a connection type from compatible open interfaces. The link order determines which router interface remains available for each transit connection.

  • Select the gold lightning-bolt connection tool before each link.
  • Connect PC-A to SW-A.
  • Connect SW-A to CPE-A.
  • Connect CPE-A to ISP-CORE.
  • Connect PC-B to SW-B.
  • Connect SW-B to CPE-B.
  • Connect ISP-CORE to CPE-B.

You should see one continuous seven-device path across the Logical workspace.

  • Hover over each router-side cable endpoint to inspect its interface label.

What Should the Interfaces Show?

  • CPE-A GigabitEthernet0/0/0 faces SW-A.
  • CPE-A GigabitEthernet0/0/1 faces ISP-CORE GigabitEthernet0/0/0.
  • ISP-CORE GigabitEthernet0/0/1 faces CPE-B GigabitEthernet0/0/1.
  • CPE-B GigabitEthernet0/0/0 faces SW-B.

Interface Mapping Different?

  • Select the mismatched cable.
  • Press the Delete key.
  • Reconnect the customer LAN link before its transit link.
  • Inspect the router-side interface labels again.

Help me correct my Packet Tracer interface mapping.

  • Click File in the Packet Tracer menu bar.
  • Select Save to preserve the completed topology.

Before you reopen the file, do you expect every device name and cable to return exactly where you saved it?

  • Click the X in the top-right corner to close Cisco Packet Tracer.
  • Double-click mini-isp-ipv4.pkt on your Windows Desktop to reopen it in Cisco Packet Tracer.

What Should You See?

  • You should see three 4321 routers named CPE-A, ISP-CORE, and CPE-B.
  • You should see two 2960 switches named SW-A and SW-B.
  • You should see two PC-PT devices named PC-A and PC-B.
  • You should see the complete path from PC-A through ISP-CORE to PC-B.

✔️ Awesome, I've got everything!

Your reopened lab contains all seven named devices. The complete ISP path is saved in mini-isp-ipv4.pkt.

ⓧ I'd like to double check the full code

The complete saved state has these details.

  • The file is named mini-isp-ipv4.pkt.
  • The Logical workspace contains 4321 routers CPE-A, ISP-CORE, and CPE-B.
  • The Logical workspace contains 2960 switches SW-A and SW-B.
  • The Logical workspace contains PC-PT devices PC-A and PC-B.
  • The connected path runs from PC-A to SW-A to CPE-A to ISP-CORE to CPE-B to SW-B to PC-B.
  • CPE-A Gi0/0/0 faces SW-A.
  • CPE-A Gi0/0/1 faces ISP-CORE Gi0/0/0.
  • ISP-CORE Gi0/0/1 faces CPE-B Gi0/0/1.
  • CPE-B Gi0/0/0 faces SW-B.
  • No IPv4 addressing or static routing is configured.

Your topology is saved. Next up, you'll map the IPv4 plan onto this layout. Then you'll bring Customer A online.

Bring Customer A Online

The topology from the last step gives every device a clear place in the path. Its links still need IPv4 identities before PC-A can communicate.

PC-A's first checkpoint is its local gateway. A successful test proves the Customer A LAN works before provider routing is involved.

In this step, get ready to:
  • Document the four IPv4 subnets on the topology.
  • Configure CPE-A's Customer A interfaces.
  • Configure PC-A so it can reach its default gateway.
Label the IPv4 plan

An IPv4 address plan maps each link to a unique subnet. Labeling the topology makes later routing failures easier to trace.

Why use /24 and /30 networks?

A /24 gives each customer LAN space for hosts such as PCs plus its router interface.

A /30 keeps each transit segment compact because it connects two router interfaces.

  • Switch back to Cisco Packet Tracer in the Logical workspace from earlier.
  • Select the note icon from the workspace toolbar.
  • Place a note beside the Customer A LAN containing Customer A LAN 192.168.10.0/24.
  • Place a note beside the CPE-A transit link containing CPE-A-to-ISP-CORE transit 10.0.0.0/30.
  • Place a note beside the CPE-B transit link containing ISP-CORE-to-CPE-B transit 10.0.0.4/30.
  • Place a note beside the Customer B LAN containing Customer B LAN 192.168.20.0/24.
  • Return to the selection tool in the workspace toolbar.
  • Click the save icon in the top toolbar to save mini-isp-ipv4.pkt.

You should see one network label beside each LAN or transit link. The four labels now provide a map for every address you configure.

Configure CPE-A

Each router interface needs an address from the subnet it faces. It must also be enabled before it can pass traffic.

  • Click CPE-A in the Logical workspace.
  • Select the CLI tab in the router window.
  • Decline any initial configuration dialog using its on-screen choice.
  • Configure CPE-A's identity and interfaces by entering these commands in order:
enable
configure terminal
hostname CPE-A
interface GigabitEthernet0/0/0
ip address 192.168.10.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/0/1
ip address 10.0.0.1 255.255.255.252
no shutdown
end
show ip interface brief

What does this configuration do?

  • The hostname line makes the router prompt identify CPE-A.
  • The GigabitEthernet0/0/0 interface uses 192.168.10.1 as the Customer A gateway.
  • The GigabitEthernet0/0/1 interface uses 10.0.0.1 as the CPE-A transit endpoint.
  • The enabling lines activate both interfaces administratively.
  • The final check summarizes the configured interface addresses plus their current states.
  • Confirm GigabitEthernet0/0/0 lists 192.168.10.1.
  • Confirm the Customer A LAN interface shows an active state.
  • Confirm GigabitEthernet0/0/1 lists 10.0.0.1.
  • Click the save icon in the top toolbar to save mini-isp-ipv4.pkt.

That is the router side complete. CPE-A now holds the two addresses assigned to Customer A.

Missing an address or active interface?

  • Return to the CLI prompt to compare the interface names plus addresses with the command block.
  • Re-enter the enabling line inside any interface that remains disabled.
  • Check that the Customer A cable connects to GigabitEthernet0/0/0 on CPE-A.

Help me troubleshoot the CPE-A interface configuration.

✔️ Awesome, I've got everything!

Your CPE-A interface summary includes both configured IPv4 addresses. Keep the router window available for comparison.

ⓧ I'd like to double check the full code

Compare your CPE-A configuration sequence with this complete reference:

enable
configure terminal
hostname CPE-A
interface GigabitEthernet0/0/0
ip address 192.168.10.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/0/1
ip address 10.0.0.1 255.255.255.252
no shutdown
end
show ip interface brief

How to use this reference

Check each interface name before comparing its address plus mask. The order keeps each enabling line inside the correct interface configuration.

Configure and test PC-A

PC-A needs a host address from Customer A's /24 subnet. Its default gateway points to CPE-A's LAN interface.

  • Click PC-A in the Logical workspace.
  • Select the Desktop tab.
  • Select IP Configuration.
  • Enter 192.168.10.10 in the IPv4 Address field.
  • Enter 255.255.255.0 in the Subnet Mask field.
  • Enter 192.168.10.1 in the Default Gateway field.

The three fields should now show PC-A's host address, Customer A's subnet mask, plus CPE-A's LAN address.

  • Return to the Desktop tab.
  • Select Command Prompt from the Desktop tools.

Before you run the test, ask yourself whether PC-A can reach its gateway while ISP-CORE remains unconfigured.

  • Test PC-A's local gateway by running this command:
ping 192.168.10.1

What does this ping test?

Ping uses ICMP traffic to test whether PC-A can exchange packets with CPE-A's LAN interface.

Successful replies prove the host settings, switch path, cable, plus router interface work together.

You should see successful reply lines from 192.168.10.1. You now have the first working customer LAN in your mini ISP network.

Not receiving gateway replies?

  • Compare PC-A's three IP Configuration values with the values above.
  • Check that PC-A connects through SW-A to CPE-A's GigabitEthernet0/0/0 interface.
  • Return to the CPE-A CLI to confirm the LAN interface remains enabled.

Help me diagnose the failed gateway ping.

  • Click the save icon in Packet Tracer's top toolbar to save the working mini-isp-ipv4.pkt lab.

Customer A Checkpoint

Your saved lab now shows four network notes. CPE-A holds 192.168.10.1/24 on its Customer A LAN interface.

CPE-A's transit interface holds 10.0.0.1/30. PC-A uses 192.168.10.10/24 with 192.168.10.1 as its gateway.

ISP-CORE, CPE-B, plus PC-B remain unaddressed. No static routes exist.

Customer A now has a working local gateway. Next up, you'll activate the provider transit links so CPE-A can reach ISP-CORE.

Activate the Provider Transit Links

Customer A can already reach its gateway. Its packets still stop at CPE-A because ISP-CORE has no active IPv4 transit interfaces.

In this step, you will activate both transit interfaces on Cisco Packet Tracer's provider router. You will then add a default route that sends Customer A's unknown destinations toward the provider.

In this step, get ready to:
  • Configure the two ISP-CORE transit interfaces.
  • Add a default static route on CPE-A.
  • Prove the first provider transit network is operational.
Configure the provider transit interfaces

The ISP-CORE router sits between two transit networks. Each interface gives the provider core an address on one point-to-point link.

  • Switch back to the Logical workspace in Cisco Packet Tracer.
  • Select the ISP-CORE router in the topology.
  • Select the CLI tab.
  • Configure the core's two transit interfaces by entering these commands:
enable
configure terminal
hostname ISP-CORE
interface GigabitEthernet0/0/0
ip address 10.0.0.2 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/0/1
ip address 10.0.0.5 255.255.255.252
no shutdown
end
show ip interface brief

What Does This Configuration Do?

  • The hostname ISP-CORE command gives the provider router its topology name.
  • GigabitEthernet0/0/0 receives 10.0.0.2 on the transit link toward CPE-A.
  • GigabitEthernet0/0/1 receives 10.0.0.5 on the transit link toward CPE-B.
  • The no shutdown commands enable both provider interfaces.
  • The show ip interface brief command displays each interface address and its current state.

You should see 10.0.0.2 beside GigabitEthernet0/0/0. The CPE-B-facing interface should show 10.0.0.5 with an enabled state.

  • Select File from the Packet Tracer menu.
  • Select Save to update mini-isp-ipv4.pkt.

Do Not See the Transit Addresses?

  • Confirm that the selected router is ISP-CORE.
  • Check that the interface identifiers are GigabitEthernet0/0/0 and GigabitEthernet0/0/1.
  • Confirm that each interface received its own no shutdown command.

Help me diagnose the ISP-CORE interface configuration.

Add Customer A's default route

CPE-A currently knows only its directly connected networks. A default route sends every other destination toward ISP-CORE at 10.0.0.2.

  • Return to the CPE-A router in the Logical workspace.
  • Select the CLI tab.
  • Add Customer A's default route by entering these commands:
enable
configure terminal
ip route 0.0.0.0 0.0.0.0 10.0.0.2
end
show ip route

What Does This Route Do?

  • The ip route command creates a static path for destinations that do not match a more specific route.
  • The next hop 10.0.0.2 tells CPE-A to forward those packets to ISP-CORE.
  • The show ip route command displays the router's routing table for verification.

You should see a static default route pointing to 10.0.0.2. Good progress. CPE-A now knows where to send traffic for networks beyond its directly connected links.

  • Select File from the Packet Tracer menu.
  • Select Save to preserve the default route in mini-isp-ipv4.pkt.

Default Route Missing?

  • Confirm that you entered the route on CPE-A.
  • Check that the next-hop address is 10.0.0.2.
  • Run show ip route again after leaving configuration mode.

Help me find the missing CPE-A default route.

Verify the first transit network

A ping uses ICMP to test whether CPE-A can exchange packets with ISP-CORE. The routing table then confirms which paths CPE-A can use.

Before you run the checks, decide whether CPE-A can now reach 10.0.0.2.

  • Test the first transit link by entering these commands in CPE-A's CLI:
ping 10.0.0.2
show ip route

What Do These Checks Prove?

  • The ping 10.0.0.2 command proves that CPE-A can exchange packets with ISP-CORE across the first transit link.
  • The show ip route command confirms that 10.0.0.0/30 is directly connected.
  • The same route check confirms that unknown destinations use 10.0.0.2 as the next hop.

You should see replies from ISP-CORE at 10.0.0.2 followed by CPE-A's routes. That is the first provider handoff live and carrying traffic.

  • Select File from the Packet Tracer menu.
  • Select Save to store the working transit link in mini-isp-ipv4.pkt.

Ping Still Failing?

  • Confirm that CPE-A uses 10.0.0.1 255.255.255.252 on GigabitEthernet0/0/1.
  • Confirm that ISP-CORE uses 10.0.0.2 255.255.255.252 on GigabitEthernet0/0/0.
  • Check that both transit interfaces received the no shutdown command.

Help me isolate the failed CPE-A to ISP-CORE ping.

Your saved lab should now match this provider transit checkpoint.

✔️ Awesome, I've got everything

Your mini-isp-ipv4.pkt file now contains the active provider transit interfaces and Customer A's default route.

ⓧ I'd like to double check the full code

  • Confirm that the four topology notes still label Customer A LAN 192.168.10.0/24, CPE-A-to-ISP-CORE transit 10.0.0.0/30, ISP-CORE-to-CPE-B transit 10.0.0.4/30, and Customer B LAN 192.168.20.0/24.
  • Confirm that CPE-A retains 192.168.10.1 255.255.255.0 on GigabitEthernet0/0/0 with the interface up.
  • Confirm that CPE-A retains 10.0.0.1 255.255.255.252 on GigabitEthernet0/0/1 with the interface up.
  • Confirm that CPE-A has a default static route using 10.0.0.2 as the next hop.
  • Confirm that PC-A retains 192.168.10.10 with mask 255.255.255.0 and gateway 192.168.10.1.
  • Confirm that ISP-CORE uses 10.0.0.2 255.255.255.252 on GigabitEthernet0/0/0 toward CPE-A.
  • Confirm that ISP-CORE uses 10.0.0.5 255.255.255.252 on GigabitEthernet0/0/1 toward CPE-B.
  • Confirm that CPE-A receives replies when it pings 10.0.0.2.
  • Confirm that CPE-B and PC-B still have no IPv4 addressing configured.
  • Confirm that ISP-CORE still has no static routes to either customer LAN.

Your first provider transit network is operational. Next, you will bring Customer B online and watch the incomplete provider routing table block end-to-end traffic.

Add Customer B and Expose the Missing Route

Your provider transit links are active in Cisco Packet Tracer. Customer A can already reach ISP-CORE.

Now you'll bring Customer B online. The full path test then reveals whether the provider's routing table can carry traffic between both customer LANs.

In this step, get ready to:
  • Configure CPE-B with its LAN address, transit address, and default route.
  • Bring PC-B online with a working default gateway.
  • Locate the end-to-end break with reachability and path tests.
Configure CPE-B and its default route

CPE-B connects the 192.168.20.0/24 customer LAN to the 10.0.0.4/30 provider transit network. Its default route sends nonlocal traffic toward ISP-CORE.

  • Select CPE-B in the Logical workspace.
  • Select the CLI tab.
  • Decline the initial configuration flow if Packet Tracer displays one.
  • Configure both interfaces and the default route by entering these commands:
enable
configure terminal
hostname CPE-B
interface GigabitEthernet0/0/0
ip address 192.168.20.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/0/1
ip address 10.0.0.6 255.255.255.252
no shutdown
exit
ip route 0.0.0.0 0.0.0.0 10.0.0.5
end
show ip interface brief

What does this configuration do?

  • The first section names the router CPE-B.
  • The LAN interface uses 192.168.20.1 as PC-B's gateway.
  • The transit interface uses 10.0.0.6 on the link to ISP-CORE.
  • The default route points nonlocal traffic toward ISP-CORE at 10.0.0.5.
  • The final command displays a compact summary of both interfaces.
  • Inspect the interface summary printed at the end of the command sequence.

You should see both Gigabit Ethernet interfaces with their assigned IPv4 addresses. Both links should report active interface and protocol states.

Are the CPE-B interfaces inactive?

  • Compare each interface address and mask with the command block above.
  • Confirm that each interface received the activation command shown above.
  • Check that the transit cable connects CPE-B to ISP-CORE.

Ask for help with troubleshooting the CPE-B interface states.

✔️ Awesome, I've got everything!

CPE-B now has active LAN and transit interfaces. Its default route points toward ISP-CORE.

ⓧ I'd like to double check the full code

  • Compare your complete CPE-B configuration sequence with this reference:
enable
configure terminal
hostname CPE-B
interface GigabitEthernet0/0/0
ip address 192.168.20.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/0/1
ip address 10.0.0.6 255.255.255.252
no shutdown
exit
ip route 0.0.0.0 0.0.0.0 10.0.0.5
end
show ip interface brief

What should this configure?

  • The first section sets CPE-B's hostname.
  • The next two sections address its LAN and transit interfaces.
  • The final route sends nonlocal traffic toward ISP-CORE.
Configure PC-B and test its gateway

PC-B needs an IPv4 address from the Customer B LAN. Its gateway test checks the complete local path through SW-B to CPE-B.

  • Select PC-B in the Logical workspace.
  • Select the Desktop tab.
  • Select IP Configuration.
  • Enter 192.168.20.10 in the IPv4 Address field.
  • Enter 255.255.255.0 in the Subnet Mask field.

The address fields should now show PC-B inside the 192.168.20.0/24 customer LAN.

  • Enter 192.168.20.1 in the Default Gateway field.

The gateway field now points PC-B toward CPE-B for traffic beyond the local LAN.

  • Close the IP Configuration window.
  • Select Command Prompt from the Desktop tab.

Before you run the test, do you expect PC-B to reach an address on its own LAN?

  • Test PC-B's connection to CPE-B by running this command:
ping 192.168.20.1

What does this test prove?

This ping sends traffic from PC-B to its local gateway. A reply proves that PC-B's address, subnet mask, gateway, switch path, and CPE-B LAN interface work together.

You should see replies from CPE-B at 192.168.20.1. That local reply is a solid checkpoint: PC-B can now reach its router.

No replies from the gateway?

  • Confirm that PC-B's three address fields match the values above.
  • Confirm that the cable path from PC-B through SW-B reaches CPE-B.
  • Confirm that CPE-B's LAN interface reports active interface and protocol states.

Ask for help with diagnosing the PC-B gateway test.

Test the end-to-end path

The gateway reply covers the local Customer B path. The next tests follow traffic from PC-A across both customer routers and ISP-CORE.

  • Return to PC-A in the Logical workspace.
  • Select the Desktop tab.
  • Select Command Prompt.

Before you run the end-to-end test, do you think the packet will reach PC-B or stop somewhere in the provider path?

  • Test the full path from PC-A to PC-B by running this command:
ping 192.168.20.10

What does this test cover?

This ping asks every router in the path to forward traffic toward PC-B. It also requires a complete return path to PC-A.

You should see the test fail without replies from PC-B. This failure is intentional.

Why did the full path fail?

PC-B already reached CPE-B. That evidence clears Customer B's local LAN.

ISP-CORE currently knows only its directly connected transit networks. Its routing table has no route to either customer LAN.

Before you trace the path, where do you expect the final response to come from?

  • Trace the incomplete path from PC-A to PC-B by running this command:
tracert 192.168.20.10

What does the path trace show?

The trace reveals how far packets travel before responses stop. This separates a local host problem from a forwarding problem deeper in the topology.

You should see the trace stop before it reaches PC-B. The final responding hop shows how far the working path extends.

  • Record the final responding hop here: the last responding hop.
  • Leave ISP-CORE's routing table unchanged.
  • Save the updated lab as mini-isp-ipv4.pkt.

Your Step 4 Checkpoint

  • Customer A's addressing remains unchanged. Its default route still points to 10.0.0.2.
  • ISP-CORE's two transit interfaces remain active. Its routing table still lacks routes for 192.168.10.0/24 and 192.168.20.0/24.
  • CPE-B has active LAN and transit interfaces. Its default route points to 10.0.0.5.
  • PC-B uses 192.168.20.10/24 with gateway 192.168.20.1. It can reach that gateway.
  • PC-A cannot reach PC-B. The path trace stops because ISP-CORE has no customer-LAN routes.

Customer B works on its local LAN, while the end-to-end path remains incomplete. Next, you'll use ISP-CORE's routing table to repair the exact gap your tests exposed.

Teach the ISP Core Both Customer Networks

Your Customer B LAN now reaches its own gateway in Cisco Packet Tracer. The failed PC-A test shows that ISP-CORE still has no forwarding path to either customer LAN.

ISP-CORE needs explicit next-hop static routes for both remote LANs. You will add those paths before proving end-to-end connectivity.

In this step, get ready to:
  • Confirm that ISP-CORE has no routes to the customer LANs.
  • Add a next-hop static route for each customer network.
  • Prove end-to-end connectivity from PC-A.
Inspect the ISP-CORE routing table

A routing table tells a router where to forward packets for each destination network. ISP-CORE currently knows only the networks attached directly to its transit interfaces.

  • Click ISP-CORE in the Logical workspace.
  • Select the CLI tab.
  • Inspect the current routing table by entering this command:
show ip route

What Does This Table Prove?

  • The show ip route command displays the forwarding paths currently known by ISP-CORE.
  • The table contains paths for the two directly connected transit networks.
  • Entries for 192.168.10.0/24 and 192.168.20.0/24 are absent.
  • A static route appears with the code S after it enters the routing table.

ISP-CORE has no forwarding instruction for either customer LAN. That missing information explains why the earlier PC-A test stopped inside the routed path.

Seeing the Wrong Routing Table?

  • Check that the device window is labeled ISP-CORE.
  • Return to the Logical workspace if you selected a customer router.
  • Select ISP-CORE again.

Help me inspect the correct routing table.

Add both customer routes

Each route pairs a remote customer network with the customer router that can reach it. ISP-CORE can then send Customer A traffic toward CPE-A and Customer B traffic toward CPE-B.

  • Enter this configuration at the ISP-CORE prompt:
enable
configure terminal
ip route 192.168.10.0 255.255.255.0 10.0.0.1
ip route 192.168.20.0 255.255.255.0 10.0.0.6
end
show ip route static

What Do These Commands Do?

  • The first route sends traffic for 192.168.10.0 with mask 255.255.255.0 to CPE-A at 10.0.0.1.
  • The second route sends traffic for 192.168.20.0 with mask 255.255.255.0 to CPE-B at 10.0.0.6.
  • The show ip route static command displays the static routes installed on ISP-CORE.

You should see one static entry for each customer network. The routes should use 10.0.0.1 and 10.0.0.6 as their next hops.

Missing One of the Static Routes?

  • Compare the Customer A destination with 192.168.10.0 255.255.255.0.
  • Compare the Customer B destination with 192.168.20.0 255.255.255.0.
  • Check that Customer A uses next hop 10.0.0.1.
  • Check that Customer B uses next hop 10.0.0.6.

Help me correct the ISP-CORE static routes.

Prove end-to-end connectivity

The new routes give ISP-CORE a path toward each customer LAN. A successful test now proves that traffic can travel across the provider network in both directions.

  • Click PC-A in the Logical workspace.
  • Select the Desktop tab.
  • Select Command Prompt.

Before you test again, where do you expect the path to travel now that ISP-CORE knows both customer networks?

  • Test reachability and trace the repaired path by running these commands:
ping 192.168.20.10
tracert 192.168.20.10

What Should You See?

The ICMP test should receive replies from 192.168.20.10. The path trace should cross the customer and provider routers before reaching PC-B.

These results prove that ISP-CORE can forward traffic toward both customer networks. They also prove that the return path reaches PC-A.

The earlier failure has disappeared because ISP-CORE now knows how to reach both remote LANs.

Still Unable to Reach PC-B?

  • Return to ISP-CORE.
  • Confirm that the Customer A route uses next hop 10.0.0.1.
  • Confirm that the Customer B route uses next hop 10.0.0.6.
  • Check that the router links remain up in the Logical workspace.

Help me diagnose the remaining connectivity failure.

  • Close the PC-A device window after capturing your result.
  • Click File in Packet Tracer's top menu.
  • Click Save.

Packet Tracer should show mini-isp-ipv4.pkt without an unsaved-change indicator.

Strong finish: your saved mini-isp-ipv4.pkt lab now carries traffic between both customer LANs.

Secret mission

Create and Repair a Provider-Side Outage

Your mini ISP works from end to end. Now create a provider-side outage, isolate the failed interface from packet-path evidence, and restore service without changing the working address plan or routes.

Clean Up Your Resources

Clean Up Your Resources

Your repaired lab stays entirely on your Windows PC in Cisco Packet Tracer, so it creates no ongoing cloud costs. Choose whether to keep the session active, pause it for later, or delete the saved lab.

Resources you used:

  • The open Cisco Packet Tracer session containing your repaired mini ISP topology.
  • The saved mini-isp-ipv4.pkt lab on your Windows PC.

Keep everything running

Everything is already saved. Choose this option if you want to continue testing routes or creating outages in the working topology.

  • Leave Cisco Packet Tracer open with the repaired topology in the Logical workspace.
  • Keep mini-isp-ipv4.pkt in its current saved location.

Pause - I'll come back to this later

Pausing closes the simulator without changing the saved lab. The .pkt file keeps every address, route, label, and repaired interface state.

  • Click the window close control in the top-right corner of Cisco Packet Tracer.
  • Press the Windows key to open Windows search.
  • Type mini-isp-ipv4.pkt into the search field.

You should see mini-isp-ipv4.pkt in the search results. Your lab is safely paused for another session.

Delete - I don't want to use this again

Deleting the lab clears your working topology from this PC. Cisco Packet Tracer and your Cisco account stay untouched.

  • Click the window close control in the top-right corner of Cisco Packet Tracer.
  • Press the Windows key to open Windows search.
  • Type mini-isp-ipv4.pkt into the search field.

You should see the saved lab in the search results. This confirms that you have found the exact project file.

  • Right-click the matching search result to show its context menu.
  • Open the folder containing the matching file from the context menu.
  • Select mini-isp-ipv4.pkt in that folder.

The next shortcut permanently removes the selected file instead of sending it to the Recycle Bin. Windows may ask you to confirm the deletion.

  • Press Shift+Delete to remove the selected file.
  • Confirm the deletion if Windows displays a prompt.
  • Confirm that mini-isp-ipv4.pkt no longer appears in its folder.

That closes this lab cleanly. You have removed the only saved project artifact.

Nice Work!

Nice Work!

You did it! Your saved Cisco Packet Tracer lab now carries IPv4 traffic between two customer LANs through ISP-CORE.

You've learned how to:

  • Design an ISP-style topology that separates customer LANs from point-to-point transit networks with /24 and /30 prefixes.
  • Configure router interfaces plus static routes so traffic crosses the full CPE-A to ISP-CORE to CPE-B path.
  • Use routing tables and path tests to isolate a missing provider route. Prove the fix with successful ping and tracert results.
  • Secret Mission: Create a provider-side outage on ISP-CORE. Isolate the failed link from path evidence before restoring end-to-end service.

Ready to quiz yourself?