Beej's Guide to Network Concepts

In the last Packet Tracer project, we had a single router between two subnets. In this project, we’ll have multiple routers between subnets.

This complicates matters, because each router’s routing table needs to be edited so that it knows out which connection to forward packets.

In a larger LAN, a gateway protocol could be used to quickly distribute the information the routers needed.

But in our case, to keep it conceptually simple, we’ll just manually configure the routes in each of the three routers we’ll have.

28.1 What We’re Building

28.2 Drag Out the Components

But, and this is the fun bit, the middle router is connected to two routers and another LAN!

Put straight-through copper connections between the components as shown in the diagram, above.

28.3 Setting Up the Middle Router

That router in the middle that connects to two other routers and a LAN? It doesn’t come with enough ports by default. We need to add one.

Luckily this is a virtual simulation, so you’re allowed to simulate virtual payment for the new components and won’t have to open your real wallet.

The power switch is on the right side. Scroll over there and click it. (You can’t add components until you power it down.)

Two of the Ethernet connectors are in the upper left. Just right of those, there are two more ports that we can plug components into.

From the left sidebar, drag a “GLC-T” into one of those ports, as shown below:

28.4 Set up the Three LAN Subnets

By convention, routers often are the .1 on their subnet, e.g. 192.168.2.1. This isn’t a requirement.

Assign the 2 PCs and 1 of the routers IP addresses on the subnet. Connect them all to a switch.

Make sure the correct Ethernet port on the router has been set “On” in its config!

Sanity check: all computers on a subnet should be able to ping each other and their router.

28.5 Set the Default Gateway on All PCs

Remember that when PCs send out traffic, they either know they’re sending it on the LAN (because the destination is on the same LAN), or they don’t know where the IP is. If they don’t recognize the IP as being on the same subnet, they send the traffic to their default gateway, i.e. the router that knows what to do with it.

Click on each PC in turn. Under “Config” in sidebar “Global/Settings”, set a static “Default Gateway” of that LAN’s router IP.

For example, if I’m on PC 192.168.1.2 and my router on that LAN is 192.168.1.1, I’ll set the PC’s default gateway to 192.168.1.1.

In fact, I’ll set the default gateway for all the PCs on the LAN to that value.

28.6 Setting Up the Router Subnets

In order to route properly, we need one subnet between the left and middle router, and another between the middle and right.

This means the left and right routers will have TWO IP addresses because they’re attached to two subnets.

But the middle router will have THREE IP addresses because it’s attached to three subnets! (i.e. attached to one LAN, and attached to two other routers.

Connect the subnets with copper straight-through connectors if you haven’t done so already..

28.7 Setting Up the Routing Tables

We’re almost there, but if you’re on 192.168.0.2 and you try to ping 192.168.1.2, the traffic won’t get through.

This is because the router on subnet 192.168.0.0/24 doesn’t know where to send so that it arrives at 192.168.1.2.

We’re going to manually add “static routes” to each of the routers so that they know where to send things. As mentioned earlier, in real-life LANs, it would be more common to use gateway protocols to automatically get these routing tables set up.

But this is a lab–what would the fun be in that? (Actually it would be a very useful exercise, but this one is usefuller as an introduction.)

If a packet destined for 192.168.1.2 (on the right) leaves from 192.168.0.3 on the left, how does it get there?

We can see it must travel through all three routers. But when it arrives at the first one at 192.168.0.1 (the router for the LAN), where does that router send it?

Well, from there, we’ll head out on the 10.0.0.0/16 subnet to router 10.0.0.2.

So we have to add a route for the leftmost router saying, “Hey, if you get anything for subnet 192.168.0.0/24, forward it to 10.0.0.2 because that’s the next hop on the way.”

We do this by clicking on the leftmost router, then going to “Config”, and “Routing/Static” in the left sidebar.

The “Network” and “Mask” fields are the destination, and “Next Hop” is the router we should forward that traffic to.

In the case of the diagram, we’d add a route to the leftmost router like so (recalling that a \24 network has netmask 255.255.255.0):

Network:  192.168.1.0
Mask:     255.255.255.0
Next Hop: 10.0.0.2

And that gets us partway there! But, sadly and importantly, that middle router doesn’t know where to send traffic for 192.168.1.0/24, either.

So we have to add a route to that middle router that sends it on the next hop, but this time out its 10.1.0.0/16 interface:

Network:  192.168.1.0
Mask:     255.255.255.0
Next Hop: 10.1.0.2

And now we’re there! The router with IP 10.1.0.2 has an interface that is connected to 192.168.1.0/24. Our original packet is going to 192.168.1.2, and that’s on the same subnet! The router knows it can just send the traffic out on that interface.

Add routing table entries for all the non-directly-connected subnets to each router.

Each router should have two static routing table entries so that all inbound and outbound traffic is covered.

28.8 Test It Out!

If it’s all configured correctly, you should be able to ping any PC from any other PC! The routers forward the traffic to the other LANs!

28 Project: Packet Tracer: Multiple Routers