network together? I’m stuck with using this layout because of the ancient bridges and hubs involved. I really hope you don’t run across a network like this, but it’s still really important to understand the strategic ideas that this figure represents!
See that bridge up at the top of our internetwork shown in Figure 1.4? It’s there to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. That bridge also created only three collision domains, one for each port, which means that each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is really lame and to be avoided if possible, but it’s still better than having one collision domain for all hosts! So don’t do this at home; it’s a great museum piece and a wonderful example of what not to do, but this inefficient design would be terrible for use in today’s networks! It does show us how far we’ve come though, and again, the foundational concepts it illustrates are really important for you to get.
And I want you to notice something else: The three interconnected hubs at the bottom of the figure also connect to the router. This setup creates one collision domain and one broadcast domain and makes that bridged network, with its two collision domains, look majorly better by contrast!
The best network connected to the router is the LAN switched network on the left. Why? Because each port on that switch breaks up collision domains. But it’s not all good – all devices are still in the same broadcast domain. Do you remember why this can be really bad? Because all devices must listen to all broadcasts transmitted, that’s why! And if your broadcast domains are too large, the users have less bandwidth and are required to process more broadcasts. Network response time eventually will slow to a level that could cause riots and strikes, so it’s important to keep your broadcast domains small in the vast majority of networks today.
Once there are only switches in our example network, things really change a lot! Figure 1.5 demonstrates a network you’ll typically stumble upon today.
FIGURE 1.5 Switched networks creating an internetwork
Here I’ve placed the LAN switches at the center of this network world, with the router connecting the logical networks. If I went ahead and implemented this design, I’ll have created something called virtual LANs, or VLANs, which are used when you logically break up broadcast domains in a layer 2, switched network. It’s really important to understand that even in a switched network environment, you still need a router to provide communication between VLANs. Don’t forget that!
Still, clearly the best network design is the one that’s perfectly configured to meet the business requirements of the specific company or client it serves, and it’s usually one in which LAN switches exist in harmony with routers strategically placed in the network. It’s my hope that this book will help you understand the basics of routers and switches so you can make solid, informed decisions on a case-by-case basis and be able to achieve that goal! But I digress…
So let’s go back to Figure 1.4 now for a minute and really scrutinize it because I want to ask you this question: How many collision domains and broadcast domains are really there in this internetwork? I hope you answered nine collision domains and three broadcast domains! The broadcast domains are definitely the easiest to spot because only routers break up broadcast domains by default, and since there are three interface connections, that gives you three broadcast domains. But do you see the nine collision domains? Just in case that’s a no, I’ll explain. The all-hub network at the bottom is one collision domain; the bridge network on top equals three collision domains. Add in the switch network of five collision domains – one for each switch port – and you get a total of nine!
While we’re at this, in Figure 1.5, each port on the switch is a separate collision domain, and each VLAN would be a separate broadcast domain. So how many collision domains do you see here? I’m counting 12 – remember that connections between the switches are considered a collision domain! Since the figure doesn’t show any VLAN information, we can assume the default of one broadcast domain is in place.
Before we move on to Internetworking Models, let’s take a look at a few more network devices that we’ll find in pretty much every network today as shown in Figure 1.6.
FIGURE 1.6 Other devices typically found in our internetworks today.
Taking off from the switched network in Figure 1.5, you’ll find WLAN devices, including AP’s and wireless controllers, and firewalls. You’d be hard pressed not to find these devices in your networks today.
Let’s look closer at these devices:
■ WLAN devices: These devices connect wireless devices such as computers, printers, and tablets to the network. Since pretty much every device manufactured today has a wireless NIC, you just need to configure a basic access point (AP) to connect to a traditional wired network.
■ Access Points or APs: These devices allow wireless devices to connect to a wired network and extend a collision domain from a switch, and are typically in their own broadcast domain or what we’ll refer to as a Virtual LAN (VLAN). An AP can be a simple standalone device, but today they are usually managed by wireless controllers either in house or through the internet.
■ WLAN Controllers: These are the devices that network administrators or network operations centers use to manage access points in medium to large to extremely large quantities. The WLAN controller automatically handles the configuration of wireless access points and was typically used only in larger enterprise systems. However, with Cisco’s acquisition of Meraki systems, you can easily manage a small to medium sized wireless network via the cloud using their simple to configure web controller system.
■ Firewalls: These devices are network security systems that monitor and control the incoming and outgoing network traffic based on predetermined security rules, and is usually an Intrusion Protection System (IPS). Cisco Adaptive Security Appliance (ASA) firewall typically establishes a barrier between a trusted, secure internal network and the Internet, which is not secure or trusted. Cisco’s new acquisition of Sourcefire put them in the top of the market with Next Generation Firewalls (NGFW) and Next Generation IPS (NGIPS), which Cisco now just calls Firepower. Cisco new Firepower runs on dedicated appliances, Cisco’s ASA’s, ISR routers and even on Meraki products.
Should I Replace My Existing 10/100 Mbps Switches?
Let’s say you’re a network administrator at a large company. The boss comes to you and says that he got your requisition to buy a bunch of new switches but he’s really freaking out about the price tag! Should you push it – do you really need to go this far?
Absolutely! Make your case and go for it because the newest switches add really huge capacity to a network that older 10/100 Mbps switches just can’t touch. And yes, five-year-old switches are considered pretty
