CCNA Routing and Switching Complete Study Guide. Todd LammleЧитать онлайн книгу.
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 Pleistocene these days. But in reality, most of us just don’t have an unlimited budget to buy all new gigabit switches; however, 10/100 switches are just not good enough in today’s networks.
Another good question: Do you really need low-latency 1 Gbps or better switch ports for all your users, servers, and other devices? Yes, you absolutely need new higher-end switches! This is because servers and hosts are no longer the bottlenecks of our internetworks, our routers and switches are – especially legacy ones. We now need gigabit on the desktop and on every router interface; 10 Gbps is now the minimum between switch uplinks, so go to 40 or even 100 Gbps as uplinks if you can afford it.
Go ahead. Put in that requisition for all new switches. You’ll be a hero before long!
Okay, so now that you’ve gotten a pretty thorough introduction to internetworking and the various devices that populate an internetwork, it’s time to head into exploring the internetworking models.
Internetworking Models
First a little history: When networks first came into being, computers could typically communicate only with computers from the same manufacturer. For example, companies ran either a complete DECnet solution or an IBM solution, never both together. In the late 1970s, the Open Systems Interconnection (OSI) reference model was created by the International Organization for Standardization (ISO) to break through this barrier.
The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work in peaceable accord with each other. Like world peace, it’ll probably never happen completely, but it’s still a great goal!
Anyway the OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers.
Coming up, I’ll explain the layered approach to you plus how we can use it to help us troubleshoot our internetworks.