* There are standard 'best practices' for network design. * However there are a few universal 'correct answers'. * The answer to most general questions about network design is 'it depends'. * However, to understand the networks you will be configuring and troubleshooting, it's important to know some basics of network design. ## Common Terminologies ### Star Topology  * When several devices all connect to one central device, we can draw them in a 'star' like shape. Therefore, this is often called a 'star topology'. * Note that in network diagrams the devices might not be drawn in the shape of a star. But if many devices are connected to one central device, we can call it a star topology, regardless of how the diagram is drawn. ### Full Mesh  * A **Full Mesh** topology is formed when each device is connected to each other device. ### Partial Mesh  * A **Partial Mesh** topology is formed when some devices are connected to each other, but not all. ## 2-Tier LAN Architecture * The two-tier LAN design consists of two hierarchical layers: * **Access Layer** * **Distribution Layer** * Also called a 'Collapsed Core' design because it omits a layer that is found in the Three-Tier design: the **Core Layer** ### Access Layer * The layer that end hosts connect to (PCs, printers, cameras, etc.). * Typically Access Layer Switches have lots of ports for end hosts to connect to. * QoS marking is typically done here because it's a good practice to mark traffic as early as possible in the network. * Security services like port security, DAI (Dynamic ARP Inspection), etc are typically performed here. * Switchports might be PoE-enabled for wireless Access Points, IP phones, etc. ### Distribution Layer (Aggregation Layer) * Aggregates connections from the Access Layer Switches. * Connections from Access Layer switches are usually aggregated to a redundant pair of Distribution Layer switches. * Typically is the border between Layer 2 and Layer 3 in the network. * Runs Layer 2 and Layer 3 protocols since it is concerned with both Layers. * Connects to services such as Internet, WAN, other parts of the LAN, etc. * This is not always the case, but usually the connections from the Access Layer switches to the Distribution Layer switches are Layer 2 connections, and the end hosts use the SVIs on the Distribution Layer switches as their default gateways. ### 2-Tier Campus Lan Design  * The connections between distribution switches and access layer switches are Layer 2 and loops can occur. Therefore, STP disables a few link to prevent loops. * D1 and D2 are multilayer switches, and end hosts use the SVIs on them as their default gateways. They will probably use a FHRP such as HSRP or VRRP to provide a redundant virtual IP address for hosts to use as their default gateway. * The Distribution Layer is used to connect to services such as the internet or the WAN, as well as other parts of the LAN. * Notice that each Distribution Layer switch is connected to each other Distribution Layer switch. * Distribution layer switches form a full mesh.  * In large LAN networks with many Distribution Layer switches (for example, in separate buildings), the number of connections required between Distribution Layer switches grows rapidly * This makes it much more difficult and complicated to scale the network, to make it bigger.  * To help scale large LAN networks, you can add a Core Layer. * Cisco recommends adding a Core Layer if there are more than 3 Distribution Layers in a single location. * Each Distribution Layer connects to the Core Layer so there is no need for a full mesh of direct connections between the Distribution Layer switches. * When we add a Core Layer, we obtain a three-tier LAN architecture. ## 3-Tier LAN Architecture * The three-tier LAN design consists of three hierarchical layers: * **Access Layer** * **Distribution Layer** * **Core Layer** ### Core Layer * Connects Distribution Layers together in large LAN networks. * The focus is speed ('fast transport'). Powerful switches focused on speed. * CPU-intensive operations such as security, QoS marking/classifications, etc. should be avoided at this Layer. * Connections are all Layer 3. No spanning-tree! * Should maintain connectivity throughout the LAN even if devices fail. * Because it's the backbone of the LAN, redundancy of devices and connections is very important. ### 3-Tier Campus Lan Design  * The Core switches connect to the internet routers. ## Spine-Leaf Architecture (Data Center) * Data centers are dedicated spaces/buildings used to store computer systems such as servers and network devices. * Traditional data center designs used a three-tier architecture (Access-Distribution-Core). * This worked well when most traffic in the data center was North-South. * With the precedence of virtual servers, applications are often deployed in a distributed manner (across multiple physical servers), which increases the amount of East-West traffic in the data center. * The traditional three-tier architecture led to bottlenecks in bandwidth as well as variability in the server-to-server latency depending on the path the traffic takes. * To solve this, Spine-Leaf architecture (also called Clos architecture) has become prominent in data centers.  * North-south represents traffic that goes to other parts of the LAN or out to the internet. * East-west represents traffic between devices in the same part of the network. The traffic doesn't go out to other parts of the LAN or out to the internet.  * It's a two-tier architecture, but different than the traditional two-tier design we looked at before. * There are spine switches and leaf switches. * There are some rules about Spine-Leaf architecture: * Every Leaf switch is connected to every Spine switch. * Every Spine switch is connected to every Leaf switch. * Leaf switches do not connect to other Leaf switches. * Spine switches do not connect to other Spine switches. * End hosts (servers, etc.) only connect to Leaf switches. * The path taken by traffic is randomly chosen to balance the traffic load among the Spine switches. * Each server is separated by the same number of 'hops' (except those connected to the same Leaf), providing consistent latency for East-West traffic. * It's very simple to scale because if you need to add more servers you can just add another Leaf switch and connect it to the existing Spine switches. ## SOHO (Small Office/Home Office) * Small Office/Home Office (SOHO) refers to the office of a small company, or a small home office with few devices. * It doesn't have to be an actual home office. If your home has a network connected to the internet, it is considered a SOHO network. * SOHO networks don't have complex needs, so all networking functions are typically provided by a single device, often called a 'home router' or 'wireless router'. * This one device can serve as a: * Router * Switch * Firewall * Wireless Access Point * Modem  * Very small companies or home offices don't have the need or probably the resources to hire a dedicated network engineer. * They often just rent a wireless router from the ISP and that serves their purposes.