Creating a switched building design is not the same as creating a switched campus design. A building design can contain a few hundred or thousands of network nodes or components, whereas a campus design can comprise multiple buildings and will generally contain some kind of campus backbone that services the buildings. The important thing to analyze in these design types concerns the Layer 2 and Layer 3 switching technologies used at various network layers.
Figure 4.7 – Cisco Enterprise Composite Model
The Cisco enterprise composite model, shown above in Figure 4.7, consists of multiple modules. The Access Layer block combines and aggregates all of the different end-user devices and end-user connections that interact with other parts of the network. The Distribution Layer block acts as an aggregator for the network access devices and consists of multilayer switches connected to the campus backbone.
The campus backbone interconnects with the Distribution Layer block, and its main goal is to communicate with the edge distribution submodule in order to provide access to enterprise edge services (e.g., e-commerce, Internet connectivity, remote access, or WAN). From a geographical standpoint, the Core Layer block is located in the main building and the Distribution Layer block is located in every building connected to the campus backbone (i.e., main building).
Another component in the enterprise campus module is the server farm block, which either can be located within every building or can be centrally located in the main building and available for all the buildings in the campus. This block connects the servers to the enterprise network and handles server availability and traffic load balancing.
The edge distribution submodule connects the enterprise edge applications to the network campus. The main consideration of the edge distribution submodule is security. Once this submodule is penetrated, the internal network is available for the attacker.
The network management’s availability requirements are similar to those in the server farm block but it does not need as much bandwidth. Network control, verification, troubleshooting, and monitoring logic reside in this submodule.
The paragraphs to follow will analyze some of the requirements of these different components as they apply to the enterprise campus design and Layer 2 and Layer 3 switching.
The Access Layer block connects to the Distribution Layer block (one for every building). The closer the Distribution Layer block is located to the Access Layer block, the more scalability the network must build in. Scalability should not be an issue at the Core Layer block or the enterprise edge module because hardware purchased for those levels will function for many years without needing major upgrades. However, scalability in the Access Layer block is important from a switching standpoint, especially due to the possible growth of the company (i.e., adding users, customers, or partners). When this happens, you can easily accommodate the new requirements by adding Layer 3 switches at the Distribution Layer.
In the Access Layer block, it is possible to continue to use shared technology if some of the users already use it and the budget does not allow for full switching implementation. Nevertheless, the recommendation is to have a full switching environment for all of the Access Layer areas, which will require high scalability in order to be able to add employees and to help the business expand. From an availability and performance standpoint, at the Access Layer, you should not be concerned about redundancy or high performance per port. The Access Layer devices’ performance is the lowest within the entire network infrastructure, but performance, availability, and cost requirements will grow as you get closer to the backbone area. In addition, the Distribution Layer block can integrate Layer 2 or Layer 3 switching and has medium needs for scalability, high availability, better performance, and cost per port.
One of the biggest Layer 2 design mistakes within the Access and Distribution Layers is to forget about the importance of STP and the mapping between VLANS and STP instances. Another big mistake is cutting costs in the Distribution Layer area by not purchasing modular devices. This will cause scalability problems when the number of end-users increases because the entire distribution design will have to be reconsidered.
The backbone connects all the buildings in the campus and is comprised of a combination of Layer 2 and Layer 3 switching. As mentioned, the Core Layer’s scalability factor is low because backbone hardware should not have to be replaced for a long time. On the other hand, the network core should have a high degree of availability and performance because it integrates all the other network modules and all the network traffic passes through the backbone devices. A failure in the backbone area means a disrupted service at another network layer, whereas a failure in the Access or Distribution Layers does not affect Core Layer functionality.
Another feature of the Core Layer block is the high cost per port. The connections to the Distribution Layer block and the edge distribution submodule often uses MM fiber. In the server farm block, you will generally have Layer 3 switching with the SAN using fiber technology. The server farm block should operate on Layer 3 switching technology and should have medium scalability, but high availability and performance.
The edge distribution submodule uses Layer 3 switching and fiber to connect to the backbone and to the Distribution Layer block. As for the backbone, the edge distribution submodule does not have to be very scalable. The availability, performance, and costs are comparable to the ones in the server farm block. As mentioned earlier, you must pay extra attention to the security features implemented in the edge distribution submodule (e.g., firewalls, IDSs, etc.).
The importance of different features per network components, when considering the switching infrastructure, is summarized below:
|
Components |
Scalability |
Availability |
Performance
|
Cost |
| Access Layer |
HIGH |
LOW |
LOW |
LOW |
| Distribution Layer |
HIGH |
MEDIUM |
MEDIUM |
MEDIUM |
| Backbone |
LOW |
HIGH |
HIGH |
HIGH |
| Server Farm |
MEDIUM |
HIGH |
HIGH |
HIGH |
| Edge Distribution |
LOW |
HIGH |
HIGH |
HIGH |
Analyzing Application Traffic
One of the first enterprise campus design issues refers to analyzing the application traffic as it relates to the switched network design. The traffic patterns usually fall into one of the following scenarios:
- Local within a segment/module/submodule
- Distance (remote) traffic patterns; this implies traversing different segments or crossing submodules or modules in the campus design
Networks were originally designed according to the 80/20 rule, which states that 80% of the traffic is internal and 20% is remote. This concept has changed with the evolution of enterprise networking and distributed server networking; in modern campus networks, the ratio is now 20/80, whereas 20% is for local traffic and 80% is for traffic that crosses between modules and segments. This change occurred as a result of servers no longer sitting in the workgroup areas.
Generally, the application and backbone servers are placed in a server farm area. This puts a much higher load on the backbone because much of the traffic from the client side is going to the servers in the server farm through Core Layer devices. This changes the way you will analyze application traffic.
In order to exemplify the 80/20 rule, consider a workgroup area with various devices (e.g., workstations, laptops, printers, etc.) connected to a basic Layer 2 switch using VLANs. The inter-VLAN routing is accomplished on the routers that also allow access to an e-mail server. According to the 80/20 rule, 80% of the traffic stays within the VLAN, while 20% of the traffic is going to the e-mail server, as illustrated in Figure 4.8 below:
Figure 4.8 – Example of the 80/20 Rule
On the other hand, for an example of the modern 20/80 rule, consider a situation in which there are multiple logical departments using common resources, with applications distributed throughout the organization. This means there are no dedicated servers located within the department, for example, database servers or file servers. All the data is stored in the server farm block. The end-user devices connect to a series of Layer 2 or low-end Layer 3 switches before reaching the Distribution Layer block, where the high-end Layer 3 switches with high availability are located. The data flow finally reaches the server farm block consisting of modern database servers (e.g., e-mail, applications, collaborations, or databases). In this example, the traffic distribution reflects the 20/80 rule, meaning 20% of the network traffic stays local, while 80% of the traffic is moving across the Distribution Layer and backbone of the network, as illustrated below in Figure 4.9. This is the reason you want your Distribution Layer and Core Layer links to be very highly available and fast, in order to move data across the entire enterprise.
Figure 4.9 – Example of the 20/80 Rule
The diagrams presented above represent a single building, but in a large campus enterprise, you will have multiple buildings connected by the network backbone module. The network backbone is connected to the edge distribution submodule in order to provide external access from the network (e.g., WAN, remote access, e-commerce, or VPN).
Analyzing Multicast Traffic
With the incredible advances of collaboration tools using the World Wide Web and the Internet, it is very likely that the organization will have to support multicast traffic.
The process of multicasting, opposed to the process of broadcasting or unicasting, has the advantage of saving bandwidth because it sends a single stream of data to multiple nodes. The multicasting concept is used by every modern corporation around the world to deliver data to groups via the following methods:
- Corporate meetings
- Video conferencing
- E-learning solutions
- Webcasting information
- Distributing applications
- Streaming news feeds
- Streaming stock quotes
The multicast data is sent to a multicast group, and users receive the information by joining the specific multicast group using Internet Group Management Protocol (IGMP). Cisco multicast-enabled routers can be used, running multicast routing protocols such as Protocol Independent Multicast (PIM), to forward the incoming multicast stream to a particular switch port.
Cisco switches effectively implement multicasting using two main protocols: Cisco Group Management Protocol (CGMP) and IGMP snooping. CGMP allows switches to communicate with multicast-enabled routers to figure out whether any users attached to the switches are part of any particular multicasting groups and whether they are qualified to receive the specific stream of data. IGMP snooping allows the switch to intercept the multicast receiver registration message, and based on the gathered information it makes changes to its forwarding table. IGMP snooping works only on Layer 3-aware switches because IGMP is a Layer 3 protocol.
Understanding whether the organization will use multicast traffic and the way this will be accomplished is another important issue when designing the campus switching infrastructure.
Analyzing Delay-Sensitive Traffic
If using multicasting or web streaming, e-commerce, e-learning solutions, or IP Telephony, the traffic involved in this process will be delay-sensitive and QoS techniques might be necessary to ensure that this type of traffic is treated with priority.
In Layer 3 applications such as Frame Relay environments using EIGRP, OSPF, or BGP as the routing protocols with the ISP, it is very common to use QoS techniques to shape and control traffic at the IP layer. You can also use QoS at Layer 2. When using QoS or analyzing or controlling delay-sensitive traffic at Layer 2, there are four categories of QoS techniques, as follows:
- Tagging and traffic classification
- Congestion control
- Policy and traffic shaping
- Scheduling
Figure 4.10 – Layer 2 QoS Techniques
By examining Figure 4.10 above, you can see that tagging and traffic classification can happen between the end-user nodes, through the Access Layer and up to the Distribution Layer. This is where packets are classified, grouped, and partitioned based on different priority levels or classes of service. This occurs by inspecting the Layer 2 packet headers and determining the priority of the traffic based on the type of traffic (e.g., IP Telephony, Telnet traffic, printer traffic, file services, or other traffic). In this way, the traffic can be tagged and classified and the Layer 2 frame can be changed depending on the priority.
Note: Tagging and traffic classification is also called traffic marking.
The next three techniques – congestion control, policy and traffic shaping, and scheduling – occur in the Distribution Layer block and the edge distribution submodule, primarily on Layer 3 switches. Avoid applying any QoS techniques at the Core Layer because you want as little overhead as possible on the backbone devices so they can successfully achieve their goals, which are fast connectivity, high availability, and reliability.
Congestion control involves the interfaces of the Access Layer switches and the queuing mechanisms configured on them. Queuing techniques are used in order to deal with the congestion of packets coming into and going out of the switch ports. This method ensures that traffic from critical applications will be forwarded properly, especially when using real-time applications (e.g., VoIP), while reducing the amount of jitter and latency as much as possible so the specific application will function in an optimal way, with as little delay as possible.
Policing and shaping help move important traffic to the top of the list and dynamically adjust the priorities of certain types of traffic during periods of congestion. Scheduling is the process of establishing the order in which the congested queues will be served.
QoS techniques can be used to allow Layer 2 switches to assist in the queuing process by scheduling certain processes. Much of the QoS activity happens at Layer 3, but many features can also be taken into consideration at Layer 2, and they can be implemented on higher-end switches to provide support for tagging, traffic classification, congestion control, policing and shaping traffic, and traffic scheduling.