Content

| 1.2 |  | Key Characteristics of Various Switching Technologies |
| 1.2.1 |  | Overview |

The two primary factors that govern the success of a switched network are:

- Availability
- Performance

Other factors include:

- Reliability
- Scalability
- Cost

The range and variety of services now available on modern switches are extensive, can be overwhelming, and will continue to evolve over time since these services are driven by customer needs. Focus on the core concepts of these technologies instead of expecting to master each one. Many of these technologies will be replaced by completely independent solutions in the years to come.

Modern networks should support data, voice, and video. This means the network devices must support Quality of Service (QoS) and multicast technologies. QoS is needed to provide acceptable levels of service to the end user.

The network should also have a high-speed backbone. This means one or more switches need to support large capacities on the switch backplane and offer significant Gigabit Ethernet port densities.

The network should be scalable. This means having devices that can be upgraded without needing to replace them with completely new equipment. Each switch purchased for the backbone must have a chassis containing sufficient slots to reasonably support module upgrades over time that are appropriate for the network. A switch that supports high-speed switching at Layer 3 to Layer 7 is needed. This means that the switches should support technologies such as multilayer switching (MLS) or Cisco Express Forwarding (CEF).

Finally, the network should have redundancy. This means that the switches need to support such features as HSRP, IRDP, VTP, trunking, channeling, IGMP, CGMP, and spanning tree.

These features are just some of the considerations that need to be kept in mind when purchasing a switch. A variety of other features are also supported by modern switches.

The OSI model provides a convenient way to describe processes that take place in a networking environment. Each layer in the OSI model relates to a set of specific functions and this course focuses on Layers 2, 3, and 4 of the OSI model.

Protocols can normally be categorized into one or more of the OSI layers. Each protocol exchanges information called protocol data units (PDUs) between peer layers of network devices. Figure contains a table with the specific name for each PDU at each layer and the device type that traditionally processes the PDU.

The underlying layer services each protocol peer layer. For example, TCP segments are encapsulated in Layer 3 packets which are encapsulated in Layer 2 frames. A layer-specific device processes only those PDUs for which it is responsible by inspecting the PDU header.

Switches are available that can make decisions based on PDU header information at Layers 2, 3, and 4. Content service switches such as the Cisco CSS 11800 make switching decisions based on information up through Layer 7.