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  1. Switching Introduction
  2. Summary
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This chapter from Implementing Cisco IP Switched Networks (SWITCH) Foundation Learning Guide: (CCNP SWITCH 300-115) covers the following basic switching topics as a review to CCNA: hubs and switches, bridges and switches, switches of today, broadcast domains, MAC addresses, the basic Ethernet frame format, basic switching function, VLANs, the Spanning Tree Protocol, trunking, port channels, and multilayer switching (MLS).
This chapter is from the book

Before journeying into Cisco campus networks and detail technology readouts to prepare for CCNP: Switch, this chapter quickly reviews several topics covered in CCNA and briefly introduces a few topics to ease comprehension of this book. Because each technology covered, such as spanning tree or virtual LANs (VLANs), can exist by itself, the short technology highlights in the chapter reduce cross-referencing of chapters.

If you have a very good understanding of switching terminology and a basic understanding of switching technology, you may want to skip this chapter and begin with Chapter 2, “Network Design Fundamentals.”

This chapter covers the following basic switching topics as a review to CCNA and serves as a teaser for topics covered later in chapter:

  • Hubs and switches
  • Bridges and switches
  • Switches of today
  • Broadcast domains
  • MAC addresses
  • The basic Ethernet frame format
  • Basic switching function
  • VLANs
  • The Spanning Tree Protocol
  • Trunking
  • Port channels
  • Multilayer switching (MLS)

Switching Introduction

The term LAN switching is becoming legacy. LAN switching was a popular term to describe LANs built on Cisco Catalyst switches in the 1990s to mid-2000s. In today’s networks, LANs have been segmented into distinct functional areas: data centers and campus networks.

This book focuses on campus networks. Campus networks generally take a more conservative approach to architectures, using Cisco Catalyst switches and leveraging traditional Layer 2 and Layer 3 hierarchical designs. Data centers are in a state of evolution, with the focus on applications, dev/ops, and software programmability. These architectures use bleeding-edge technologies such as FabricPath, Dynamic Fabric Allocation (DFA), Application Centric Infrastructure (ACI), and so on.

The remainder of this chapter focuses on a couple of key switching concepts in relation to campus networks that are found throughout this text. Many of these concepts are discussed in more detail in later chapters, but a quick review and definition will help you understand the following chapters. Moreover, because all campus network features are heavily intertwined, it is difficult to present topics in a serial fashion. Definitions in this chapter will ease reading in that manner as well.

Hubs and Switches

Hubs are archaic, and the terminology should be avoided. Even the simplest multiport Ethernet devices for the home are switches.

In review, hubs died off as a product because they are shared-bandwidth devices. Switches introduced dedicated bandwidth. A hub allows multiple devices to be connected to the same network segment. The devices on that segment share the bandwidth with each other. As an example with a 100-Mbps hub, and there are six devices connected to six different ports on the hub, all six devices share the 100 Mbps of bandwidth with each other. A 100-Mbps hub shares 100 Mbps of bandwidth among the connected devices. In terms of the OSI reference model, a hub is considered a Layer 1 (physical layer) device. It hears an electrical signal on the wire and passes it along to the other ports.

A switch allows multiple devices to be connected to the same network, just like a hub does, but this is where the similarity ends. A switch allows each connected device to have dedicated bandwidth instead of shared bandwidth. The bandwidth between the switch and the device is reserved for communication to and from that device alone. Six devices connected to six different ports on a 1-Gbps switch each have 1 Gbps of bandwidth to work with, instead of shared bandwidth with the other devices. A switch can greatly increase the available bandwidth in your network, which can lead to improved network performance. Switches also support additional capabilities beyond what hubs support. Later sub-sections describe some of these features.

Bridges and Switches

A basic switch is considered a Layer 2 device. When we use the word layer, we are referring to the seven-layer OSI reference model. A switch does not just pass electrical signals along, like a hub does; instead, it assembles the signals into a frame (Layer 2), and then decides what to do with the frame. A switch determines what to do with a frame by borrowing an algorithm from a previously common networking device: a transparent bridge. Logically, a switch acts just like a transparent bridge would, but it can handle frames much faster than a transparent bridge could (because of special hardware and architecture). Once a switch decides where the frame should be sent, it passes the frame out the appropriate port (or ports). You can think of a switch as a device creating instantaneous connections between various ports, on a frame-by-frame basis.

Switches of Today

Today’s switches have evolved beyond just switching frames. Most modern switches can actually route traffic. In addition, switches can prioritize traffic, support no downtime through redundancy, and provide convergence services around IP telephony and wireless networks.

In summary, to meet evolving network needs of today, Cisco Catalyst switch designs include support for the following industry-leading features beyond the legacy features found in all switches:

  • Application intelligence: This helps networks recognize many types of applications and secure and prioritize those applications to provide the best user experience.
  • Unified network services: Combining the best elements of wireless and wired networking allows you to consistently connect to any resource or person with any device. 10 Gigabit Ethernet technology and Power over Ethernet (PoE) technology support new applications and devices.
  • Nonstop communications: Features such as redundant hardware, and nonstop forwarding and stateful switchover (NSF/SSO) technology support more-reliable connections.
  • Integrated security: LAN switches provide the first line of defense against internal network attacks and prevent unauthorized intrusion.
  • Operational manageability: To more easily manage the network, IT staff must be able to remotely configure and monitor network devices from a central location.

Broadcast Domains

In a review from CCNA material, a broadcast domain is a set of network devices that receive broadcast frames originating from any device within the group. Routers typically bound broadcast domains because routers do not forward broadcast frames. VLANs are an example of broadcast domain. Broadcast domains are generally limited to a specific Layer 2 segment that contains a single IP subnet. The next section discusses the addresses used within broadcast domains.

MAC Addresses

MAC addresses are standardized data link layer addresses that are required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures. MAC addresses are 6 bytes long and are controlled by the IEEE. MAC addresses are also known as a hardware address, MAC layer address, and physical address.

A MAC address is also applied to virtual devices. Virtual machines on a server may all contain individual MAC addresses. Moreover, most devices have more than one MAC address. A simple example is your laptop; it has both a LAN MAC address and a wireless MAC address. The next section covers the basic frame structure used in Ethernet.

The Basic Ethernet Frame Format

The IEEE 802.3 standard defines a basic data frame format that is required for all MAC implementations, plus several additional optional formats that are used to extend the protocol’s basic capability. The basic data frame format contains the following seven fields, as shown in Figure 1-1.

Figure 1-1

Figure 1-1 The Basic IEEE 802.3 MAC Data Frame Format

  • Preamble (PRE): Consists of 7 bytes. The PRE is an alternating pattern of 1s and 0s that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.
  • Start-of-frame delimiter (SOF): Consists of 1 byte. The SOF is an alternating pattern of 1s and 0s, ending with two consecutive 1 bits, indicating that the next bit is the leftmost bit in the leftmost byte of the destination address.
  • Destination address (DA): Consists of 6 bytes. The DA field identifies which station(s) should receive the frame. In the first byte of the DA, the 2 least significant bits are used to indicate whether the destination is an individual address or group address (that is, multicast). The first of these 2 bits indicates whether the address is an individual address (indicated by a 0) or a group address (indicated by a 1). The second bit indicates whether the DA is globally administered (indicated by a 0) or locally administered (indicated by a 1). The remaining bits are a uniquely assigned value that identifies a single station, a defined group of stations, or all stations on the network.
  • Source addresses (SA): Consists of 6 bytes. The SA field identifies the sending station. The SA is always an individual address, and the leftmost bit in the SA field is always 0.

    Note that jumbo frames up to 9000 bytes are supported on the current-generation Cisco Catalyst switches.


Basic Switching Function

When a switch receives a frame, it must decide what to do with that frame. It could ignore the frame, it could pass the frame out one other port, or it could pass the frame out many other ports.

To know what to do with the frame, the switch learns the location of all devices on the segment. This location information is placed in a content addressable memory table (CAM, named for the type of memory used to store these tables). The CAM table shows, for each device, the MAC address of the device, out which port that MAC address can be found, and with which VLAN this port is associated. The switch continually performs this learning process as frames are received into the switch. The CAM table of the switch is continually updated. The next chapter discusses the CAM table in more detail.

This information in the CAM table is used to decide how a received frame is handled. To decide where to send a frame, the switch looks at the destination MAC address in a received frame and looks up that destination MAC address in the CAM table. The CAM table shows the port that the frame must be sent out for that frame to reach the specified destination MAC address. In brief, the basic switching function at Layer 2 adheres to these rules for determining forwarding responsibility:

  • If the destination MAC address is found in the CAM table, the switch sends the frame out the port that is associated with that destination MAC address in the CAM table. This process is called forwarding.
  • If the associated port to send the frame out is the same port that the frame originally came in on, there is no need to send the frame back out that same port, and the frame is ignored. This process is called filtering.
  • If the destination MAC address is not in the CAM table (that is, unknown unicast), the switch sends the frame out all other ports that are in the same VLAN as the received frame. This is called flooding. It does not flood the frame out the same port on which the frame was received.
  • If the destination MAC address of the received frame is the broadcast address (FFFF.FFFF.FFFF), the frame is sent out all ports that are in the same VLAN as the received frame. This is also called flooding. The only exception is the frame is not sent out the same port on which the frame was received.

The next section introduces a widely popular feature leveraged by Cisco Catalyst switches and Nexus switches to segment groups of ports into their own LAN segments.


Because the switch decides on a frame-by-frame basis which ports exchange data, it is a natural extension to put logic inside the switch to allow it to choose ports for special groupings. This grouping of ports is called a virtual local-area network (VLAN). The switch makes sure that traffic from one group of ports never gets sent to other groups of ports (which would be routing). These port groups (VLANs) can each be considered an individual LAN segment.

VLANs are also described as broadcast domains. This is because of the transparent bridging algorithm, which says that broadcast packets (packets destined for the all devices address) be sent out all ports that are in the same group (that is, in the same VLAN). All ports that are in the same VLAN are also in the same broadcast domain.

The next section introduces the legacy spanning tree technology used to build Layer 2 domains.

The Spanning Tree Protocol

As discussed previously, the switch forwarding algorithm floods unknown and broadcast frames out of all the ports that are in the same VLAN as the received frame. This causes a potential problem. If the network devices that run this algorithm are connected together in a physical loop, flooded frames (like broadcasts) are passed from switch to switch, around and around the loop, forever. Depending on the physical connections involved, the frames can actually multiply exponentially because of the flooding algorithm, which can cause serious network problems.

There is a benefit to a physical loop in your network: It can provide redundancy. If one link fails, there is still another way for the traffic to reach its destination. To allow the benefits derived from redundancy, without breaking the network because of flooding, a protocol called the Spanning Tree Protocol (STP) was created. Spanning tree was standardized in the IEEE 802.1D specification.

The purpose of STP is to identify and temporarily block the loops in a network segment or VLAN. The switches run STP, which involves electing a root bridge or switch. The other switches measure their distance from the root switch. If there is more than one way to get to the root switch, there is a loop. The switches follow the algorithm to determine which ports must be blocked to break the loop. STP is dynamic; if a link in the segment fails, ports that were originally blocking can possibly be changed to forwarding mode.

Spanning tree is covered in more detail later in this book. The next section covers how to pass multiple VLANs on a single port.


Trunking is a mechanism that is most often used to allow multiple VLANs to function independently across multiple switches. Routers and servers can use trunking, as well, which allows them to live simultaneously on multiple VLANs. If your network only has one VLAN in it, you might never need trunking; but if your network has more than one VLAN, you probably want to take advantage of the benefits of trunking.

A port on a switch normally belongs to only one VLAN; any traffic received or sent on this port is assumed to belong to the configured VLAN. A trunk port, however, is a port that can be configured to send and receive traffic for many VLANs. It accomplishes this when it attaches VLAN information to each frame, a process called tagging the frame. Also, trunking must be active on both sides of the link; the other side must expect frames that include VLAN information for proper communication to occur. As with all the section briefs in this chapter, more information is found later in this book.

Port Channels

Utilizing port channels (EtherChannels) is a technique that is used when you have multiple connections to the same device. Rather than each link functioning independently, port channels group the ports together to work as one unit. Port channels distribute traffic across all the links and provide redundancy if one or more links fail. Port channel settings must be the same on both sides of the links involved in the channel. Normally, spanning tree would block all of these parallel connections between devices because they are loops, but port channels run underneath spanning tree, so that spanning tree thinks all the ports within a given port channel are only a single port. Later chapters discuss port channels in more detail.

Multilayer Switching

Multilayer switching (MLS) is the ability of a switch to forward frames based on information in the Layer 3 and sometimes Layer 4 header. Almost all Cisco Catalyst switches model 3500 or later support MLS. MLS is becoming a legacy term due to the wide support. The most important aspect to MLS is recognizing that switches can route or switch frames at wire-rate speeds using specialized hardware. This effectively bundles the routing function into the switch and is specifically useful for routing between VLANs in the core of the network. The next chapter discusses this capability in more detail.

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