Layer 2 Switching

In Chapter 2 we covered a basic overview of bridging and switching functions, including a look at how these devices help to segment a network into a number of smaller collision domains. In this chapter we’ll go many steps further, with a look at switching methods, loop avoidance, Virtual LANs, Spanning Tree Protocol, trunk connections, frame tagging, Token Ring switching, and Cisco Catalyst switch models.

For the purpose of the CCNA exam, you’ll need to be familiar with the configuration of Cisco Catalyst 1900 series switches, the most basic manageable switches that Cisco offers. These particular switches have two different configuration modes – one menu-driven, the other command line-based. The 1900 comes with one of two software versions – the Enterprise version includes the command line interface (CLI), while the Standard Edition does not. In this chapter we’ll stick to the initial configuration of a 1900 using a terminal session and the menu interface. Getting into the command line at this point would be premature, since we haven’t examined the Cisco Internetwork Operating System (IOS) yet. For that reason, the 1900 configuration details can be found in Appendix A. By the time we get there, you’ll almost be an IOS pro.

The material to be covered in this chapter includes:

  • Cisco switching methods
  • Redundancy and loop avoidance on bridged and switches networks
  • Spanning Tree Protocol
  • Virtual LANs (VLANs)
  • Trunking and VLAN identification
  • Cisco Catalyst 1900 initial configuration
  • Layer 2 multicasting techniques
  • Cisco switching equipment

Switching Reviewed

By this point, you should be familiar with the basic benefits that a switch provides. You know that a switch segments a network into smaller collision domains, and this helps to provide better performance and throughput. You also have an awareness of how a MAC address table is built, and how a bridge or switch makes forwarding/filtering decisions. Finally, you should recall that when a computer is plugged directly into its own switch port, full duplex communication becomes possible. The ability to communicate without collisions makes full bandwidth available to systems. For example, if two Fast Ethernet network cards are plugged into individual 100 Mbps switch ports, they become capable of simultaneously sending and receiving 100 at Mbps. While some people (including marketers, of course!) will suggest that this means the card is capable of 200 Mbps performance, this is stretching the truth. The card can only transmit or receive in either direction at a maximum of 100 Mbps, even if it can do both at the same time.

While a switch goes a long way towards providing better network performance, you have to remember that at the end of the day, it’s still only a Layer 2 device. Because of that, all broadcasts and multicasts will still be forwarded (or flooded) out all switch ports, along with frames whose destination address isn’t yet known to the switch.

Does a switch allow networks to grow larger than when hubs are used? Certainly. However, in order to provide a higher level of performance and cut down on broadcast traffic, a large network will still need to be split into different broadcast domains. This still requires the use of routers– keep this in mind when you’re thinking about network switching.

Tip: Remember that a switch or a bridge will create a larger number of smaller collision domains.