10 Chapter 1: LAN Switching Review and Configuring Cisco 2950 LAN Switches

Foundation Topics

Cisco’s LAN switch revenues surpassed router revenues about the time the CCNA exam was first announced, in 1998, so there is little doubt about the importance of LAN switches to Cisco. Not surprisingly, both CCNA exams cover LAN switching concepts extensively.

This chapter starts with a brief review of LAN switching topics. The majority of this chapter is devoted to basic switch configuration for a Cisco 2950 switch.

Brief Review of LAN Switching

LAN switches forward Ethernet frames—they just have to decide when to forward them and when not to. Most switch logic relates somehow to the source and destination MAC addresses inside the Ethernet frame headers of the frames sent through the LAN. Switch logic is also dependent on the type of MAC addresses used. So, a brief review of Ethernet addresses can help shed some light on how LAN switches work.

The IEEE defines three general categories of MAC addresses on Ethernet:

■Unicast addresses—A MAC address that identifies a single LAN interface card. Today, most cards use the MAC address that is burned into the card.

■Broadcast addresses—The most often used IEEE group MAC address, the broadcast address, has a value of FFFF.FFFF.FFFF (in hexadecimal notation). The broadcast address implies that all devices on the LAN should receive and process a frame sent to the broadcast address.

■Multicast addresses—Frames sent to unicast addresses are destined for a single device; frames sent to a broadcast address are sent to all devices on the LAN. Frames sent to multicast addresses are meant for all devices that care to receive the frame, meaning that all devices might receive the frame, none, or some number in between. Some applications need to communicate with multiple other devices. By sending one frame, all the devices that care about receiving the data sent by that application can process the data, and the rest can ignore it.

With these reminders of the three types of Ethernet MAC addresses, you can appreciate the logic used by a LAN switch. A switch listens for frames that enter all its interfaces. After receiving a frame, about a switch decides whether to forward a frame and, if so, out which port(s). Switches basically perform three tasks:

■Learning—The switch learns MAC addresses by examining the source MAC address of each frame the bridge receives. By learning, the switch can make good forwarding choices in the future.

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■Forwarding or filtering—The switch decides when to forward a frame or when to filter (not forward) it based on the destination MAC address. The switch looks at the previously learned MAC addresses in an address table to decide where to forward the frames.

■Loop prevention—The switch creates a loop-free environment with other bridges by using Spanning Tree Protocol (STP). Having physically redundant links helps LAN availability, and STP prevents the switch logic from letting frames loop around the network indefinitely, congesting the LAN.

The next few sections take you through the first two tasks a switch performs. The third task, loop prevention, is performed using STP, which is covered in depth in Chapter 2, “Spanning Tree Protocol.”

The Forward-Versus-Filter Decision

Switches reduce network overhead by forwarding traffic from one segment to another only when necessary. To decide whether to forward a frame, the switch uses a dynamically built table called a bridge table or MAC address table. The switch examines the address table to decide whether it should forward a frame.

For example, consider the simple network shown in Figure 1-1. Fred first sends a frame to Barney and then one to Wilma.

The switch decides to filter (in other words, to not forward) the frame that Fred sends to Barney. Fred sends a frame with a destination MAC address of 0200.2222.2222, which is Barney’s MAC address. The switch overhears the frame, because it is attached to Hub1. The switch then decides what common sense tells you from looking at the figure—it should not forward the frame, because Barney, attached to Hub1 as well, already has received the frame. (Hubs simply repeat the signal out all ports, for all frames, so the switch receives everything sent by either Barney or Fred.) But how does the switch know to not forward the frame? The switch decides to filter the frame because it received the frame on port E0, and it knows that Barney’s MAC is also located out E0.

Conversely, the switch decides to forward the frame that Fred sends to Wilma in the bottom part of the figure. The frame enters the switch’s E0 interface, and the switch knows that the destination address, 0200.3333.3333, is located somewhere out its E1 interface. So the switch forwards the frame.

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Figure 1-2 Switch Learning: Adding Two Entries to an Empty Table

Barney 0200.2222.2222

As shown in the figure, after Fred sends his first frame to Barney, the switch adds an entry for 0200.1111.1111, Fred’s MAC address, associated with interface E0. When Barney replies in Step 2, the switch adds a second entry, this one for 0200.2222.2222, Barney’s MAC address. Learning always occurs by looking at the source MAC address in the frame.

Forwarding Unknown Unicasts and Broadcasts

Bridges forward LAN broadcast frames, and unknown unicast frames, out all ports. LAN broadcasts, by definition, are received by all devices on the same LAN. So the switch simply forwards broadcasts out all ports, except the one on which the broadcast was received. Switches forward unknown unicast frames, which are frames whose destination MAC addresses are not yet in the bridging table, out all ports as well. The switch floods the frame with the hope that the unknown device will be on some other Ethernet segment, it will reply, and the switch will build a correct entry in the address table.

Generally speaking, switches also forward LAN multicast frames out all ports, just like they do for broadcasts. However, a few multicast features in switches limit the flooding of multicasts, such as Internet Group Management Protocol (IGMP) snooping.