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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.

Brief Review of LAN Switching 11

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.

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

Figure 1-1 Switch Forwarding and Filtering Decision

Frame Sent to 0200.2222.2222… Came in E0.

I Should FILTER it, Because

Fred Destination Is on Port E0. 0200.1111.1111

Wilma 0200.3333.3333

E0

E1

Hub1

Hub2

Barney 0200.2222.2222

Frame sent to 0200.3333.3333… Came in E0.

I should FORWARD it, because

Fred destination is off port E1. 0200.1111.1111

E0

E1

Hub1

Hub2

Betty 0200.4444.4444

MAC Address Table

0200.1111.1111 E0

0200.2222.2222 E0

0200.3333.3333 E1

0200.4444.4444 E1

Wilma 0200.3333.3333

Barney

Betty

0200.2222.2222

0200.4444.4444

How Switches Learn MAC Addresses

The filter-versus-forward decision works best when the switch knows where all the MAC addresses are in the network. Switches dynamically learn the MAC addresses in the network to build its MAC address table. With a full, accurate MAC address table, the switch can make accurate forwarding and filtering decisions.

Switches build the MAC address table by listening to incoming frames and examining the frame’s source MAC address. If a frame enters the switch, and the source MAC address is not in the address table, the switch creates an entry in the table. The MAC address is placed in the table, along with the interface in which the frame arrived. Switch learning logic is that simple!

Figure 1-2 shows the same network as Figure 1-1, but before the switch has built any bridge table entries. This figure shows the first two frames sent in this network—a frame from Fred, addressed to Barney, and then Barney’s response, addressed to Fred.

Brief Review of LAN Switching 13

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

 

 

 

 

 

 

 

 

 

 

I Learned Fred’s MAC When He Sent

 

 

 

 

 

 

 

 

 

 

 

Fred

 

Frame 1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I Learned Barney’s MAC When He

 

Address Table: Before Either Frame Is Sent

0200.1111.1111

 

Sent Frame 2!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Wilma

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0200.3333.3333

 

 

 

 

 

 

 

 

 

 

 

 

 

1

 

 

Address Table: After Frame 1 (Fred to Barney)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Address:

 

 

Interface

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E0

E1

 

 

 

 

 

 

 

 

0200.1111.1111

E0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Address Table: After Frame 2 (Barney to Fred)

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Address:

 

Interface

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0200.1111.1111

E0

 

 

 

 

 

 

 

 

 

 

 

Betty

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0200.2222.2222

E0

 

 

 

 

 

 

 

 

 

 

 

0200.4444.4444

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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.