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Chapter 2. Configure, verify, and troubl... > Explain basic switching concepts and...

2.4. Explain basic switching concepts and the operation of Cisco switches

Unlike bridges, which use software to create and manage a filter table, switches use application-specific integrated circuits (ASICs) to build and maintain their filter tables. But it's still okay to think of a layer 2 switch as a multiport bridge because their basic reason for being is the same: to break up collision domains.

Layer 2 switches and bridges are faster than routers because they don't take up time looking at the Network layer header information. Instead, they look at the frame's hardware addresses before deciding to either forward, flood, or drop the frame.

Switches create private, dedicated collision domains and provide independent bandwidth on each port, unlike hubs. Figure 2.11 shows five hosts connected to a switch—all running 10Mbps half-duplex to the server. Unlike with a hub, each host has 10Mbps dedicated communication to the server.

Figure 2.11. Switches create private domains.

Layer 2 switching provides the following:

  • Hardware-based bridging (ASIC)

  • Wire speed

  • Low latency

  • Low cost

What makes layer 2 switching so efficient is that no modification to the data packet takes place. The device only reads the frame encapsulating the packet, which makes the switching process considerably faster and less error-prone than routing processes are.

And if you use layer 2 switching for both workgroup connectivity and network segmentation (breaking up collision domains), you can create a flatter network design with more network segments than you can with traditional routed networks.

Plus, layer 2 switching increases bandwidth for each user because, again, each connection (interface) into the switch is its own collision domain. This feature makes it possible for you to connect multiple devices to each interface.

In the following sections, I will dive deeper into the layer 2 switching technology.

2.4.1. Limitations of Layer 2 Switching

Since we commonly stick layer 2 switching into the same category as bridged networks, we also tend to think it has the same hang-ups and issues that bridged networks do. Keep in mind that bridges are good and helpful things if we design the network correctly, keeping their features as well as their limitations in mind. And to design well with bridges, these are the two most important considerations:

  • We absolutely must break up the collision domains correctly.

  • The right way to create a functional bridged network is to make sure that its users spend 80 percent of their time on the local segment.

Bridged networks break up collision domains, but remember, that network is still one large broadcast domain. Neither layer 2 switches nor bridges break up broadcast domains by default—something that not only limits your network's size and growth potential but also can reduce its overall performance.

Broadcasts and multicasts, along with the slow convergence time of spanning trees, can give you some major grief as your network grows. These are the big reasons that layer 2 switches and bridges cannot completely replace routers (layer 3 devices) in the internetwork.

2.4.2. Bridging vs. LAN Switching

It's true—layer 2 switches really are pretty much just bridges that give us a lot more ports, but there are some important differences you should always keep in mind:

  • Bridges are software based, while switches are hardware based because they use ASIC chips to help make filtering decisions.

  • A switch can be viewed as a multiport bridge.

  • There can be only one spanning-tree instance per bridge, while switches can have many. (I'm going to tell you all about spanning trees in a bit.)

  • Switches have a higher number of ports than most bridges.

  • Both bridges and switches forward layer 2 broadcasts.

  • Bridges and switches learn MAC addresses by examining the source address of each frame received.

  • Both bridges and switches make forwarding decisions based on layer 2 addresses.

2.4.3. Three Switch Functions at Layer 2

There are three distinct functions of layer 2 switching (you need to remember these!): address learning, forward/filter decisions, and loop avoidance.

Address learning

Layer 2 switches and bridges remember the source hardware address of each frame received on an interface, and they enter this information into a MAC database called a forward/filter table.

Forward/filter decisions

When a frame is received on an interface, the switch looks at the destination hardware address and finds the exit interface in the MAC database. The frame is only forwarded out the specified destination port.

Loop avoidance

If multiple connections between switches are created for redundancy purposes, network loops can occur. Spanning Tree Protocol (STP) is used to stop network loops while still permitting redundancy.

I'm going to talk about address learning, forward/filtering decisions, and loop avoidance in detail in the next sections. Address Learning

When a switch is first powered on, the MAC forward/filter table is empty, as shown in Figure 2.12.

Figure 2.12. Empty forward/filter table on a switch

When a device transmits and an interface receives a frame, the switch places the frame's source address in the MAC forward/filter table, allowing it to remember which interface the sending device is located on. The switch then has no choice but to flood the network with this frame out of every port except the source port because it has no idea where the destination device is actually located.

If a device answers this flooded frame and sends a frame back, then the switch will take the source address from that frame and place that MAC address in its database as well, associating this address with the interface that received the frame. Since the switch now has both of the relevant MAC addresses in its filtering table, the two devices can now make a point-to-point connection. The switch doesn't need to flood the frame as it did the first time because now the frames can and will be forwarded only between the two devices. This is exactly the thing that makes layer 2 switches better than hubs. In a hub network, all frames are forwarded out all ports every time—no matter what. Figure 2.13 shows the processes involved in building a MAC database.

In this figure, you can see four hosts attached to a switch. When the switch is powered on, it has nothing in its MAC address forward/filter table, just as in Figure 2.5. But when the hosts start communicating, the switch places the source hardware address of each frame in the table along with the port that the frame's address corresponds to.

Figure 2.13. How switches learn hosts' locations

Let me give you an example of how a forward/filter table is populated:

  1. Host A sends a frame to Host B. Host A's MAC address is 0000.8c01.000A; Host B's MAC address is 0000.8c01.000B.

  2. The switch receives the frame on the E0/0 interface and places the source address in the MAC address table.

  3. Since the destination address is not in the MAC database, the frame is forwarded out all interfaces—except the source port.

  4. Host B receives the frame and responds to Host A. The switch receives this frame on interface E0/1 and places the source hardware address in the MAC database.

  5. Host A and Host B can now make a point-to-point connection and only the two devices will receive the frames. Hosts C and D will not see the frames, nor are their MAC addresses found in the database because they haven't yet sent a frame to the switch.

If Host A and Host B don't communicate to the switch again within a certain amount of time, the switch will flush their entries from the database to keep it as current as possible. Forward/Filter Decisions

When a frame arrives at a switch interface, the destination hardware address is compared to the forward/filter MAC database. If the destination hardware address is known and listed in the database, the frame is only sent out the correct exit interface. The switch doesn't transmit the frame out any interface except for the destination interface. This preserves bandwidth on the other network segments and is called frame filtering.

But if the destination hardware address is not listed in the MAC database, then the frame is flooded out all active interfaces except the interface the frame was received on. If a device answers the flooded frame, the MAC database is updated with the device's location (interface).

If a host or server sends a broadcast on the LAN, the switch will flood the frame out all active ports except the source port by default. Remember, the switch creates smaller collision domains, but it's still one large broadcast domain by default.

In Figure 2.14, Host A sends a data frame to Host D. What will the switch do when it receives the frame from Host A?

Figure 2.14. Forward/filter table

Since Host A's MAC address is not in the forward/filter table, the switch will add the source address and port to the MAC address table and then forward the frame to Host D. If Host D's MAC address was not in the forward/filter table, the switch would have flooded the frame out all ports except for port Fa0/3.

Now let's take a look at the output of a show mac address-table:

Switch#sh mac address-table
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0005.dccb.d74b    DYNAMIC     Fa0/1
   1    000a.f467.9e80    DYNAMIC     Fa0/3
   1    000a.f467.9e8b    DYNAMIC     Fa0/4
   1    000a.f467.9e8c    DYNAMIC     Fa0/3
   1    0010.7b7f.c2b0    DYNAMIC     Fa0/3
   1    0030.80dc.460b    DYNAMIC     Fa0/3
   1    0030.9492.a5dd    DYNAMIC     Fa0/1
   1    00d0.58ad.05f4    DYNAMIC     Fa0/1

Suppose the preceding switch received a frame with the following MAC addresses:

Source MAC: 0005.dccb.d74b
Destination MAC: 000a.f467.9e8c

How will the switch handle this frame? Answer: The destination MAC address will be found in the MAC address table and the frame will be forwarded out Fa0/3 only. Remember that if the destination MAC address is not found in the forward/filter table, it will forward the frame out all ports of the switch looking for the destination device.

2.4.4. Exam Objectives

Remember the three switch functions.

Address learning, forward/filter decisions, and loop avoidance are the functions of a switch.

Remember the command

show mac address-table

The command show mac address-table will show you the forward/filter table used on the LAN switch.

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