Semester 3 Chapter 2 LAN Switching
Overview |
Today, network designers are moving away from using bridges and hubs and are primarily using switches and routers to build networks. Chapter 1, "Review: The OSI Reference Model and Routing," provides a review of the OSI reference model and an overview of network planning and design considerations related to routing. This chapter discusses problems in a local-area network (LAN) and possible solutions that can improve LAN performance. You will learn about LAN congestion and its effect on network performance and the advantages of LAN segmentation in a network. In addition, you will learn about the advantages and disadvantages of using bridges, switches, and routers for LAN segmentation and the effects of switching, bridging, and routing on network throughput. Finally, you will learn about Ethernet, Fast Ethernet, and VLANs, and the benefits of these technologies.
Washington Project: LAN Switching As you read through this chapter on LAN switching, consider why switches are being introduced. Imagine where switches might be used in an Elementary school site LAN design. |
2.1 Various LAN Communication Problems | |
2.1.1 Factors that impact network performance | |
Today's LANs are becoming increasingly congested and overburdened. In addition to an ever-growing population of network users, several other factors have combined to expand the capabilities of traditional LANs:
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2.1.2 Elements of Ethernet/802.3 networks |
The most common LAN architecture is Ethernet. Ethernet is used to transport data between devices on a network, such as computers, printers, and file servers. As shown in the graphic, all the devices are connected to the same delivery medium. Ethernet media uses a data frame broadcast method of transmitting and receiving data to all nodes on the shared media. The performance of a shared-medium Ethernet/802.3 LAN can be negatively affected by several factors:
Ethernet using CSMA/CD and a shared medium can support data transmission rates of up to 100 Mbps. CSMA/CD is an access method that allows only one station to transmit at a time. The goal of Ethernet is to provide a best-effort delivery service and allow all devices on the shared medium to transmit on an equal basis. As shown in Figure , one of the inherent problems with CSMA/CD technology is collisions. |
2.1.3 Half-duplex Ethernet |
Ethernet is a half-duplex technology. Each Ethernet host checks the network to see whether data is being transmitted before it transmits additional data. If the network is already in use, the transmission is delayed. Despite transmission deferral, two or more Ethernet hosts can transmit at the same time, which results in a collision. When a collision occurs, the host that first detects the collision will send out a jam signal. Upon hearing the jam signal, each host will wait a random period of time before attempting to transmit. The NIC's backoff algorithm generates this random period of time. As more hosts are added to the network and begin transmitting, collisions are more likely to occur. Ethernet LANs become saturated because users run network-intensive software, such as client/server applications, which cause hosts to transmit more often and for longer periods of time. The physical connector (e.g. NIC) used by devices on an Ethernet LAN provides several circuits so that communications between devices can occur. |
2.1.4 Network congestion |
Technology advances are producing faster and more intelligent desktop computers and workstations. The combination of more powerful computers/workstations and network-intensive applications has created a need for network capacity, or bandwidth, that is much greater than the 10 Mbps that is available on shared Ethernet/802.3 LANs. Today's networks are experiencing an increase in the transmission of large graphics files, images, full-motion video, and multimedia applications, as well as an increase in the number of users on a network. All these factors place an even greater strain on Ethernet's 10-Mbps bandwidth. As more people utilize a network to share large files, access file servers, and connect to the Internet, network congestion occurs. This can result in slower response times, longer file transfers, and network users becoming less productive due to network delays. To relieve network congestion, more bandwidth is needed or the available bandwidth must be used more efficiently. The methods used to implement these solutions are discussed later in the chapter. |
2.1.5 Network latency |
Latency, sometimes called delay, is the time a frame (or a packet) takes to travel from the source station (node) to its final destination on the network. It is important to quantify the total latency of the path between source and destination for LANs and WANs. In the specific case of an Ethernet LAN, understanding latency and its effect on network timing is crucial to determining whether CSMA/CD for detecting collisions and negotiating transmissions will work properly. Latency has at least three sources. First, there is the time it takes the source NIC to place voltage pulses on the wire and the time it takes the receiving NIC to interpret these pulses. This is sometimes called NIC delay (it is typically around 1 microsecond for 10BASE-T NICs). Second, there is the actual propagation delay as the signal takes time -- albeit a very short time -- to actually travel down the cable (it is typically about .556 microseconds per 100 m for Cat 5 UTP). The longer the cable, the more propagation delay; the slower the nominal velocity of propagation (NVP) of the cable, the more the propagation delay. Third, latency is added according to which networking devices -- whether they be Layer 1, 2, or 3 (and how they are configured) -- are added in the path between the two communicating computers. The actual transmission time (the duration of the host actually sending bits) must also be included in understanding timing on networks. Latency does not depend solely on distance and number of devices. For example, if three properly configured switches separate two workstations, the workstations may experience less latency than if two properly configured routers separated them. This is because generally routers conduct more complex and time consuming decision-making functions (recall that routers are layer 3 devices and look at more encapsulation data than layer 2 switches do) |
2.1.6 Ethernet 10BASE-T transmission time |
All networks have what is called a "bit time" or a "slot time". Many LAN technologies, such as Ethernet, define a bit time as the basic unit of time in which ONE bit can be sent. In order for the electronic or optical devices to recognize a binary one or a binary zero, there must be some minimum duration during which the bit is "on" or "off". Transmission time equals the number of bits being sent times the bit time for a given technology. Another way to think about transmission time is as the time it takes a frame to actually be transmitted (small frames take a shorter amount of time, large frames take a longer amount of time to be transmitted). The table shows the transmission times for four different 10BASE-T frame sizes. Each 10 Mbps Ethernet bit has a 100 ns window for transmitting (the bit time). A byte equals 8 bits. Therfore, 1 byte takes a minimum of 800 ns to transmit. A 64-byte frame (the smallest allowable 10BASE-T frame, so that CSMA/CD will work properly) takes 51, 200 ns, or 51.2 microseconds, to transmit (64 bytes at 800 ns per byte equals 51, 200 ns and 51, 200 ns divided by 1000 equals 51.2 microseconds). Transmission time of an entire 1000-byte frame from the source station requires 800 microseconds just to complete the frame. The time at which the frame actually arrives at the destination station depends on the additional latency (delay) introduced by the network. This latency can is due to NIC delays, propagation delays, and layer 1, 2, or 3 device delays. |
2.1.7 The benefit of using repeaters |
The distance that a LAN can cover is limited due to attenuation; attenuation means that the signal weakens (that is, attenuates) as it travels through the network. Attenuation is caused by the resistance in the cable, or medium. An Ethernet repeater is a physical-layer device on the network that boosts or regenerates the signal on an Ethernet LAN. When you use an Ethernet repeater to extend the distance of a LAN, a single network can cover a greater distance and more users can share that same network, as shown. However, using repeaters and multiport repeaters, known as hubs, compounds the issue of broadcasts and collisions and has a negative effect on the overall performance of the shared-media LAN. |
2.2 Full-Duplex Transmitting, Fast Ethernet Standard and LAN Segmentation | |
2.2.1 Full-duplex Ethernet | |
Full-duplex Ethernet allows the transmission of a packet and the reception of a different packet at the same time. This simultaneous transmission and reception requires the use of two pairs of wires in the cable and a switched connection between each node. This connection is considered point-to-point and is collision free. Because both nodes can transmit and receive at the same time, there are no negotiations for bandwidth. Full-duplex Ethernet can use an existing shared medium as long as the medium meets minimum Ethernet standards. To transmit and receive simultaneously, a dedicated port is required for each node. Full-duplex connections can use 10BASE-T, 100BASE-TX, or 100BASE-FX media to create point-to-point connections. The network interface cards (NICs) on both ends need to have full-duplex capabilities. The full-duplex Ethernet switch takes advantage of the two pairs of wires in the cable. This is done by creating a direct connection between the transmit (TX) at one end of the circuit and the receive (RX) at the other end. With these two stations connected this way, a collision-free domain is created because the transmission and receipt of data occurs on separate non-competitive circuits. Ethernet usually can only use 50%-60% of the 10-Mbps available bandwidth because of collisions and latency. Full-duplex Ethernet offers 100% of the bandwidth in both directions. This produces a potential 20-Mbps throughput- 10-Mbps TX and 10-Mbps RX. | |
2.2.2 LAN segmentation |
A network can be divided into smaller units called segments. Each segment uses the CSMA/CD access method and maintains traffic between users on the segment. Figure shows an example of a segmented Ethernet network. The entire network has 15 computers (6 file severs and 9 PCs). By using segments in a network, fewer users/devices are sharing the same 10 Mbps when communicating to one another within the segment. Each segment is its own collision domain. By dividing the network into three segments, a network manager can decrease network congestion within each segment. When transmitting data within a segment, the five devices within each segment are sharing the 10-Mbps bandwidth per segment. In a segmented Ethernet LAN, data passed between segments is transmitted on the backbone of the network using a bridge, router, or switch. |
2.2.3 LAN segmentation with bridges |
Ethernet LANs that use a bridge to segment the LAN provide more bandwidth per user because there are fewer users on each segment. In contrast, LANs that do not use bridges for segmentation provide less bandwidth per user because there are more users on a nonsegmented LAN. Bridges "learn" a network's segmentation by building address tables that contain the address of each network device and which segment to use to reach that device. Bridges are Layer 2 devices that forward data frames according to the frames' Media Access Control (MAC) addresses. In addition, bridges are transparent to the other devices on the network. Bridges increase the latency in a network by 10%-30%. This latency is due to the decision making required of the bridge or bridges in transmitting data. A bridge is considered a store-and-forward device because it must examine the destination address field and calculate the CRC in the frame check sequence field, before forwarding the frame to all ports. If the Destination port is busy, the bridge can temporarily store the frame until the port is available. The time it takes to perform these tasks slows the network transmissions causing increased latency. |
2.2.4 The pros and cons of LAN segmentation with routers |
Routers are more advanced than typical bridges. A bridge is passive on the network and operates at the data link layer. A router operates at the network layer and bases all its decisions about forwarding between segments on the network-layer protocol address. Routers create the highest level of segmentation by forwarding data to the hub, to which workstations are connected. A router makes forwarding decisions to segments by examining the destination address on the data packet and looking in its routing table for forwarding instructions. A router must examine a packet to determine the best path for forwarding that packet to its destination. This process takes time. Protocols that require an acknowledgement from the receiver to the sender for every packet as it is delivered (known as acknowledgement-oriented protocols) have a 30%-40% loss of throughput. Protocols that require minimal acknowledgements (sliding-window protocols) suffer a 20%-30% loss of throughput. This is due to the fact that there is less data traffic between the sender and receiver (that is, fewer acknowledgements). |
2.2.5 Pros and cons of LAN segmentation with switches |
LAN switching eases bandwidth shortages and network bottlenecks, such as those between several PCs and a remote file server. A switch can segment a LAN into microsegments, which are single host segments. This creates collision-free domains from one larger collision domain. Although the LAN switch eliminates collision domains, all hosts connected to the switch are still in the same broadcast domain. Therefore, all nodes connected through the LAN switch can see a broadcast from just one node. Switched Ethernet is based on Ethernet. Each node is directly connected to one of its ports or a segment that is connected to one of the switch's ports. This creates a 10-Mbps bandwidth connection between each node and each segment on the switch. A computer connected directly to an Ethernet switch is its own collision domain and accesses the full 10 Mbps. A LAN that uses a Switched Ethernet topology creates a network that behaves as though it has only two nodes-the sending node and the receiving node. These two nodes share the 10-Mbps bandwidth between them, which means that nearly all the bandwidth is available for the transmission of data. Because a Switched Ethernet LAN uses bandwidth so efficiently, it can provide more throughput than Ethernet LANs connected by bridges or hubs. In a Switched Ethernet implementation, the available bandwidth can reach close to 100%. Ethernet switching increases the bandwidth available on a network by creating dedicated network segments (that is, point-to-point connections) and connecting those segments in a virtual network within the switch. This virtual network circuit exists only when two nodes need to communicate. This is why it is called a virtual circuit, it exists only when needed and is established within the switch. One drawback of switches is that they cost more than hubs. However, many businesses implement switch technology slowly by connecting hubs to switches to such a time that the hubs can be replaced. |
2.3 Switching and VLANs | |
2.3.1 Describe the two basic operations of a switch | |
Switching is a technology that decreases congestion in Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) LANs by reducing traffic and increasing bandwidth. LAN switches are often used to replace shared hubs. They are designed to work with cable infrastructures that already exist so that they can be installed without disrupting existing network traffic. Today in data communications, all switching equipment perform two basic operations:
The term bridging refers to a technology in which a device known as a bridge connects two or more LAN segments. A bridge transmits datagrams from one segment to their destinations on other segments. When a bridge is powered on and begins to operate, it examines the MAC address of the incoming datagrams and builds a table of known destinations. If the bridge knows that the destination of a datagram is on the same segment as the source of the datagram, it drops the datagram because there is no need to transmit it. If the bridge knows that the destination is on another segment, it transmits the datagram on that segment only. If the bridge does not know the destination segment, the bridge transmits the datagram on all segments except the source segment (a technique known as flooding). The primary benefit of bridging is that it limits traffic to certain network segments. Both bridges and switches connect LAN segments, use a table of MAC addresses to determine the segment on which a datagram needs to be transmitted, and reduce traffic. Switches are more functional in todays networks than bridges because they operate at much higher speeds than bridges and can support new functionality, such as virtual LANs (VLANs). Bridges typically switch using software; switches typically switch using hardware. | |
2.3.2 Ethernet switch latency |
Each switch used on a 10 Mbps Ethernet LAN adds latency to the network. However, latency depends on the brand of the switch and upon the type of switching used. As will be explained later, the different switching modes ("store and forward", "fragment free", and "fast forward") differ on when the decision to switch an incoming frame is made. The latency due to the switch "making its decision" must be added to the time for the frame to enter and exit the switch ports and determines the total latency of the switch. Note that a hub, which is simply forwarding frames (with no filtering or decision-making), has only a port-to-port latency. All of these fractions of a second may not seem to matter much, but recall that we are sending data a rates of 10 Mbps (1 bit per 1 ten-millionth of a second), 100 Mbps (1 bit per 1 100 ten-millionth of a second), or 1000 Mbps (1 Gbps, 1 bit per 1 billionth of a second). So networking devices are operating at incredibly high speeds, so every nanosecond can matter! |
2.3.3 Layer 2 and Layer 3 switching |
There are two methods of switching data frames-Layer 2 and Layer 3 switching. Switching is the process of taking an incoming frame from one interface and delivering it out through another interface. Routers use Layer 3 switching to route a packet; switches (Layer 2 switches) use Layer 2 switching to forward frames. The difference between Layer 2 and Layer 3 switching is the type of information inside the frame that is used to determine the correct output interface. With Layer 2 switching, frames are switched based on MAC address information. With Layer 3 switching, frames are switched based on network-layer information. Layer 2 switching does not look inside a packet for network-layer information as does Layer 3 switching. Layer 2 switching looks at a destination MAC address within a frame. It sends the information to the appropriate interface if it knows the destination address location. Layer 2 switching builds and maintains a switching table that keeps track of the MAC addresses that belong to each port or interface. If the Layer 2 switch does not know where to send the frame, it broadcasts the frame out all its ports to the network to learn the correct destination. When the frame's reply is returned, the switch learns the location of the new address and adds the information to the switching table. The manufacturer of the data communications equipment determines the Layer 2 addresses. They are unique addresses that are derived in two parts- the manufacturing (MFG) code and the unique identifier. The Institute of Electrical and Electronic Engineers (IEEE) assigns the MFG code to each vendor. The vendor assigns a unique identifier. Except in Systems Network Architecture (SNA) networks, users have little or no control over Layer 2 addressing because Layer 2 addresses are fixed with a device, whereas Layer 3 addresses can be changed. In addition, Layer 2 addresses assume a flat address space with universally unique addresses. Layer 3 switching operates at the network layer. It examines packet information and forwards packets based on their network-layer destination addresses. Layer 3 switching also supports router functionality. For the most part, the network administrator determines the Layer 3 addresses. Protocols such as IP, IPX, and AppleTalk use Layer 3 addressing. By creating Layer 3 addresses, a network administrator creates local areas that act as single addressing units (similar to streets, cities, states, and countries) and assigns a number to each local entity. If users move to another building, their end stations obtain new Layer 3 addresses, but their Layer 2 addresses remain the same. Because routers operate at Layer 3 of the OSI reference model, they can adhere to and create a hierarchical addressing structure. Therefore, a routed network can tie a logical addressing structure to a physical infrastructure, for example, through TCP/IP subnets or IPX networks for each segment. Traffic flow in a switched (that is, flat) network is therefore inherently different from traffic flow in a routed (that is, hierarchical) network. Hierarchical networks offer more flexible traffic flow than flat networks because they can use the network hierarchy to determine optimal paths and contain broadcast domains. |
2.3.4 Microsegmentation |
The increasing power of desktop processors and the requirements of client/server and multimedia applications have created an increased need for greater bandwidth in traditional shared-media environments. These requirements are prompting network designers to replace hubs in wiring closets with switches. Layer 2 switches use microsegmentation to satisfy the demands for more bandwidth and increased performance, but network designers are now faced with increasing demands for intersubnet communication. For example, every time a user accesses servers and other resources that are located on different subnets, the traffic must go through a Layer 3 device. Potentially, there is a tremendous bottleneck, which can threaten network performance. To avoid this bottleneck, network designers can add Layer 3 capabilities throughout the network, which alleviates the burden on centralized routers. Therefore, a switch improves bandwidth by separating collision domains and selectively forwarding traffic to the appropriate segments of a network. |
2.3.5 How a switch learns addresses |
An Ethernet switch can learn the address of each device on the network by reading the source address of each packet transmitted and noting the port where the frame entered the switch. The switch then adds this information to its forwarding database. Addresses are learned dynamically. This means that as new addresses are read, they are learned and stored in content-addressable memory (CAM). When a source is read that is not found in CAM, it is learned and stored for future use. Each time an address is stored, it is time stamped. This allows for addresses to be stored for a set period of time. Each time an address is referenced or found in CAM, it receives a new time stamp. Addresses that are not referenced during a set period of time are removed from the list. By removing aged or old addresses, CAM maintains an accurate and functional forwarding database. |
2.3.6 Benefits of LAN switching |
Switches have many benefits. A LAN switch allows many users to communicate in parallel through the use of virtual circuits and dedicated network segments in a collision-free environment. This maximizes the bandwidth available on the shared medium. Also, moving to a switched LAN environment is very cost-effective because you can reuse existing hardware and cabling. Finally, the power of the switch combined with the software to configure LANs give network administrators great flexibility in managing the network. |
2.3.7 Symmetric and asymmetric switching |
Symmetric switching is one way to characterize a LAN switch according to the bandwidth allocated to each port on the switch. A symmetric switch provides switched connections between ports with the same bandwidth, such as all 10-Mbps ports or all 100-Mbps ports. As shown in Figure an asymmetric LAN switch provides switched connections between ports of unlike bandwidth, such as a combination of 10- Mbps and 100-Mbps ports. Asymmetric switching makes the most of client/server network traffic flows where multiple clients are communicating with a server at the same time, requiring more bandwidth dedicated to the switch port that the server is connected to in order to prevent a bottleneck at that port. As you will learn in the next section, memory buffering in an asymmetric switch is required to allow traffic from the 100-Mbps port to be sent to a 10-Mbps port without causing too much congestion at the 10-Mbps port.
In this lab you will examine an Ethernet Switch to gather information about its physical characteristics and begin to appreciate the function of switches in a network. You will determine the model number and features of a specific switch including which interfaces are present and to which cabling and devices they are connected. |
2.3.8 Memory buffering |
An Ethernet switch may use a buffering technique to store and forward packets to the correct port or ports. Buffering may also be used when the destination port is busy. The area of memory where the switch stores the data is called the memory buffer. This memory buffer can use two methods for forwarding packets--port-based memory buffering and shared memory buffering. In port-based memory buffering, packets are stored in queues that are linked to specific incoming ports. A packet is transmitted to the outgoing port only when all the packets ahead of it in the queue have been successfully transmitted. It is possible for a single packet to delay the transmission of all the packets in memory because of a busy destination port. This delay occurs even if the other packets can be transmitted to open destination ports. Shared memory buffering deposits all packets into a common memory buffer that is shared by all the ports on the switch. The amount of memory allocated to a port is determined by how much is required by each port. This is called dynamic allocation of buffer memory. The packets in the buffer are then linked dynamically to the transmit port--the packet is linked to the memory allocation of that transmit port. This allows the packet to be received on one port and transmitted on another port, without moving it into a different queue. The switch maintains a map of the ports to which a packet needs to be transmitted. The switch clears out this map of destination ports only after the packet has been successfully transmitted. Because the memory buffer is shared, the packet is restricted by the size of the entire memory buffer, not just the allocation to one port. This means that larger packets can be transmitted with fewer dropped packets. This is important to 10/100 switching, where a 100-Mbps port can forward a packet to a 10-Mbps port. |
2.3.9 Two switching methods |
Two switching modes can be used to forward a frame through a switch:
The latency of each switching mode depends on how the switch forwards the frames. The faster the switching mode, the smaller the latency in the switch. To accomplish faster frame forwarding, the switch takes less time to check for errors. The tradeoff is less error checking, which can lead to a higher number of retransmissions. |
2.3.10 How to set up VLANs |
An Ethernet switch physically segments a LAN into individual collision domains. However, each segment is still part of one broadcast domain. The total number of segments on a switch equals one broadcast domain. This means that all nodes on all segments can see a broadcast from a node on one segment. A VLAN is a logical grouping of network devices or users that are not restricted to a physical switch segment. The devices or users in a VLAN can be grouped by function, department, application, and so on, regardless of their physical segment location. A VLAN creates a single broadcast domain that is not restricted to a physical segment and is treated like a subnet. VLAN setup is done in the switch by software. VLANs are now standardized according to IEEE 802.1Q, but implementations vary between vendors.
This lab will help develop a basic understanding of Ethernet switch management and will help prepare for more advanced switching lessons such as VLANs. You will work with the Switch Management Console User Interface Menus to configure some basic switch options. Switch management can be done through a menu-driven interface such as the Management Console or through a command line interface (CLI) as with most routers.
In this lab you will work with the Management Console interface menus to configure a switch to operate in Fragment-Free switching mode. You will also configure a port to enable Full Duplex and Port Fast operation. Most switches can be configured with these options.
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2.4 The Spanning-Tree Protocol | |
2.4.1 Overview of the spanning-tree protocol | |
The main function of the Spanning-Tree Protocol is to allow duplicate switched/bridged paths without incurring the latency effects of loops in the network. Bridges and switches make their forwarding decisions for unicast frames based on the destination MAC address in the frame. If the MAC address is unknown, the device floods the frame out all ports in an attempt to reach the desired destination. It also does this for all broadcast frames. The Spanning-Tree Algorithm, implemented by the Spanning-Tree Protocol, prevents loops by calculating a stable spanning-tree network topology. When creating fault-tolerant networks, a loop-free path must exist between all Ethernet nodes in the network. The Spanning-Tree Algorithm is used to calculate a loop-free path. Spanning-tree frames, called bridge protocol data units (BPDUs), are sent and received by all switches in the network at regular intervals and are used to determine the spanning-tree topology. A switch uses Spanning-Tree Protocol on all Ethernet and Fast Ethernet based VLANs. Spanning-Tree Protocol detects and breaks loops by placing some connections in a standby mode, which are activated in the event of an active connection failure. A separate instance of Spanning-Tree Protocol runs within each configured VLAN, ensuring Ethernet topologies that conform to industry standards throughout the network.
Spanning Tree Protocol
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2.4.2 Describe the five spanning-tree protocol states |
The Spanning-Tree Protocol states are as follows:
The state for each VLAN is initially set by the configuration and later modified by the Spanning-Tree Protocol process. You can determine the status, cost, and priority of ports and VLANs by using the show spantree command. After the port-to-VLAN state is set, Spanning-Tree Protocol determines whether the port forwards or blocks frames. Ports can be configured to immediately enter Spanning-Tree Protocol forwarding mode when a connection is made, instead of following the usual sequence of blocking, learning, and then forwarding. The capability to quickly switch states from blocking to forwarding rather than going through the transitional port states is useful in situations where immediate access to a server is required.
This lab will provide an opportunity to configure a switch for IP and HTTP (Hypertext Transfer Protocol) access. By assigning an IP address to the switch you will be able to ping it and Telnet to it. You will also be able to use your workstation browser (Netscape or Internet Explorer) to connect to the switch and check switch settings and port statistics. |
Summary |
Now that you completed this chapter, you should have a firm understanding of the following:
Threaded Case Study Washington School District Project Task: User Requirements and Project Teams After this chapter, you will begin studying the TCS Overview for the Washington School District Network Design Project. You will also be grouped in project teams, with each team assigned one school site. You need to complete the following tasks:
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Acacia swf
Desert View swf
Mountain Sky swf
RE Miller swf
Royal Palm swf
Sunnyslope swf Sunset swf
All Zipped swf
CCNA Certification Exam Learning Objectives (*** are explicit CCNA Exam objectives; unmarked are knowledge assumed by the exam): LAN Switching
Ethernet
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