Switches and broadcast domains

Switches and broadcast domains 
4.3.8 This page will describe the three methods of data transmission that are used in a network.
Communication in a network occurs in three ways. The most common way of communication is by unicast transmissions. In a unicast transmission, one transmitter tries to reach one receiver.
Another way to communicate is known as a multicast transmission. Multicast transmission occurs when one transmitter tries to reach only a subset, or a group, of the entire segment. 
The final way to communicate is by broadcasting. Broadcasting is when one transmitter tries to reach all the receivers in the network. The server station sends out one message and everyone on that segment receives the message.
When a device wants to send out a Layer 2 broadcast, the destination MAC address in the frame is set to all ones. A MAC address of all ones is FF:FF:FF:FF:FF:FF in hexadecimal. By setting the destination to this value, all the devices will accept and process the broadcasted frame.
The broadcast domain at Layer 2 in referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive frame broadcasts by a host to all other machines on the LAN.
A switch is a Layer 2 device. When a switch receives a broadcast, it forwards it to each port on the switch except the incoming port. Each attached device must process the broadcast frame. This leads to reduced network efficiency, because available bandwidth is used for broadcasting purposes. 
When two switches are connected, the broadcast domain is increased. In this example a broadcast frame is forwarded to all connected ports on Switch 1. Switch 1 is connected to Switch 2. The frame is propagated to all devices connected to Switch 2. 
The overall result is a reduction in available bandwidth. This happens because all devices in the broadcast domain must receive and process the broadcast frame.
Routers are Layer 3 devices. Routers do not propagate broadcasts. Routers are used to segment both collision and broadcast domains.
The next page will explain how a workstation connects to a LAN.

Switches and collision domains

Switches and collision domains 
4.3.7 This page will discuss collisions, which is a major disadvantage of Ethernet 802.3 networks.
A major disadvantage of Ethernet 802.3 networks is collisions. Collisions occur when two hosts transmit frames simultaneously. When a collision occurs, the transmitted frames are corrupted or destroyed in the collision. The sending hosts stop sending further transmissions for a random period of time, based on the Ethernet 802.3 rules of CSMA/CD. Excessive collisions cause networks to be unproductive.
The network area where frames originate and collide is called the collision domain. All shared media environments are collision domains. When a host is connected to a switch port, the switch creates a dedicated connection. This connection is considered to be an individual collision domain. For example, if a twelve-port switch has a device connected to each port then twelve collision domains are created.
A switch builds a switching table by learning the MAC addresses of the hosts that are connected to each switch port. When two connected hosts want to communicate with each other, the switch looks up the switching table and establishes a virtual connection between the ports. The virtual circuit is maintained until the session is terminated.
In Figure , Host B and Host C want to communicate with each other. The switch creates the virtual connection which is referred to as a microsegment. The microsegment behaves as if the network has only two hosts, one host sending and one receiving providing maximum utilization of the available bandwidth.
Switches reduce collisions and increase bandwidth on network segments because they provide dedicated bandwidth to each network segment.
The next page will discuss three methods of data transmission in a network. 

Microsegmentation implementation

Microsegmentation implementation 
4.3.6 This page will explain the functions of a switch in a LAN due to microsegmentation.
LAN switches are considered multi-port bridges with no collision domain, because of microsegmentation. Data is exchanged at high speeds by switching the frame to its destination. By reading the destination MAC address Layer 2 information, switches can achieve high-speed data transfers, much like a bridge does. This process leads to low latency levels and a high rate of speed for frame forwarding.  
Ethernet switching increases the bandwidth available on a network. It does this by creating dedicated network segments, or point-to-point connections, and connecting these segments in a virtual network within the switch. This virtual network circuit exists only when two nodes need to communicate. This is called a virtual circuit because it exists only when needed, and is established within the switch.
Even though the LAN switch reduces the size of collision domains, all hosts connected to the switch are still in the same broadcast domain. Therefore, a broadcast from one node will still be seen by all the other nodes connected through the LAN switch.
Switches are data link layer devices that, like bridges, enable multiple physical LAN segments to be interconnected into a single larger network. Similar to bridges, switches forward and flood traffic based on MAC addresses. Because switching is performed in hardware instead of in software, it is significantly faster. Each switch port can be considered a micro-bridge acting as a separate bridge and gives the full bandwidth of the medium to each host.
The next page will discuss collisions.

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Why segment LANs?

Why segment LANs? 
4.3.5 Highlight that there are two primary reasons for segmenting a LAN. The first is to isolate traffic between segments. The second reason is to achieve more bandwidth per user by creating smaller collision domains. By this stage, students have heard of this term several times but instructors are encouraged to make sure that students understand the difference between collision and broadcast domains. The three figures are particularly useful.

This page will explain the two main reasons to segment a LAN.
There are two primary reasons for segmenting a LAN. The first is to isolate traffic between segments. The second reason is to achieve more bandwidth per user by creating smaller collision domains.
Without LAN segmentation, LANs larger than a small workgroup could quickly become clogged with traffic and collisions.
LAN segmentation can be implemented through the utilization of bridges, switches, and routers. Each of these devices has particular pros and cons.
With the addition of devices like bridges, switches, and routers the LAN is segmented into a number of smaller collision domains. In the example shown, four collision domains have been created.
By dividing large networks into self-contained units, bridges and switches provide several advantages. Bridges and switches will diminish the traffic experienced by devices on all connected segments, because only a certain percentage of traffic is forwarded. Bridges and switches reduce the collision domain but not the broadcast domain.
Each interface on the router connects to a separate network. Therefore the insertion of the router into a LAN will create smaller collision domains and smaller broadcast domains. This occurs because routers do not forward broadcasts unless programmed to do so.
A switch employs "microsegmentation" to reduce the collision domain on a LAN. The switch does this by creating dedicated network segments, or point-to-point connections. The switch connects these segments in a virtual network within the switch.
This virtual network circuit exists only when two nodes need to communicate. This is called a virtual circuit as it exists only when needed, and is established within the switch.
The next page will discuss microsegmentation.

How switches and bridges filter frames

How switches and bridges filter frames 
4.3.4 If the frame is addressed for another LAN, the bridge copies the frame onto the second LAN. Ignoring a frame is called filtering. Copying the frame is called forwarding.
Emphasize that a bridge is considered a store-and-forward device because it must examine the destination address field and calculate the cyclic redundancy check (CRC) in the frame check sequence field before forwarding the frame to all ports. Students may need a further explanation of the term CRC. Encourage them to check the glossary for an explanation of this term. 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 and causes increased latency.

This page will explain how switches and bridges filter frames. In this discussion, the terms “switch” and “bridge” are synonymous.
Most switches are capable of filtering frames based on any Layer 2 frame field. For example, a switch can be programmed to reject, not forward, all frames sourced from a particular network. Because link layer information often includes a reference to an upper-layer protocol, switches can usually filter on this parameter. Furthermore, filters can be helpful in dealing with unnecessary broadcast and multicast packets.
Once the switch has built the local address table, it is ready to operate. When it receives a frame, it examines the destination address. If the frame address is local, the switch ignores it. If the frame is addressed for another LAN segment, the switch copies the frame onto the second segment.

• Ignoring a frame is called filtering.
• Copying the frame is called forwarding.

Basic filtering keeps local frames local and sends remote frames to another LAN segment.
Filtering on specific source and destination addresses performs the following actions:

• Stopping one station from sending frames outside of its local LAN segment
• Stopping all "outside" frames destined for a particular station, thereby restricting the other stations with which it can communicate

Both types of filtering provide some control over internetwork traffic and can offer improved security.
Most Ethernet switches can now filter broadcast and multicast frames. Bridges and switches that can filter frames based on MAC addresses can also be used to filter Ethernet frames by multicast and broadcast addresses. This filtering is achieved through the implementation of virtual local-area networks or VLANs. VLANs allow network administrators to prevent the transmission of unnecessary multicast and broadcast messages throughout a network. Occasionally, a device will malfunction and continually send out broadcast frames, which are copied around the network. This is called a broadcast storm and it can significantly reduce network performance. A switch that can filter broadcast frames makes a broadcast storm less harmful.
Today, switches are also able to filter according to the network-layer protocol. This blurs the demarcation between switches and routers. A router operates on the network layer using a routing protocol to direct traffic around the network. A switch that implements advanced filtering techniques is usually called a brouter. Brouters filter by looking at network layer information but they do not use a routing protocol.
The next page will explain how switches are used to segment a LAN. 

How switches and bridges learn addresses

How switches and bridges learn addresses 
4.3.3 This page will explain how bridges and switches learn addresses and forward frames.
Bridges and switches only forward frames that need to travel from one LAN segment to another. To accomplish this task, they must learn which devices are connected to which LAN segment. 
A bridge is considered an intelligent device because it can make decisions based on MAC addresses. To do this, a bridge refers to an address table. When a bridge is turned on, broadcast messages are transmitted asking all the stations on the local segment of the network to respond. As the stations return the broadcast message, the bridge builds a table of local addresses. This process is called learning.
Bridges and switches learn in the following ways:

• Reading the source MAC address of each received frame or datagram
• Recording the port on which the MAC address was received

In this way, the bridge or switch learns which addresses belong to the devices connected to each port.
The learned addresses and associated port or interface are stored in the addressing table. The bridge examines the destination address of all received frames. The bridge then scans the address table searching for the destination address.
The switching table is stored using Content Addressable Memory (CAM). CAM is used in switch applications to perform the following functions:

• To take out and process the address information from incoming data packets
• To compare the destination address with a table of addresses stored within it

The CAM stores host MAC addresses and associated port numbers. The CAM compares the received destination MAC address against the CAM table contents. If the comparison yields a match, the port is provided, and the switch forwards the packet to the correct port and address. 
An Ethernet switch can learn the address of each device on the network by reading the source address of each frame 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 CAM. When a source address 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.
The processes followed by the CAM are as follows:

1. If the address is not found, the bridge forwards the frame out all ports except the port on which it was received. This process is called flooding. The address may also have been deleted by the bridge because the bridge software was recently restarted, ran short of address entries in the address table, or deleted the address because it was too old. Since the bridge does not know which port to use to forward the frame, it will send it to out all ports, except the one from which it was received. It is clearly unnecessary to send it back to the same cable segment from which it was received, since any other computer or bridges on this cable must already have received the packet.
2. If the address is found in an address table and the address is associated with the port on which it was received, the frame is discarded. It must already have been received by the destination.
3. If the address is found in an address table and the address is not associated with the port on which it was received, the bridge forwards the frame to the port associated with the address.

If the address is found in an address table and the address is not associated with the port on which it was received, the bridge forwards the frame to the port associated with the address.
The next page will describe the process that is used to filter frames.