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CCNA 1

Module 8: Ethernet Switching

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Layer 2 Bridging •By increasing the number of nodes on a single segment, the probability of collisions increases, resulting in more retransmissions. A solution to the problem is to break the large segment into parts and separate it into isolated collision domains.

•A bridge keeps a table of MAC addresses and the associated ports. The bridge then forwards or discards frames based on the table entries

•A bridge has only two ports and divides a collision domain into two parts. All decisions made by a bridge are based on MAC or Layer 2 addressing. A bridge will create more collision domains but will not add broadcast domains.

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Layer 2 Switching•A switch is essentially a fast, multi-port bridge, which can contain dozens of ports.  Rather than creating two collision domains, each port creates its own collision domain.

•In full duplex mode, there is no contention for the media. Thus, a collision domain no longer exists. Theoretically, the bandwidth is doubled when using full duplex.

• A switch dynamically builds and maintains a Content-Addressable Memory

(CAM) table, holding all of the necessary MAC information for each port.

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Latency•Latency is the delay between the time a frame first starts to leave the source device and the time the first part of the frame reaches its destination

•Source of delay

– Media delays

– Circuit delays

– Software delays

– Delays caused by the content of the frame

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Switch Modes•Store-and-forward

– A switch can receive the entire frame before sending it out the destination port. This gives the switch software an opportunity to verify the Frame Check Sum (FCS) to ensure that the frame was reliably received before sending it to

the destination •Cut-through switching

– A switch can start to transfer the frame as soon as the destination MAC address is received which results in the lowest latency through the switch. However, no error checking is available.

•Fragment –free

– Fragment-free reads the first 64 bytes, which includes the frame header, and switching begins before the entire data field and checksum are read. This mode verifies the reliability of the addressing and Logical Link Control (LLC) protocol information to ensure the destination and handling of the data will be

•Asymmetric switching provides switched connections between ports of unlike bandwidths

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•Switched networks are often designed with redundant paths to provide for reliability and fault tolerance.

•Switching loops can occur by design or by accident, and they can lead to broadcast storms that will rapidly overwhelm a network.

•Each switch in a LAN using STP sends special messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence and to elect a root bridge for the network. The switches then use the Spanning-Tree Algorithm (STA) to resolve and shut down the redundant paths.

•Each port on a switch using Spanning-Tree Protocol exists in one of the following five states: Blocking, Listening, Learning, Forwarding, Disabled

Spanning Tree Protocol (STP)

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Directly Connected NetworksThere are three situations of directly connected networks:

shared media extended shared media point to point

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Directly Connected Networks•Shared media environments occur when multiple hosts have

access to the same medium. Ethernet is a shared-media

environment.

•A special case of shared media environments is the extended

shared media environment. Using networking devices can

extend the medium which is being "shared". Even more multiple-

access, or more users, can be accommodated.

•Point to point network is most widely used in WANs where one

networking device is connected to precisely one other device via

a link.

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CollisionsUsing many computers connected to the same network wanting to communicate billions of bits every second means we must consider what happens when two bits are on the wire, or optical fiber, or on the same wireless frequency at the same time. •With Ethernet, only one data packet can be on the cable at any one time. •If more than one node attempts to transmit at the same time, a collision will occur. •When a collision occurs, the data from each device impact and are damaged. •The network area within which data packets originate and collide is called a collision domain.

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Collisions

•Both packets are "destroyed", bit by bit, (this signals that there was a collisions) and some process for handling the competition for the medium, also called contention, must be implemented.

•The digital system can only tell two voltage or light or electromagnetic wave states, and in a collision, the signals interfere, or collide, with each other. This creates a third voltage which is unrecognizable.

•A certain amount of collisions are a natural function of a shared media environment because large numbers of computers are all trying to communicate with each other, at the same time, by using the same wire.

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Collision Domains

If you have N computers connected to a single medium with no other networking devices, a basic shared access situation, you have a collision domain.

Basically, all computers on a single shared access media are a collision domain.

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Collision Domains•Since repeaters amplify and re-time bits, with no filtering based on source and destination addresses of the packets of bits which pass through them, using a repeater simply extends the collision domain.

•Thus the network on both sides of the repeater is one larger collision domain.

•Adding one or more repeatersdoes not change the numberof collision domains, it onlymakes it larger.

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Collision Domains•Any signal which comes in one port of the hub is amplified and re-timed is sent out every other port.

•Hubs, which are so useful for connecting large number of computers, begin to have diminished performance if all the computers have large bandwidth demands simultaneously.

•Remember another name for the hub -- the multiport repeater.

•The hub doesn’t create another collision domain, it simply extends it.

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Collision Domains•To assure that a repeated 10BASE-T network will function properly, the round-trip delay calculation must be within certain limits otherwise all the workstations will not be able to hear all the collisions on the network.

•Repeater latency, propagation delay, and NIC latency all contribute to the four repeater rule.

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Separating Collision Domains•The size of collision domains can be reduced by using intelligent networking devices that break up the domains.

•Examples of this type of networking device are bridges, switches,

and routers. •Separating collision domains by using bridges, switches, and routers is called segmentation

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•When a node needs to communicate with all hosts on the network, it sends a broadcast frame with a destination MAC address 0xFFFFFFFFFFFF

•If too many broadcasts are sent out over the network a broadcast storm can result

•It can cause network time-outs

•It causes traffic slowdowns

•It causes the network to operate at less than optimal

performance.

Layer 2 broadcasts

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•A broadcast domain is a grouping of collision domains that are connected by Layer 2 devices

•Broadcasts have to be controlled at Layer 3, as Layer 2 and Layer 1 devices have no way of controlling them because routers do not forward broadcasts.

•Routers actually work at Layers 1, 2, and 3. They, like all Layer 1 devices, have a physical connection to, and transmit data onto, the media. They have a Layer 2 encapsulation on all interfaces and perform just like any other Layer 2 device. It is Layer 3 that allows the router to segment broadcast domains

•Layer 3 forwarding is based on the destination IP address and not the MAC address.

Broadcast Domains

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Data Flow through a network•Data flow through a routed IP based network, involves data moving across traffic management devices at Layers 1, 2, and 3 of the OSI model. Layer 1 is used for transmission across the physical media, Layer 2 for collision domain management, and Layer 3 for broadcast domain management.

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Network Segment