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www.ciscopress.comCopyright 2003
CCNA 1 Chapter 6, part 2
Ethernet Switching
By
Your Name
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Objectives
• Ethernet switching
• Collision domains and broadcast domains
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Layer 2 Bridging
• Based on the Ethernet frame design, the need arose for a device that could hold a learned list of MAC addresses and direct frames to a specific port, based on the MAC address of the destination node.
• Describe the process.
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Bridge Types
A bridge connects network segments and makes intelligent decisions about whether to pass signals on to the next segment.
Source route bridges are used primarily with Token Ring network segments.
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Bridge Operations
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Switches
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Switch Benefits
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Switching Table
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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.
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Switch Modes
• Cut-through switching — A switch can start to transfer the frame as soon as the destination MAC address is received.
• Store-and-forward switching — The 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).
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Spanning Tree Protocol
• Loops can occur when extra switches and bridges are added to provide redundant paths for reliability and fault tolerance.
• A switch sends special messages called bridge protocol data units (BPDUs) out all its ports to let other switches know of its existence.
• The switches use a spanning-tree algorithm (STA) to resolve and shut down the redundant paths
• The protocol used to resolve and eliminate loops is known as
the Spanning Tree Protocol (STP).
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Collision Domains and Broadcast Domains
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Shared Media Environments
• It is important to be able to identify a shared media environment, because collisions only occur in a shared environment.
• Some networks are directly connected and all hosts share Layer 1:– Shared media– Extended shared media– Point-to-point network
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Collisions and Collision Domains
• Shared media environment• Collisions and collision domains• Signals in a collision• Repeaters, hubs, and collision domains• The four-repeater rule• Segmenting collision domains
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Types of Networks
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Data Collisions
When two bits are propagated at the same time on the same network, a collision will occur.
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Collisions and Collision Domains
• Collisions are not inherently bad.– They are a normal function of Legacy Ethernet.
• Data on the network during a collision is lost and usually must be retransmitted.– Increased collisions indicate congestion.
• All devices on a network that would cause a collision if they transmitted simultaneously are in a collision domain.
• Networks with only Layer 1 components are a single collision domain.
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Shared Media, Repeaters, Hubs, and Collision Domains
Shared media
Extended by a repeater
Extended by a hub
Extended by a huband repeater
Each is a single collision domain!
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The Four-Repeater Rule
Legacy Ethernet network: No more than 4 repeaters or repeating hubs can be between any 2 computers on the network.– From A to B is 4 repeaters.
A
B
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Four-Repeater Rule Example
• The 5-story building shown violates the four-repeater rule because host A and B are 5 repeaters apart.
• Hubs would cause the same result.
• Even if all servers were on the third floor, and A and B would never communicate directly; they are too far to hear each other transmit and can cause data collisions.
• What are implications for taller buildings?
A
B
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Four-Repeater Rule Example Layer 1 Solution
• The hub added, which could be on any floor, allows us to comply with the four-repeater rule.
• No 2 hosts are more than 3 repeaters apart.
• What are implications for taller buildings? It really wouldn’t matter if each floor connects to the hub.
• How many collision domains do we have? Still only one and getting bigger with each floor.
A
B
Hub
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Too Much Traffic / Too Many Hosts in a Collision Domain…
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Ethernet LAN Segmentation
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Segmenting with Bridges
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Segmenting with Switches
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Segmenting with Routers
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Microsegmentation
Hubs
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Layer 2 Broadcasts
• Layer 2 devices must flood all broadcast and multicast traffic.
• The accumulation of broadcast and multicast traffic from each device in the network is referred to as broadcast radiation.
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The only devices that can segment collision domains are bridges, switches (both Layer 2), and routers (Layer 3).
Broadcast Domains
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Data Flow
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Network Segment
• If the segment is used in TCP, it would be defined as a separate piece of the data.
• If segment is being used in the context of physical networking media in a routed network, it would be seen as one of the parts or sections of the total network.