1. Module 8: Ethernet Switching
James Chen
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Outline
8.1 Ethernet Switching
Layer 2 bridging
Layer 2 switching
Switch operation
Latency
Switch modes
Spanning-Tree Protocol
8.2 Collision Domains and Broadcast Domains
Shared media environments
Collision domains
Segmentation
Layer 2 broadcasts
Broadcast domains
Introduction to data flow
What is a network segment?
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8.1 Ethernet Switching
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Layer 2 bridging
Ethernet is a shared media.
Only one node can transmit data at a time.
Within Ethernet physical segment
more nodes
more contention
more retransmissions
Break the large segment into parts and separate it into isolated collision domains.
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Layer 2 bridging (cont.)
Example :
Host A is pinging Host B.
The address of Host A is added to its bridge table.
The address of Host B has not been recorded yet as only the source address of a frame is recorded.
Host B processes the ping request and transmits a ping reply back to Host A.
The address of Host B is added to its bridge table.
Host A is now going to ping Host C.
The address of Host C has not been recorded yet as only the source address of a frame is recorded.
Host C processes the ping request and transmits a ping reply back to Host A.
The address of Host C is added to its bridge table.
When Host D transmits data, its MAC address will also be recorded in the bridge table.
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Layer 2 bridging (cont.)
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Layer 2 switching
Generally, 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 and do not affect the logical or Layer 3 addressing.
A switch dynamically builds and maintains a Content-Addressable Memory (CAM) table, holding all of the necessary MAC information for each port.
A bridge will divide a collision domain but has no effect on a logical or broadcast domain.
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Switch operation
A switch is essentially a multi-port bridge.
When only one host is connected to a switch port, the two nodes (the switch port & host) share this small segment, or collision domain. The small physical segment is called microsegment.
Most switches are capable of supporting full duplex.
No contention for the full duplex media.
The bandwidth is doubled when using full duplex.
Content-addressable memory (CAM) is memory that essentially works backwards compared to conventional memory.
Entering data into the memory will return the associated address.
Using CAM allows a switch to directly find the port that is associated with a MAC address without using search algorithms.
Application-specific integrated circuit (ASIC) -> speed up
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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.
A wide variety of conditions can cause delays as a frame travels from source to destination:
Media delays caused by the finite speed (10/100/1000Mbps) that signals can travel through the physical media.
Circuit delays caused by the electronics that process the signal along the path.
Software delays caused by the decisions that software must make to implement switching and protocols.
Delays caused by the content of the frame.
For example, a device cannot route a frame to a destination until the destination MAC address has been read. (RARP in routers)
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Switch modes
How a frame is switched to the destination port is a trade off between latency and reliability.
Cut-through
A switch can start to transfer the frame as soon as the destination MAC address is received.
Store-and-forward
The switch receives the entire frame before sending it out the destination port.
To verify the Frame Check Sum (FCS).
Fail > it is discarded.
Fragment-free
The switch reads the first 64 bytes (frame header).
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 correct.
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Switch modes (cont.)
Synchronous switching
Both the source port and destination port must be operating at the same bit rate.
cut-through
Asynchronous switching
The bit rates of both sides are not the same, the frame must be stored at one bit rate before it is sent out at the other bit rate.
store-and-forward
Asymmetric switching
It provides switched connections between ports of unlike bandwidths.
It is optimized for client/server traffic flows in which multiple clients simultaneously communicate with a server, requiring more bandwidth dedicated to the server port to prevent a bottleneck at that port.
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12. Spanning-Tree Protocol
To prevent switch loops and broadcast storms.
Usually caused by design errors or accident.
redundant paths ： to provide for reliability and fault tolerance
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:
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13. Spanning-Tree Protocol(cont.)
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8.2 Collision Domains and Broadcast Domains
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15. Shared media environments
Layer 1 media and topologies :
Shared media environment
Extended shared media environment
Accommodate for multiple access or longer cable distances.
Point-to-point network environment
dialup network connections.
Collisions only occur in a shared environment.
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Collision domains
Collisions cause the network to be inefficient.
All transmission stops for a period of time.
The length of this period of time without transmissions varies and is determined by a backoff algorithm for each network device. 
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17. Collision domains (cont.)
Layer 1 devices do not break up collision domains,
Layer 2 and Layer 3 devices do break up collision domains.
Breaking up, or increasing the number of collision domains with Layer 2 and 3 devices is also known as segmentation. 
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18. Collision domains (cont.)
In a small network a single collosion domain can work just fine as there is little contention for the network media. This type of network is fine for an isolated network that does not require much data transmission.
But as the network starts to grow, the contention for the line becomes greater and a larger number of collisions start to occur.
As the network continues to grow, the contention for the line becomes greater and even starts to effect the performance of the computers on the network.
Finally when the collision domain becomes too big and network transmission demands become too great. The number of collisions practically shuts the network down.
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19. Collision domains (cont.)
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.
A late collision is when a collision happens after the first 64 bytes (512 bits) of the frame are transmitted.
The chipsets in NICs are not required to retransmit automatically when a late collision occurs.
The rule :
5 segments of network media
4 repeaters or hubs
3 host segments of the network
2 link sections (no hosts)
1 large collision domain
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Round_Trip Delay
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Segmentation
Layer 2 devices segment or divide collision domains.
Keep tracking of the MAC addresses and which segment they are on.
Layer 3 devices, like Layer 2 devices, do not forward collisions.
Layer 3 devices and their functions will be covered in more depth in the section on broadcast domains. 
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Layer 2 broadcasts
Destination MAC address 0xFFFFFFFFFFFF
Layer 2 devices must flood all broadcast and multicast traffic.
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23. Layer 2 broadcasts (cont.)
Because the NIC must interrupt the CPU to process each broadcast or multicast group it belongs to (no discard), broadcast radiation affects the performance of hosts in the network.
Workstations broadcast an Address Resolution Protocol (ARP) request every time they need to locate a MAC address that is not in the ARP table.
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Broadcast domains
Broadcasts are forwarded by Layer 2 devices.
Broadcast domains are controlled at Layer 3 because routers do not forward broadcasts.
Layer 3 forwarding is based on the destination IP address and not the MAC address.
Use router to segment broadcast domains.
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25. Introduction to data flow
Layer 1 devices do no filtering, so everything that is received is passed on to the next segment.
Layer 2 devices filter data frames based on the destination MAC address.
Layer 3 devices filter data packets based on IP destination address.
Data flow through a routed IP based network.
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26. What is a network segment?
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END
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