Transcript Chapter 8

Semester 1 Module 8
Ethernet Switching
Andres, Wen-Yuan Liao
Department of Computer Science and
Engineering
De Lin Institute of Technology
[email protected]
http://www.cse.dlit.edu.tw/~andres
Overview
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Define bridging and switching
Define and describe the content-addressable
memory (CAM) table
Define latency
Describe store-and-forward and cut-through
packet switching modes
Explain Spanning-Tree Protocol (STP)
Define collisions, broadcasts, collision domains,
and broadcast domains
Identify the Layers 1, 2, and 3 devices used to
create collision domains and broadcast domains
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Discuss data flow and problems with broadcasts
Explain network segmentation and list the
devices used to create segments
Outline
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Ethernet Switching
Collision Domains and Broadcast
Domains
Layer 2 bridging
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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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Host A is pinging Host B.
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Host A is now going to ping Host C.
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The address of Host A is added to its bridge table.
The address of Host B has not been recorded yet.
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.
The address of Host C has not been recorded yet.
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.
Layer 2 switching
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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.
Switch operation
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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 media.
The bandwidth is doubled when using full duplex.
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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.
Latency
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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.
Switch modes
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How a frame is switched to the destination port
is a trade off between latency and reliability.
Cut-through
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A switch can start to transfer the frame as soon as the
destination MAC address is received.
No error checking.
Lowest latency.
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Store-and-forward
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The switch receives the entire frame before sending it
out the destination port.
To verify the Frame Check Sum (FCS).
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If the frame is invalid, it is discarded at the switch. .
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Fragment-free
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The switch reads the first 64 bytes (include 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.
Spanning-Tree Protocol
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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.
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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:
Outline
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Ethernet Switching
Collision Domains and Broadcast
Domains
Shared media environments
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Layer 1 media and topologies :
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Shared media environment.
Extended shared media environment.
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Point-to-point network environment
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Accommodate for multiple access or longer cable
distances.
Dialup network connections.
Collisions only occur in a shared environment.
Collision domains
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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.
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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The 5-4-3-2-1 rule (repeater):
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5 segments of network media
4 repeaters or hubs
3 host segments of the network
2 link sections (no hosts)
1 large collision domain
Segmentation
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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.
Layer 2 broadcasts
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Destination MAC address 0xFFFFFFFFFFFF.
Layer 2 devices must flood all broadcast and
multicast traffic.
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.
Broadcast domains
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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.
Introduction to data flow
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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.
What is a network segment?
Good luck in your exams !