Chapter 7 - TechnologyProServices

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Chapter 7
-
Ethernet LAN Switching Concepts

•
LAN Switching Concepts

•
LAN Design Considerations

LAN Switching Concepts

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Hubs

–
Hubs led to too much congestion

–
Only one device could send at a time

–
Shared bandwidth

•
Bridges

–
Usually two ports

–
Segment

network into 2 collision domains

–
Reduce collisions and improve network performance

•
More effective bandwidth

Switching Logic

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Based on source and dest MAC address

•
Unicast, Broadcast or Multicast

1.
Deciding when to forward a frame or when to
filter (not forward) a frame, based on the
destination MAC address

2.
Learning MAC addresses by examining the
source MAC address of each frame received by
the bridge

3.
Creating a (Layer 2) loop
-
free environment with
other bridges by using Spanning Tree Protocol
(STP)

How Switches Learn Addresses

•
Listen to incoming Frames

–
Examine source MAC address

•
If not in table
-
add it and associate with interface


Flooding Frames

•
If no MAC addresses match the
destination
-
send to everyone

–
Unknown Unicast frames

•
Flooding

•
Interactivity Timer

–
Track how long since MAC address has been
used

–
Discard oldest when full

Spanning Tree Protocol

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Prevent Loops

•
Networks often built with redundant links

–
Good design

•
How to shut down the redundant links to
prevent broadcast loops or flood loops

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Ports are
Forwarding

or
Blocking

Internal Processing

•
How do we process the frames

•
store
-
and
-
forward processing
.

–
switch must receive the entire frame before forwarding the first bit of the
frame.

•
cut
-
through

–
switch starts sending the frame out the output port as soon as possible.
Although this might reduce latency, it also propagates errors. Because
the frame check sequence (FCS) is in the Ethernet trailer, the switch
cannot determine if the frame had any errors before starting to forward
the frame

•
Fragment
-
free processing

–
works similarly to cut
-
through, but it tries to reduce the number of
errored frames that it forwards. One interesting fact about Ethernet
carrier sense multiple access with collision detection (CSMA/CD) logic
is that collisions should be detected within the first 64 bytes of a frame.
Fragment
-
free processing works like cut
-
through logic, but it waits to
receive the first 64 bytes before forwarding a frame.

Switching Summary

•
Switch ports connected to a single device microsegment
the LAN, providing dedicated bandwidth to that single
device.

•
Switches allow multiple simultaneous conversations
between devices on different ports.

•
Switch ports connected to a single device support full
duplex, in effect doubling the amount of bandwidth
available to the device.

•
Switches support rate adaptation, which means that
devices that use different Ethernet speeds can
communicate through the switch (hubs cannot).


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Design Considerations

•
Collision Domains and Broadcast Domains

•
A
collision domain
is a set of network interface
cards (NIC) for which a frame sent by one NIC
could result in a collision with a frame sent by
any other NIC in the same collision domain.

•
A
broadcast domain
is a set of NICs for which a
broadcast frame sent by one NIC is received by
all other NICs in the same broadcast domain.

VLANS

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A LAN is all devices in the same
Broadcast Domain

•
VLANS let you assign switch ports to
different Broadcast Domain

–
Acts like separate switches

–
Need a router to connect broadcast domains


VLAN Benefits

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To create more flexible designs that group users
by department, or by groups that work together,
instead of by physical location

•
To segment devices into smaller LANs
(broadcast domains) to reduce overhead caused
to each host in the VLAN

•
To reduce the workload for STP by limiting a
VLAN to a single access switch

•
To enforce better security by keeping hosts that
work with sensitive data on a separate VLAN

•
To separate traffic sent by an IP phone from
traffic sent by PCs connected to the phones

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LAN Terminology

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Access, Distribution and Core

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Access:
Provides a connection point (access)
for end
-
user devices. Does not forward frames
between two other access switches under
normal circumstances.

•
Distribution:
Provides an aggregation point for
access switches, forwarding framesbetween
switches, but not connecting directly to end
-
user
devices.

•
Core:
Aggregates distribution switches in very
large campus LANs, providing very high
forwarding rates.

Ethernet Standards and Cable Lengths

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