Chapter 7 - TechnologyProServices

canoeornithologistNetworking and Communications

Oct 26, 2013 (4 years and 8 months ago)


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Chapter 7
Ethernet LAN Switching Concepts

LAN Switching Concepts

LAN Design Considerations

LAN Switching Concepts


Hubs led to too much congestion

Only one device could send at a time

Shared bandwidth


Usually two ports


network into 2 collision domains

Reduce collisions and improve network performance

More effective bandwidth

Switching Logic

Based on source and dest MAC address

Unicast, Broadcast or Multicast

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

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

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

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
send to everyone

Unknown Unicast frames


Interactivity Timer

Track how long since MAC address has been

Discard oldest when full

Spanning Tree Protocol

Prevent Loops

Networks often built with redundant links

Good design

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

Ports are


Internal Processing

How do we process the frames

forward processing

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


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

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.
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

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

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.

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.


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

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

Access, Distribution and Core

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

Provides an aggregation point for
access switches, forwarding framesbetween
switches, but not connecting directly to end

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

Ethernet Standards and Cable Lengths

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