Chapter 7 - TechnologyProServices

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26 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Chapter 7
-
Ethernet LAN Switching Concepts


LAN Switching Concepts


LAN Design Considerations

LAN Switching Concepts


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


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


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
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).


Any Questions?

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


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

Any Questions?

LAN Terminology


Access, Distribution and Core


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

Any Questions?