S3C2 – LAN Switching - YSU Computer Science & Information ...

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26 Οκτ 2013 (πριν από 3 χρόνια και 8 μήνες)

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S3C2


LAN Switching

Addressing LAN Problems

Congestion is Caused By


Multitasking,


Faster operating systems,


More Web
-
based applications


Client
-
Server


client/server applications allow administrators to
centralize information, thus making it easy to maintain
and protect.


Point
-
to
-
point (host to host) connection is collision
free

LANS Impacted By:


The data frame broadcast delivery nature of
Ethernet/802.3 LANs


Carrier sense multiple access collision detect
(CSMA/CD) access methods allowing only one
station to transmit at a time


Multimedia applications with higher bandwidth
demand such as video and the Internet, coupled
with the broadcast nature of Ethernet, can create
network congestion.



Impact Issues Continued


Normal latency as the frames travel across
the Layer 1 medium and through Layer 1, 2,
and 3 networking devices, and the latency
added by the extension of Ethernet/802.3
LANs by adding repeaters


Extending the distances of the
Ethernet/802.3 LANs by using Layer 1
repeaters

Latency


Latency is caused by:


the time it takes the source NIC to place voltage
pulses on the wire and the time it takes the
receiving NIC to interpret these pulses. This is
sometimes called NIC delay (it is typically
around 1 microsecond for10BASE
-
T NICs).


A
byte takes a minimum of 800 ns

to transmit


Latency Continued


Second, there is the actual propagation delay as the
signal takes time
--

albeit a very short time
--

to actually
travel down the cable (it is typically about .556
microseconds per 100 m for Cat 5 UTP). The longer the
cable, the more propagation delay; the slower the
nominal velocity of propagation (NVP) of the cable, the
more the propagation delay.



Third, latency is added according to which networking
devices
--

whether they be Layer 1, 2, or 3 (and how
they are configured)
--

are added in the path between
the two communicating computers. The actual
transmission time (the duration of the host actually
sending bits) must also be included in understanding
timing on networks.


Half Duplex or Duplex


Ethernet LANs are half
-
duplex technology


Full
-
duplex Ethernet allows the transmission of a
packet and the reception of a different packet at
the same time


requires full duplex NIC card


This simultaneous transmission and reception requires
the use of two pairs of wires in the cable and a switched
connection between each node. This connection is
considered point
-
to
-
point and is collision free.



Because both nodes can transmit and receive at the
same time, there are no negotiations for bandwidth.
Full
-
duplex Ethernet can use an existing shared
medium as long as the medium meets minimum
Ethernet standards. Requires 2 pairs of wires and
switched connection

Benefit of Full Duplex


Ethernet usually can only use 50%
-
60% of the 10
-
Mbps available bandwidth because of collisions
and latency.


Full
-
duplex Ethernet offers 100% of the
bandwidth in both directions.


This produces a potential 20
-
Mbps throughput
-

10
-
Mbps TX and 10
-
Mbps RX.


Remember


Transmit connects to Receive


Think SIMPLE LAN

Why Segment


Isolates certain traffic


Creates smaller collision domains


Data are passed between segments using
bridges, switches, or routers


IMPORTANT


Decreasing size of collision domains increases
the number of collision domains

Bridges


Create tables to match segments and MAC
addresses


Layer 3 protocol independent


Stores and then forwards based on MAC
address


Increases latency on network (10
-
30%)


Creates smaller collision domains but
increases number of collision domains

Routers


Operate at Layer 3 using IP addresses


More manageable, greater functionality, multiple
paths


Smaller collision domains


Introduce latency


Protocols that provide acknowledgements introduce
30
-
40% delays


Protocols that provide minimal acknowledgements have
20%
-
30% loss in throughput


So routers introduce 20
-
40% delay depending on
protocol

Switches/Switched Ethernet


Low latency and high frame
-
forwarding rates


Eliminates impact of collisions through micro
-
segmentation


Works with existing 802.3 standards


Create dedicated network segments (point to
point) with full bandwidth


virtual circuits


Create collision free domains


Cost more than bridges or routers

Bridges vs Switches


Both bridges and switches connect LAN
segments, use a table of MAC addresses to
determine the segment on which a datagram needs
to be transmitted, and reduce traffic.


Switches are more functional in today’s networks
than bridges because they operate at much higher
speeds than bridges and can support new
functionality, such as virtual LANs (VLANs).



Bridges typically switch using hardware; switches
typically switch using software.


Layer 2 Switching


With Layer 2 switching, frames are switched
based on MAC address information.


If the Layer 2 switch does not know where to send
the frame, it broadcasts the frame out all its ports
to the network to learn the correct destination


the switch learns the location of the new address
and adds the information to the switching table


MAC Address and ports

Virtual LANs


dedicated paths between sending and
receiving hosts within the switch are
temporary. The switch’s power comes from
the fact that it can rapidly make and break
these 1 to 1 connections through its
various ports, depending upon the data in
its switching table.

How A Switch Learns Addresses


Examines source address


Sends out all ports except incoming port when
address is unknown, multicast, or broadcast


Forwards when the destination is at a different
interface


Filters when the destination is on the same
interface


Date stamps each address


discards after a certain
time period


Addresses stored in CAM


Content Addressable
Memory

Benefits of Switching


Number of collisions reduced


Simultaneous multiple communications


High speed uplinks


Improved network response


Increased user productivity

Symmetric/Asymmetric
Switching


Symmetric switching provides switching
between like bandwidths


Multiple simultaneous conversations increase
throughput


Asymmetric provides switching between
unlike bandwidths


Requires the switch to use memory buffering



Spanning Tree Protocol


Switches forward broadcast frames


Prevents loops


Loops can cause broadcast storms and exponentially
proliferate fragments


Allows redundant links


Prunes topology to a minimal spanning tree


Resilient to topology changes and device failures


Spanning Tree Frames are called bridge



protocol data units (BPDUs)


Spanning Tree enabled by default on catalyst
switch

Spanning Tree States


States are initially set and then modified by STP


Blocking


Listening


Learning


Forwarding


Disabled


Server ports can be configured to immediately
enter SPT forwarding mode



You can determine the status, cost, and priority of
ports and VLANs by using the
show spantree

command


Listening and learning create latency

Switching Modes


Store and Forward


Entire frame received before forwarding takes place


causes more latency but error detection is high


Cut Though (Read first 6 bytes)


Switch reads destination address before receiving entire
frame and it is forwarded


decreases latency but higher
error rate


Fast forward immediately forwards


Fragment Free filters out collision fragments


Fragment Free (Read first 64 bytes)


Ensures frame is not a runt and probably not an error


Buffering


Two methods


Port
-
based


Packets stored in queues that are linked to incoming ports


packets forwarded when queue is clear


Shared Memory buffering


Deposits all packets into common memory buffer shared by all
ports


Dynamic location assigns port areas


Switch maintains a map of ports and clears when packet
is switched


Virtual LAN


Logical network independent of their members’
physical locations


Administratively defined broadcast domain


Users reassigned to different VLAN using
software



Can be grouped by function, department,
application


Creates a single broadcast domain that is not
restricted to physical segment


INSTANT LEVEL OF SECURITY