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

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26 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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LAN Switching

Addressing LAN Problems

Congestion is Caused By


Faster operating systems,

More Web
based applications


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

point (host to host) connection is collision

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


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).

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

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

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
point and is collision free.

Because both nodes can transmit and receive at the
same time, there are no negotiations for bandwidth.
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.

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

This produces a potential 20
Mbps throughput

Mbps TX and 10
Mbps RX.


Transmit connects to Receive


Why Segment

Isolates certain traffic

Creates smaller collision domains

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


Decreasing size of collision domains increases
the number of collision domains


Create tables to match segments and MAC

Layer 3 protocol independent

Stores and then forwards based on MAC

Increases latency on network (10

Creates smaller collision domains but
increases number of collision domains


Operate at Layer 3 using IP addresses

More manageable, greater functionality, multiple

Smaller collision domains

Introduce latency

Protocols that provide acknowledgements introduce
40% delays

Protocols that provide minimal acknowledgements have
30% loss in throughput

So routers introduce 20
40% delay depending on

Switches/Switched Ethernet

Low latency and high frame
forwarding rates

Eliminates impact of collisions through micro

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

Filters when the destination is on the same

Date stamps each address

discards after a certain
time period

Addresses stored in CAM

Content Addressable

Benefits of Switching

Number of collisions reduced

Simultaneous multiple communications

High speed uplinks

Improved network response

Increased user productivity


Symmetric switching provides switching
between like bandwidths

Multiple simultaneous conversations increase

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

Spanning Tree States

States are initially set and then modified by STP






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


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


Two methods


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

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

Can be grouped by function, department,

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