LAN Switching
Semester 3, Chapter 2
Table of Contents
Go There!
LAN Communication Problems
Go There!
Full-Duplex, Fast Ethernet,
and Segmentation
Go There!
Switching and VLANs
Go There!
The Spanning-Tree Protocol
LAN Communication
Problems
Table of Contents
Network Performance
Network congestion has increased
significantly since the mid-90s due to:
Multitasking Operating Systems
multiple simultaneous network transactions (e.g., ftp download &
browsing)
Faster Processing Power
1980s: 1 MIPS; Today: over 75 MIPS
Network-intensive Applications
accessing network servers to use applications, files, etc.
Elements of Ethernet/802.3
Characteristics
Most common LAN architecture
Used to transport data between devices connected to the
same delivery medium
Uses a data frame broadcast method
Negative effects of a shared LAN
broadcast delivery of all frames
CSMA/CD: collisions are inherent
distance limitation requires using repeaters to extend
Half-Duplex Ethernet
Properties
Only one host can transmit at a time because the NIC
needs to listen for collisions
The NIC provides several circuits. Most important are:
receive (RX), transmit (TX), and collision detection
bandwidth usage = 50% to 60%
CSMA/CD
Operation
Devices on shared media listen for a carrier before transmitting
If no carrier is sensed for a specific period of time, a device can
transmit
If two devices transmit simultaneously, a collision occurs. The NIC
senses this because it is transmitting and receiving at the same time
The first device to detect the collision will generate a jam signal
(colliding devices continue to transmit so that all devices will hear the
collision)
All devices calculate a backoff algorithm which will delay
transmission for a random length of time.
First device who’s delay time expires can attempt to transmit data.
Network Congestion
Occurs as more people utilize a network
to...
Share large files (e.g. databases, applications, etc.)
Access file servers
connect to the Internet
Relieving congestion requires
Increasing the amount of bandwidth and/or
Using available bandwidth more efficiently
Network Latency
Latency explained
Represents the time it takes a frame to travel from is
source device to its final destination on the network (also
know as propagation delay)
Latency can also be described as the delay between the
time a device requests access to a network and the time it
is granted permission to transmit
For switches and routers, latency is the amount of delay
between the time when the device receives the frame on
one interface and forwards that frame out another interface
Routers have more inherent latency than a switch. Why?
Ethernet Transmission Time
Defined
Transmission time is the time necessary to move a packet from the
data link layer to the physical layer
10BaseT Transmission Time
Each bit has a 100ns window for transmission
 ns-nanosecond (1 billionth of a second)
So each byte has what size window?
A 64 byte frame (the smallest allowed frame) requires 51,200 ns or
51.2 microseconds
Just to frame a 1000 byte packet requires 800 microseconds
Additional latency will be added propagating the frame down the wire
and by any additional devices the frame has to go through before
reaching the destination
Using Repeaters
What is attenuation?
Loss of signal strength as it travels through the network;
caused by resistance inherent in the medium
Benefits of Using a Repeater
a layer 1 device that cleans up and boosts the signal
extends the coverage area of a LAN segment
Negative Effects of Using a Repeater
increases the collision domain size
increases the broadcast domain size
can’t filter traffic based on Layer 2 or 3 addressing
Full-Duplex,
Fast Ethernet,
and Segmentation
Table of Contents
Full-Duplex Ethernet
Simultaneous TX and RX
allows the transmission of a packet and the reception of a
different packet at the same time.
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.
100% of bandwidth is available: 10 Mbps increases to 20
Mbps of potential throughput (10 Mbps TX & 10 Mbps RX)
LAN Segmentation
Benefits of Segmenting the Network
By segmenting a LAN
fewer devices are
sharing the same
bandwidth, improving
performance of a
shared media LAN
Each segment is
considered its own
collision domain
How many broadcast
domains in graphic?
Segmenting with Bridges
Bridge Operation
Bridges “learn” a network’s segmentation by building
address tables that contain:
Bridge interface that will reach that device
Each device’s MAC address
Segmenting with Bridges
Generic Frame Format
Frame can be any length depending on technology
Ethernet frame can be up to 1522 bytes long
Address section is 12 bytes (6 bytes for each MAC)
FCS contain the CRC to check frame for errors
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
Segmenting with Bridges
Bridge Performance
adds 10% to 30% latency due to decision-making process
considered a store-and-forward device because it must
calculate the CRC at the end of the frame to check it for
errors before forwarding
if the bridge does not have an entry for the destination
MAC, it...
adds the source MAC to its bridging table
forwards the frame out all interfaces except the one it was
received on
when a reply returns, it adds the destination MAC to the table
Segmenting with Routers
Router Operation
Routers...
use layer 3 addressing (IP, IPX) and routing protocols (RIP,
IGRP) to determine the path and
switch the packet out the correct interface to the destination
because a router must open the packet to read Layer 3
addressing, it adds latency
In addition, protocols like TCP which require
acknowledgments of every packet can increase latency,
reducing throughput from 20% to 40%
Segmenting with Routers
Router Benefits
Like switches, routers
segment collision
domains.
However, since a router
will not forward
broadcasts, it also
segments broadcast
domains.
Each router interface
represents its own
broadcast domain.
Segmenting with Switches
Switching Benefits
a switch is simply a multi-port bridge, making forwarding
decisions based on MAC addresses
so, like a bridge, segmenting a LAN with a switch creates
more collision domains
replacing hubs with switches therefore decreases
congestion and increases available bandwidth.
a switch can microsegment a LAN creating collision-free
domains but still be in the same broadcast domain.
switch creates a virtual circuits, allowing many users to
communicate in parallel.
Switching and VLANs
Table of Contents
Switch Operation
Switches perform two basic functions:
Building and maintaining switching tables (similar to a
bridge table) based on MAC addresses
Switching frames out the interface to the destination
Differences between switches & bridges
Switches operate at higher speeds
Switches are capable of creating virtual LANs (VLANs)
through microsegmentation
Bridges switch using software; switches typically switch
using hardware (called the “switch fabric”)
Switch Latency
A switch adds 21 microseconds of latency.
This can be reduced by using a different switching method
As opposed to store-and-forward, the switch can use cutthrough switching which switches the packet as soon as the
destination MAC is read.
How a LAN Switch Learns Addresses
MAC addresses are learned dynamically and are stored in
CAM (content-addressable memory)
Each time a switch stores an address entry in the table, it is
time-stamped.
The time-stamp is updated each time a frame is received
Addresses whose time-stamp expires are deleted from the
table
This keeps switching tables small
Benefits of LAN Switching
Cost-effective;
switches only cost 3 to
5 times that of a hub
Allows the creation of
virtual circuits
More flexibility in
managing the network
Reduces number of
collisions
Works with existing
802.3 cabling
Symmetric Switching
symmetric switching
provides switched
connections between
ports with the same
bandwidth (10/10 Mbps
or 100/100 Mbps)
can cause bottlenecks
as users try to access
servers on other
segments.
potential bottlenecks
Asymmetric Switching
asymmetric switching
reduces the likelihood
of a potential bottleneck
at the server by
attaching the segment
with the server to a
higher bandwidth port
(100 Mbps)
asymmetric switching
requires memory
buffering in the switch
Memory Buffering
Defined
Area of memory in a switch where destination and transmission data
are stored until it can be switched out the correct port.
Two types
Port-based memory buffering
 packets are stored in a queue on each port
 possible for one packet to delay transmission of other packets because of a
busy destination port
Shared memory buffering
 common memory buffering shared by all ports
 allows packets to be RX on one port and TX out another port without changing
it to a different queue.
Two Switching Methods
Store-and-Forward
The switch receives the entire
frame, calculating the CRC at the
end, before sending it to the
destination
Cut-through
Fast forward switching--only checks
the destination MAC before
immediately forwarding the frame
Fragment Free--reads the first 64
bytes to reduce errors before
forwarding the frame
VLANs (IEEE 802.1q)
Characteristics
A logical grouping of network devices or users that are not
restricted to a physical switch segment.
The devices or users in a VLAN can be grouped by
function, department, application, and so on, regardless of
their physical segment location.
A VLAN creates a single broadcast domain that is not
restricted to a physical segment and is treated like a
subnet.
VLAN setup is done in the switch by the network
administrator using the vendor’s software.
The Spanning-Tree
Protocol
Table of Contents
Overview of STP
Elements of the Spanning Tree Protocol
Main function of STP is to allow redundant paths in a
switched/bridged network without incurring latency from the
effects of loops.
STP prevents loops by calculating a stable spanning-tree
network topology (similar to OSPF operation)
Spanning-tree frames (called bridge protocol data units-BPDUs) are sent and received by all switches in the
network and are used to determine the spanning-tree
topology
STP operation is covered in detail in Semester 7 of the
CCNP curriculum.
Five STP States
States are established by configuring each port according to
policy
Then the STP modifies the states based on traffic patterns
and potential loops
The default order of STP states are:
Blocking--no frames forwarded, BPDUs heard
Listening--no frames forwarded, listening for data frames
Learning--no frames forwarded, learning addresses
Forwarding--frames forwarded, learning addresses
Disabled--no frames forwarded, no BPDUs heard
Required Labs for this Chapter
Spend your lab time completing all four
labs in this Chapter
Lab 2.3.7--Switching Characteristics
Lab 2.3.10.1--Switch Management Console
Lab 2.3.10.2--Switch Port Options
Lab 2.4.2--Switch browser configuration
Recommendation:
DO NOT TAKE THE TEST UNTIL YOU’VE COMPLETED
THE LABS!!
Table of Contents
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