CCNA3 3.1-04 LAN Switching-pgb

Local Area Network (LAN) Switching & VLANSAdvantages of LAN segmentation in a network
Advantages and disadvantages of using bridges, switches, and routers for LAN segmentation
Effects of switching, bridging, and routing on network throughput
Fast Ethernet technology and its benefits
CCNA3 Module 4 Brierley
Uses a data frame broadcast method
Uses Carrier Sense Multiple Access/ Collision Detection (CSMA/CD)
Allows only one station to transmit at a time.
Network congestion due to increased bandwidth demands.
Normal latency Extending the distances
Ethernet/802.3 LAN Performance can be negatively effected by several factors.
Most important are receive (RX), transmit (TX), and collision detection
Loopback
Tx
Rx
Collision
Detection
Ethernet
Controller
Tx
Rx
Collision
Detection
Ethernet
Controller
Transmit
Receive
Loopback
The transmit (TX) circuit is active at the transmitting station
The receive (RX) circuit is active at the receiving station
Both devices are contending for the right to use the single shared medium.
The collision detection circuit contends for the use of the network
When a collision occurs, a host will resume transmitting based on a back-off algorithm
The problem could be broadcasts, chatty protocols, applications traffic, etc.
Latency, also known as propagation delay.
Propagation delay is the time a frame or packet takes to travel from the source node to its destination
Adding hosts increases collisions, (increases jam signals)
A byte is eight bits
One byte takes a minimum of 800ns to transmit (8 bits at 100ns per equals 800ns)
A 64 byte frame takes 51,200 ns or 51.2 microseconds to transmit
(64 bytes at 800ns = 51,200ns,
51,200ns/1000 = 51.2 microseconds)
Extending Shared Media LANs using Repeaters
Signal attenuation – the signal weakens as it travels, from the resistance found in the medium.
An Ethernet repeater can be used to extend the distance of a LAN
Move to full duplex transmitting
Upgrade to Fast Ethernet
Each segment is considered to be its own collision domain
Why Segment LANs?
In a segmented LAN frames are passed between segments on the backbone using a bridge, switch, or router
The backbone is its own collision domain and uses CSMA/CD to provide service between segments
Segmentation with Bridges
Layer 2 devices,
independent of Layer 3 protocols data frames are passed along regardless of which Layer 3 protocol is being used
increase the latency (delay) in a network by 10-30%
Why?
A bridge is a store and forward device
it must examine the destination address (MAC) to determine which interface to forward the frame
If there is no match in the table, the frame is flooded out all other interfaces
Bridges learn network’s segmentation by building address tables that contain the (MAC) address of each network device and which segment to use to reach that device
Smaller collision domains are created, not broadcast domains
Segmentation with Routers
Operates at the network layer and base all forwarding decisions on the Layer 3 protocol address
Because routers perform more functions than bridges they cause additional latency
Segment collision domains
Segmentation with LAN Switches
A switch segments a LAN into microsegments creating collision free domains from one larger collision domain, not broadcast domains
With a switched Ethernet implementation the available bandwidth can reach closer to 100%.
LAN Switch Latency
Each switch used on an Ethernet LAN adds latency to the network
However, the type of switching used can help overcome the built in latency of some switches
Only 1 host can transmit at a time
Collisions – jam signal generated, back-off algorithm before retransmission
50-60% bandwidth available
No collisions
Full-Duplex Ethernet Overview
Full duplex Ethernet allows the transmission of a packet and the reception of a different packet at the same time
Requires two pairs of conductors and a switched connection between each node
Simultaneous transmission and reception of frames is called bidirectional traffic (both directions) and yields 20Mbps of throughput
The network interface cards (NICs) on both ends need full duplex capabilities
No collisions
Uses a single port for each full-duplex
connection
TX
Full
Duplex
Ethernet
Controller
Loopback
Tx
Rx
Collision
Detection
RX
Full
Duplex
Ethernet
Controller
Loopback
Tx
Rx
Collision
Detection
Have installed network interface card that supports
full duplex
Full-Duplex Ethernet Design
Standard Ethernet normally can only use 50-60% of the 10Mbps available bandwidth
This is due to collisions and latency.
Full duplex Ethernet offers 100% of the bandwidth in both directions
This produces a potential 20Mbps throughput – 10Mbps TX and 10Mbps RX
This virtual network circuit exists only when two nodes need to communicate
This is why it is called a virtual circuit it exists only when needed and is established within the switch
Allows multiple users to communicate in parallel via these virtual circuits
How a LAN Switch Learns Addresses
New addresses are read, learned and stored in Content Address Memory (CAM)
Each time an address is stored it is time stamped
This allows addresses to be stored for a set period of time.
through the use of virtual circuits
in dedicated network segments
Cost effective
Symmetric Switching
A symmetric switch is optimized through even distribution of network traffic across the entire network
Memory Buffering
The area of memory where the switch stores the destination and transmission data is called the memory buffer
It can make use of two methods for forwarding packets
port based memory buffering or
shared memory buffering
Port based memory buffering
Packets are stored in queues that are linked to specific incoming ports
Problem: One port may fill while another is empty
Shared memory buffering
Deposits all packets into a common memory buffer that is shared by all the ports on the switch
Store and Forward - the entire frame is received before forwarding
Latency occurs while the frame is being received; the latency is greater with larger frames
Error detection is high due to the time available to check for errors waiting for the entire frame to be received
Cut-through
The switch reads the destination address before receiving the entire frame
The frame is then forwarded before the entire frame arrives
This mode decreases the latency of the transmission BUT has poor error detection
Fragment-Free Switching
Switch reads only the 1st 64 bytes of the incoming frame before forwarding
Combines cut through with store and forward
The switch uses cut-through until there are a given number of errors
Then the switch will change to store and forward method

Documents

CCNA3 3.1-04 LAN Switching-pgb