CCNA 3 Week 4 Switching Concepts. Copyright © 2005 University of Bolton Introduction Lan design has moved away from using shared media, hubs and repeaters

CCNA 3 Week 4

Switching Concepts

Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton

Introduction

Lan design has moved away from using shared media, hubs and repeaters (5-4-3 rule)

Switched networks offer many benefits– Increased bandwidth to end users– Microsegmentation– Collision-free operation


Devices at layer 1 and 2

Hubs and repeaters are layer 1 devices– Allow us to connect more devices to a LAN– Increase congestion and competition for bandwidth

Bridges and Switches are layer 2 devices– Restrict traffic based on layer 2 addresses (MAC)– Selectively forward frames– Create smaller collision domains– Will still forward broadcast frames to all


LAN performance factors

Multitasking OS lead to simulataneous network access

Wide use of centralised server-based computing

Leads to greater use of network bandwidth to transmit data


Delays and Issues

The data frame delivery of Ethernet/802.3 LANs is of a broadcast nature.

The carrier sense multiple access/collision detect (CSMA/CD) allows only 1 station to transmit once

Multimedia applications with higher bandwidth demand such as video and the Internet create congestion.

Normal latency occurs as frames travel across the network medium and through network devices


Half-duplex design

On shared medium network only one device can transmit at once

If collision occurs, JAM signal send and devices back off before re-transmitting

Increasing numbers of devices increase chance of collision


Bandwidth Demands

Particular uses which increase network use are:– Large graphics files – Full-motion video – Multimedia applications

Possible to increase total bandwidth – switch from 10MB to 100MB for example

Increase efficiency by making better use of existing bandwidth


Latency

Delay involved in transmission of information

Time taken by NICs to encode and decode voltages on medium (around 1μs for 10Base-T)

Propagation delay as the signal takes time to travel through the cable. Typically, this is about 0.556 microseconds per 100 m for Cat 5 UTP.

Latency is added based on network devices that are in the path between two computers


10 Base-T Transmission time

Bit time is 100ns (800ns for a byte)– 64 byte frame takes 51.2 μs– 1000 byte frame takes 800 μs

Delays (latency) added by routers, switches, repeaters etc decrease performance further


Ethernet Repeater

Maximum cable lengths relate to physical limitations such as attenuation

Repeaters regenerate the signal before transmitting on another segment

Tend to increase size of collision domain and have negative effect on performance


Full-Duplex

10 Base-T and 100 Base-TX cables have two pairs of wires

Can be used simultaneously to TX and RX

Requires dedicated cable and full-duplex device at each end

– Switch not hub


Segmentation


Bridges

Store and forward devices, read frame and calculate CRC

20-30% increase in latency


Routers

Operate at layer 3 making decisions based on network layer (IP) address

De-encapsulate packets

Increase of 20-30% latency over switched networks

Do not forward layer 2 broadcasts


Switched networks

Microsegmentation increases number of collision domains

No change to broadcast domains


Switch functions

The following are the two basic operations that switches perform:

Switch data frames - The process of receiving a frame on a switch interface, selecting the correct forwarding switch port(s), and forwarding the frame

Maintain switch operations - Switches build and maintain forwarding tables. Switches also construct and maintain a loop-free topology across the LAN


Switch Latency

Period of time between data entering and exiting a switch

High speed of networks makes delay significant


Layer 2 and Layer 3 Switching

Make decisions based on different addresses– Layer 2: Mac Address– Layer 3: IP address

Layer 3 switch vs Router– L3 Switch makes decisions in hardware (ASIC)– Router makes decisions using software

L2 Switches use Content Addressable Memory (CAM)


Symmetric vs Asymmetric

Symmetric switch interfaces are all the same speed

Asymmetric switches allow different port speeds– Faster ports can be dedicated to servers

With Asymmetric switches, frames may need to be buffered while waiting for slower interfaces


Memory Buffers

Port based buffering– Frames stored in queues associated with ports– Single queue means frames may be delayed even if

destination port clear

Shared memory buffering– Frames stored in common buffer– Port queues dynamically allocated– Frames transmitted when outbound port clear


Switching Methods

Store and forward– Entire frame received and checked before sent on– Increased latency, greater reliability

Cut-through– Frame transmitted before completely received– Fast-forward mode sends as soon as destination

address received– Fragment-free mode waits until at least 64 bytes

received (ie not a collision)


Layer 2 Switching

Switches improve efficiency by filtering layer 2 addresses

Each switch port is effectively a single collision domain – frames forwarded between domains at speed

Microsegmentation can provide high bandwidth by running one host per segment (full duplex)


Transmission Modes

Fast-forward– As soon as dest received, low latency, higher error

Fragment-free– Waits to ensure not a collision

Store-and-forward– Receives and checks entire frame, slower but reliable

Adaptive cut-through– Switches to store-and-forward in event of high error rate


Bridge Operation

Bridges and switches learn in the following ways:– Reading the source MAC address of each received frame or

datagram – Recording the port on which the MAC address was received

When first turned on, Bridges may broadcast to learn addresses

New frames are checked against bridging table before forwarding

Can filter traffic by port, protocol, destination


Microsegmentation

Isolating segments increases available bandwidth by reducing size of collision domains

Bridges ought to only forward %age of all traffic

Virtual circuits created within a switch to forward traffic as needed


Ethernet Communication

Excessive collisions sap bandwidth

Each switch port is a separate collision domain

Messages sent to FF:FF:FF:FF:FF:FF are broadcasts – switches forward broadcasts

Switches can extend a broadcast domain (need a router to reduce them)


Network Cabling

Use a cross-over cable– PC to PC– Switch to Switch– Switch to hub – Hub to hub – Router to router – Router to PC

Use a straight-through cable– Switch to router – Switch to workstation or

server – Hub to workstation or server

Documents

CCNA 3 Week 4 Switching Concepts. Copyright © 2005 University of Bolton Introduction Lan design has moved away from using shared media, hubs and repeaters