CCNA 3 Week 4 Switching Concepts

Copyright © 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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 microseconds per 100 m for Cat 5 UTP. Latency is added based on network devices that are in the path between two computers

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton Segmentation

Copyright © 2005 University of Bolton Bridges Store and forward devices, read frame and calculate CRC 20-30% increase in latency

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton Switched networks Microsegmentation increases number of collision domains No change to broadcast domains

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton Switch Latency Period of time between data entering and exiting a switch High speed of networks makes delay significant

Copyright © 2005 University of Bolton 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)

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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)

Copyright © 2005 University of Bolton 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)

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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

Copyright © 2005 University of Bolton 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)

Copyright © 2005 University of Bolton 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