Transcript Switches
Module 8 Ethernet Switching Version 3 1 Ethernet Switching • Ethernet is a shared media – One node can transmit data at a time • More nodes increases the demands on the available bandwidth – The probability of collisions increases, resulting in more retransmissions • A solution to the problem is to segment. • Segmenting creates more collision domains Version 3 2 Shared Media Environment • Shared media environment – – multiple hosts have access to the same medium • Extended shared media environment – – Using networking devices extends the environment to accommodate multiple access or longer cable distances • Point-to-point network environment – – one device is connected to only one other device (ex. dialup network connections) Version 3 3 Shared media environments Version 3 4 Layer 1 Devices • Layer 1 devices – repeaters and hubs • Extend collision domains • Primary function is extending cable segments • Additional hosts increase the amount of traffic • More traffic = greater chances of collisions – This results in diminished performance Version 3 5 Repeater Rule • Four repeater rule: – No more than four repeaters between any two computers – Contributing Factors • Repeater latency • Propagation delay • NIC latency – Late collision frames add delay that is referred to as consumption delay Version 3 6 Collision Domains • Collision Domains – Connected physical network segments where collisions can occur • Collisions cause: – The network to be inefficient – Transmissions to stops for a period of time Version 3 7 Collision domains Version 3 8 Collision Domains • The types of devices that interconnect the media segments define collision domains • Classified as OSI Layer 1, 2 or 3 devices • Layer 1 devices do not break up collision domains • Layer 2 and Layer 3 devices break up collision domains – Increasing the number of collision domains is known as segmentation Version 3 9 Segmentation Version 3 10 Network segment Version 3 11 Layer 2 Devices • Layer 2 devices – Bridges and Switches – Segments collision domains – Controls frame propagation using the MAC address – Tracks the MAC addresses and segment they are on Version 3 12 Layer 2 Bridging Version 3 13 Bridges • Has only two ports and divides a collision domain into two parts • Entire network will share the same logical broadcast address space • Creates more collision domains but will not add broadcast domains • All decisions made are based on MAC or Layer 2 addressing • No effect on the logical or Layer 3 addressing Version 3 14 Layer 2 Switching Version 3 15 Switches • A switch is a fast, multi-port bridge • Each port creates its own collision domain • A switch dynamically builds and maintains a ContentAddressable Memory (CAM) table • The CAM holds all of the necessary MAC information for each port Version 3 16 Switch Operation • Micro-segments consist of the switch port and the host connected to it • Communication in both directions at once is known as full duplex • Most switches are capable of supporting full duplex, as are most network interface cards (NICs) • In full duplex mode, there is no contention for the media. – A collision domain no longer exists – Theoretically, the bandwidth is doubled when using full duplex Version 3 17 Version 3 18 Switch Modes • Cut-through switching – A switch transfers the frame as soon as the destination MAC address is received – lowest latency – no error checking Version 3 19 Switch Modes • Store-and-forward switching – Higher latency – The switch receives the entire frame before sending it out – Verifies the Frame Check Sum (FCS) – Invalid frames are discarded at the switch Version 3 20 Switch Modes • Fragment-free switching • A compromise between cut-through and store-and-forward switching • Switching begins before the entire data field and checksum are read • Reads the first 64 bytes • Including the frame header • Verifies the reliability of: • Addressing • Logical Link Control (LLC) protocol Version 3 21 Switch Modes • Synchronous switching – The source port and destination port must be operating at the same bit rate • Asynchronous switching – The bit rates are not the same – The frame must be stored at one bit rate before it is sent out at the other bit rate – Store-and-forward must be used Version 3 22 Switch Modes • Asymmetric switching – Switched connections between ports of unlike bandwidths – Asymmetric switching is optimized for client/server – A server requires more bandwidth dedicated to the server port to prevent a bottleneck at that port Version 3 23 Spanning Tree Protocol • Switching loops can lead to broadcast storms that will overwhelm a network. • To counteract loops, switches are provided with the Spanning-Tree Protocol (STP) • Switches in a LAN using STP – Send Bridge Protocol Data Units (BPDUs) out all its ports – Lets other switches know of its existence – Elect a root bridge (switch) for the network – Switches use the Spanning-Tree Algorithm (STA) to resolve and shut down the redundant paths Version 3 24 STP • Each port using Spanning-Tree Protocol is in one of the following five states: – Blocking – Listening – Learning – Forwarding – Disabled Version 3 25 STP • A port moves through five states as follows: – From initialization to blocking – From blocking to listening or to disabled – From listening to learning or to disabled – From learning to forwarding or to disabled – From forwarding to disabled • Resolving and eliminating loops creates a logical hierarchical tree with no loops • The alternate paths are available if needed Version 3 26 Spanning tree protocol Version 3 27 Layer 2 Broadcasts • Ethernet Broadcasts – When a node needs to communicate with all hosts on the network – A broadcast frame with a destination MAC address 0xFFFFFFFFFFFF is sent – The network interface card (NIC) of every host must respond Version 3 28 Layer 2 Broadcasts • Layer 2 devices must flood all broadcast and multicast traffic • Broadcast Radiation – The accumulation of broadcast and multicast traffic from each device • Broadcast storm – Circulation of broadcast radiation that saturates the network – There is no bandwidth left for application data Version 3 29 Layer 2 Broadcasts • The three sources of broadcasts and multicasts: – Workstations – Routers – Multicast Applications Version 3 30 Broadcast & Collision Domain Collision Domain Collision Domain Version 3 31 Layer 3 Devices • Layer 3 devices – Routers – Do not forward collisions – Breaks up collision domains – Broadcast domains are controlled Version 3 32 Broadcast domain Version 3 33 Broadcast Domain • Broadcast Domain – A grouping of collision domains – All the nodes that are a part of that network segment bounded by a layer three device – Broadcasts have to be controlled at Layer 3 devices – Layer 2 and Layer 1 devices do not control broadcasts Version 3 34 Data Flow • Layer 2 devices filter data frames based on the destination MAC address – A Layer 2 device will forward the frame unless something prevents it from doing so • Layer 3 devices filter data packets based on IP destination address – A Layer 3 device will not forward the frame unless it has to – Layer 3 device creates multiple collision and broadcast domains Version 3 35 Dataflow Version 3 36 Latency The delay between the time a frame leaves the source device and the time the frame reaches its destination • The following conditions can cause delays: – Physical media – Circuit delays • Electronics that process the signal along the path – Software delays • Decisions that must be made to implement switching and protocols – Delays caused by the content of the frame • Destination MAC address has to be read Version 3 37 Latency Version 3 38