1 Version 3 Module 8 Ethernet Switching

2 Version 3 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

3 Version 3 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)

4 Version 3 Shared media environments

5 Version 3 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

6 Version 3 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

7 Version 3 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

8 Version 3 Collision domains

9 Version 3 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

10 Version 3 Segmentation

11 Version 3 Network segment

12 Version 3 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

13 Version 3 Layer 2 Bridging

14 Version 3 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

15 Version 3 Layer 2 Switching

16 Version 3 Switches A switch is a fast, multi-port bridge Each port creates its own collision domain A switch dynamically builds and maintains a Content- Addressable Memory (CAM) table The CAM holds all of the necessary MAC information for each port

17 Version 3 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

18 Version 3 Switch modes Store and Forward Cut through

19 Version 3 Switch Modes Cut-through switching –A switch transfers the frame as soon as the destination MAC address is received –lowest latency –no error checking

20 Version 3 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

21 Version 3 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

22 Version 3 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

23 Version 3 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

24 Version 3 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

25 Version 3 STP Each port using Spanning-Tree Protocol is in one of the following five states: –Blocking –Listening –Learning –Forwarding –Disabled

26 Version 3 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

27 Version 3 Spanning tree protocol

28 Version 3 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

29 Version 3 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

30 Version 3 Layer 2 Broadcasts The three sources of broadcasts and multicasts: –Workstations –Routers –Multicast Applications

31 Version 3 Broadcast & Collision Domain Collision Domain

32 Version 3 Layer 3 Devices Layer 3 devices –Routers –Do not forward collisions –Breaks up collision domains –Broadcast domains are controlled

33 Version 3 Broadcast domain

34 Version 3 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

35 Version 3 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

36 Version 3 Dataflow

37 Version 3 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

38 Version 3 Latency