DCN286 Introduction to Data Communication Technology Session 8

Review 1) What is the purpose of the design rule? a) Provide time for CSMA/CD to work b) Ensure that CSMA/CD works correctly in a 100BASE-T full-duplex networkby making sure that collisions are heard in a reasonable amount of time c) Provide 5 LAN segments for hostsd) Provide 4 LAN segments for hosts 2) Identify two Gigabit Ethernet over fibre cable specifications a) 1000BASE-TXb) 1000BASE-FX c) 1000BASE-SXd) 1000BASE-LX e) 1000BASE-GX 3) What transmitter and cable type are used for 1000BASE-SX? a) LED over SMF and MMFb) Electrical transmitter using CAT5 UTP c) Electrical transmitter using balanced, d) Laser over MMF shielded, 150ohm, two pair STP 4) Although 1000BASE-T standards support CSMA/CD and half-duplex, why are you unlikely to see them on a network? a) This requires the use of 1000BASE-Tb) 100BASE-T is still adequate for network hubs, and no one sells thembackbones c) The rule prevents implementationd) The collisions slow down throughput to in large networksunacceptable rates 5) What is the maximum distance supported by 1000BASE-SX Ethernet? a) 82mb) 100m c) 220m d) 5000m

Objectives Layer 2 Bridge and Switching Switch Internal Processing Spanning Tree Protocol LAN Design: Collision Domains and Broadcast Domains Devices and OSI Layers

What is meant by layer 2? How does a bridge handle frames it receives? What address does it store? What does it do to addresses is has seen before? What does it do to addresses it has not seen before? Forward – what does this mean? (wrt Bridges / Switches) Filter - what does this mean? (wrt Bridges / Switches) Unknown Unicast – what is it? Flooding Unicasts – What process is being performed? MAC address is stored in table for a period of time, then it must be refreshed (housekeeping) 4 Layer 2 Bridge and Switching

What is a broadcast address? Multicast Range of MAC addresses (01:00:5E:xx:xx:xx or 01:00:5F:xx:xx:xx) Multicast frames are delivered to a subset of devices on the network – those are configured with the multicast address Set up in software – e.g. video conference over corporate LAN, some employees listen in, others do not. Switch can treat multicast frames the same as broadcast frames (flooding) or it can treat multicast frames so that they only go to their indented destinations (forwarding) 5 Layer 2 Bridge and Switching

Latency Refers to how long it takes to get a frame/packet to progress through a network from one device to another. What is Propagation Delay? Queuing Delay – imagine a switch with many packets queued to be sent to port 1. Your packet arrives at the end of the queue and must wait for the other packets to be processed Processing time within the switch to search for a MAC address 6 Switch Internal Processing

Store-and-Forward Switching Switch must receive entire frame before beginning to act upon it. The Frame Check Sequence is validated – and switch can discard bad frames without forwarding them on Validate the data length field matches the data and pad field's length Ports can run at different speeds (10Mbps and 100Mbps ports can exchange frames) – called Asymmetric Switching Greater amount of Latency then Cut-Through Switching and Fragment-Free Switching 7 Switch Internal Processing

Cut-Through Switching All the switch needs in order to perform its forwarding is the Destination Address – which follows immediately after the preamble and start field delimiter (SFD). The switch can start forwarding the frame out the destination port before the entire message has been received at the input port However, FCS is not checked, bad frames get forwarded Frames fragments get forwarded before collisions have occurred Works only with Symmetric Switching where both ports are running at the same speed 8 Switch Internal Processing

Fragment-Free Switching Similar to Cut-through Switching, however ensure the first 64 bytes have been received before starting to forward the frame. Remember, that the smallest frame is 64 bytes – therefore in a properly designed LAN, collisions occur within the first 64 bytes being transmitted. Also is Symmetric Switching 9 Switch Internal Processing

Recall IEEE802.1 LAN Management and control functions Management and control functions that could be used by any IEEE802 LAN 10 Spanning Tree Protocol

Consider this design to implement redundancy PC1 sends a message to PC3 The forwarding tables are empty, so flooding unicast What do tables look like after first iteration? If unchecked it is called a Broadcast Storm 11 Spanning Tree Protocol PC1 PC3 PC2

12 Spanning Tree Protocol PC1 PC3 PC2 PC1PC1 PC1PC1 PC1PC1 PC1PC1 PC1PC1 PC1PC1 PC1PC1

STP (Spanning Tree Protocol) Blocking Configure port(s) to not perform unicast flooding What happens now when PC1 sends a message to PC3? 13 Spanning Tree Protocol PC1 PC3 PC2 STP BLOCKING STATE

STP States – Stable States STP BLOCKING Interface does not process incoming frames, does not send frames on this port STP FORWARDING Normal state for port – forwards and receives frames STP DISABLED Port disabled due to failure, or administrator. Frames are not received or sent on this interface 14 Spanning Tree Protocol

STP States – Transitory States STP LISTENING Waiting for forwarding table entries to time out. Forwarding does not take place in this state STP LEARNING Switch is leaning new table entries based on newly received frames. Forwarding does not take place in this state 15 Spanning Tree Protocol

STP Bridge Protocol Data Units BPDUs – Switches send BPDUs to each other to exchange information of the topology Each switch uses this to determine which interfaces should forward and which interfaces should block Changing Topology Due to interfaces failing or being administratively disabled, or links failing the switches must change their STP topology Listening State – lasts 15s. Allows the table on the interface to time out and therefore clear Learning State – lasts 15s. Allows interface to load the table without forwarding them (and potentially causing looping frames Forward State – port is in a stable functioning state 16 Spanning Tree Protocol

Collision Domains What is a Collision Domain? What rule applies to Collision Domains when designing a Network? Rule: 5: segments of network media 4: repeaters/hubs at most between any 2 end-user devices 3: links at most, between any 2 end-user devices, may have end-user devices connected 2: If 5 segments exist between 2 end-user devices, 2 segments must not have any end-user devices connected to them 1: large collision domain 17 LAN Design: Collision Domains and Broadcast Domains

Collision Domains 100 devices sharing a 10Mbps LAN average 100Kbps bandwidth As the number of devices increases in a Collision Domain, the throughput decreases as collisions increase. Expect no better than 35%-40% throughput on the bandwidth Large collision domains should not be used: Shared bandwidth – as collision domain grows (in number of devices) the amount of bandwidth available to each-end user device decreases Higher utilization – the more devices on a single collision domain, the better the chance of driving utilization higher, resulting in worsening performance 18 LAN Design: Collision Domains and Broadcast Domains

Segmentation Breaking a large collision domain into smaller collision domains rule applies to a single collision domain smaller collision domains, less likely to reach 35% utilization shared bandwidth occurs only within collision domain smallest collision domain is two interfaces – where full duplex can be enabled 19 LAN Design: Collision Domains and Broadcast Domains

Layer 2 Broadcast Domains The set of LAN interfaces (including NICs and network devices) for which a broadcast frame sent by any one device will be forwarded to all other interfaces in that same broadcast domain Repeaters, Hubs, Bridges and Switches all forward broadcast frames Routers do not forward broadcast frames 20 LAN Design: Collision Domains and Broadcast Domains

Layer 2 Broadcast Domains 21 LAN Design: Collision Domains and Broadcast Domains

Layer 2 Broadcast Domains Broadcast and Multicast frames must be interpreted by the in software by the CPU – as the NIC card does not know how to handle them. Too many broadcast / multicast frames can seriously impede PC performance as each frame is evaluated Unicast frames either match the unicast address or don't. If they match, they are acted on, otherwise they are discarded 22 LAN Design: Collision Domains and Broadcast Domains

Devices by OSI Layer D eviceOSI Separates LAN intoSeparates LAN into LayerCollision DomainsBroadcast Domains Repeater1No No Hub1No No Bridge2YesNo Switch2YesNo Router3Yes Yes 23 LAN Design: Collision Domains and Broadcast Domains

What happens to a message passing through a network? SwitchRouterHub 24 OSI Layers and Devices Session Presentation Application Transport Network Data Link Physical Data Link Physical Network Data Link Physical Session Presentation Application Transport Network Data Link Physical