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LAN Switching

LAN Switching. Semester 3, Chapter 2. Table of Contents. LAN Communication Problems. Full-Duplex, Fast Ethernet, and Segmentation. Switching and VLANs. The Spanning-Tree Protocol. LAN Communication Problems. Network Performance.

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LAN Switching

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  1. LAN Switching Semester 3, Chapter 2

  2. Table of Contents • LAN Communication Problems • Full-Duplex, Fast Ethernet, and Segmentation • Switching and VLANs • The Spanning-Tree Protocol

  3. LAN Communication Problems

  4. Network Performance • Network congestion has increased significantly since the mid-90s due to: • Multitasking Operating Systems • multiple simultaneous network transactions (e.g., ftp download & browsing) • Faster Processing Power • 1980s: 1 MIPS; Today: over 75 MIPS • Network-intensive Applications • accessing network servers to use applications, files, etc.

  5. Elements of Ethernet/802.3 • Characteristics • Most common LAN architecture • Used to transport data between devices connected to the same delivery medium • Uses a data frame broadcast method • Negative effects of a shared LAN • broadcast delivery of all frames • CSMA/CD: collisions are inherent • distance limitation requires using repeaters to extend

  6. Half-Duplex Ethernet • Properties • Only one host can transmit at a time because the NIC needs to listen for collisions • The NIC provides several circuits. Most important are: • receive (RX), transmit (TX), and collision detection • bandwidth usage = 50% to 60%

  7. CSMA/CD • Operation • Devices on shared media listen for a carrier before transmitting • If no carrier is sensed for a specific period of time, a device can transmit • If two devices transmit simultaneously, a collision occurs. The NIC senses this because it is transmitting and receiving at the same time • The first device to detect the collision will generate a jam signal (colliding devices continue to transmit so that all devices will hear the collision) • All devices calculate a backoff algorithm which will delay transmission for a random length of time. • First device who’s delay time expires can attempt to transmit data.

  8. Network Congestion • Occurs as more people utilize a network to... • Share large files (e.g. databases, applications, etc.) • Access file servers • connect to the Internet • Relieving congestion requires • Increasing the amount of bandwidth and/or • Using available bandwidth more efficiently

  9. Network Latency • Latency explained • Represents the time it takes a frame to travel from is source device to its final destination on the network(also know as propagation delay) • Latency can also be described as the delay between the time a device requests access to a network and the time it is granted permission to transmit • For switches and routers, latency is the amount of delay between the time when the device receives the frame on one interface and forwards that frame out another interface • Routers have more inherent latency than a switch. Why?

  10. Ethernet Transmission Time • Defined • Transmission time is the time necessary to move a packet from the data link layer to the physical layer • 10BaseT Transmission Time • Each bit has a 100ns window for transmission • ns-nanosecond (1 billionth of a second) • So each byte has what size window? • A 64 byte frame (the smallest allowed frame) requires 51,200 ns or 51.2 microseconds • Just to frame a 1000 byte packet requires 800 microseconds • Additional latency will be added propagating the frame down the wire and by any additional devices the frame has to go through before reaching the destination

  11. Using Repeaters • What is attenuation? • Loss of signal strength as it travels through the network; caused by resistance inherent in the medium • Benefits of Using a Repeater • a layer 1 device that cleans up and boosts the signal • extends the coverage area of a LAN segment • Negative Effects of Using a Repeater • increases the collision domain size • increases the broadcast domain size • can’t filter traffic based on Layer 2 or 3 addressing

  12. Full-Duplex, Fast Ethernet,and Segmentation

  13. Full-Duplex Ethernet • Simultaneous TX and RX • allows the transmission of a packet and the reception of a different packet at the same time. • requires the use of two pairs of wires in the cable and a switched connection between each node. • this connection is considered point-to-point and is collision free. • because both nodes can transmit and receive at the same time, there are no negotiations for bandwidth. • 100% of bandwidth is available: 10 Mbps increases to 20 Mbps of potential throughput (10 Mbps TX & 10 Mbps RX)

  14. LAN Segmentation • By segmenting a LAN fewer devices are sharing the same bandwidth, improving performance of a shared media LAN • Each segment is considered its own collision domain • How many broadcast domains in graphic? • Benefits of Segmenting the Network

  15. Segmenting with Bridges • Bridge Operation • Bridges “learn” a network’s segmentation by building address tables that contain: • Bridge interface that will reach that device • Each device’s MAC address

  16. Type/ Length Start Frame Stop Frame Address Data FCS Segmenting with Bridges • Generic Frame Format • Frame can be any length depending on technology • Ethernet frame can be up to 1522 bytes long • Address section is 12 bytes (6 bytes for each MAC) • FCS contain the CRC to check frame for errors

  17. Segmenting with Bridges • Bridge Performance • adds 10% to 30% latency due to decision-making process • considered a store-and-forward device because it must calculate the CRC at the end of the frame to check it for errors before forwarding • if the bridge does not have an entry for the destination MAC, it... • adds the source MAC to its bridging table • forwards the frame out all interfaces except the one it was received on • when a reply returns, it adds the destination MAC to the table

  18. Segmenting with Routers • Router Operation • Routers... • use layer 3 addressing (IP, IPX) and routing protocols (RIP, IGRP) to determine the path and • switch the packet out the correct interface to the destination • because a router must open the packet to read Layer 3 addressing, it adds latency • In addition, protocols like TCP which require acknowledgments of every packet can increase latency, reducing throughput from 20% to 40%

  19. Segmenting with Routers • Router Benefits • Like switches, routers segment collision domains. • However, since a router will not forward broadcasts, it also segments broadcast domains. • Each router interface represents its own broadcast domain.

  20. Segmenting with Switches • Switching Benefits • a switch is simply a multi-port bridge, making forwarding decisions based on MAC addresses • so, like a bridge, segmenting a LAN with a switch creates more collision domains • replacing hubs with switches therefore decreases congestion and increases available bandwidth. • a switch can microsegment a LAN creating collision-free domains but still be in the same broadcast domain. • switch creates a virtual circuits, allowing many users to communicate in parallel.

  21. Switching and VLANs

  22. Switch Operation • Switches perform two basic functions: • Building and maintaining switching tables (similar to a bridge table) based on MAC addresses • Switching frames out the interface to the destination • Differences between switches & bridges • Switches operate at higher speeds • Switches are capable of creating virtual LANs (VLANs) through microsegmentation • Bridges switch using software; switches typically switch using hardware (called the “switch fabric”)

  23. Switch Latency (Delay) • A switch adds 21 microseconds of latency. • This can be reduced by using a different switching method • As opposed to store-and-forward, the switch can use cut-through switching which switches the packet as soon as the destination MAC is read.

  24. How a LAN Switch Learns Addresses • MAC addresses are learned dynamically and are stored in CAM (content-addressable memory) • Each time a switch stores an address entry in the table, it is time-stamped. • The time-stamp is updated each time a frame is received • Addresses whose time-stamp expires are deleted from the table • This keeps switching tables small

  25. Benefits of LAN Switching • Cost-effective; switches only cost 3 to 5 times that of a hub • Allows the creation of virtual circuits • More flexibility in managing the network • Reduces number of collisions • Works with existing 802.3 cabling

  26. potential bottlenecks Symmetric Switching • symmetric switching provides switched connections between ports with the same bandwidth (10/10 Mbps or 100/100 Mbps) • can cause bottlenecks as users try to access servers on other segments.

  27. Asymmetric Switching • asymmetric switching reduces the likelihood of a potential bottleneck at the server by attaching the segment with the server to a higher bandwidth port (100 Mbps) • asymmetric switching requires memory buffering in the switch

  28. Memory Buffering • Defined • Area of memory in a switch where destination and transmission data are stored until it can be switched out the correct port. • Two types • Port-based memory buffering • packets are stored in a queue on each port • possible for one packet to delay transmission of other packets because of a busy destination port • Shared memory buffering • common memory buffering shared by all ports • allows packets to be RX on one port and TX out another port without changing it to a different queue.

  29. Two Switching Methods • Store-and-Forward • The switch receives the entire frame, calculating the CRC at the end, before sending it to the destination • Cut-through • Fast forward switching--only checks the destination MAC before immediately forwarding the frame • Fragment Free--reads the first 64 bytes to reduce errors before forwarding the frame

  30. VLANs (IEEE 802.1q) • Characteristics • A logical grouping of network devices or users that are not restricted to a physical switch segment. • The devices or users in a VLAN can be grouped by function, department, application, and so on, regardless of their physical segment location. • A VLAN creates a single broadcast domain that is not restricted to a physical segment and is treated like a subnet. • VLAN setup is done in the switch by the network administrator using the vendor’s software.

  31. The Spanning-Tree Protocol

  32. Overview of STP • Elements of the Spanning Tree Protocol • Main function of STP is to allow redundant paths in a switched/bridged network without incurring latency from the effects of loops. • STP prevents loops by calculating a stable spanning-tree network topology (similar to OSPF operation) • Spanning-tree frames (called bridge protocol data units--BPDUs) are sent and received by all switches in the network and are used to determine the spanning-tree topology • STP operation is covered in detail in Semester 7 of the CCNP curriculum.

  33. Five STP States • States are established by configuring each port according to policy • Then the STP modifies the states based on traffic patterns and potential loops • The default order of STP states are: • Blocking--no frames forwarded, BPDUs heard • Listening--no frames forwarded, listening for data frames • Learning--no frames forwarded, learning addresses • Forwarding--frames forwarded, learning addresses • Disabled--no frames forwarded, no BPDUs heard

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