Chapter 13


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Chapter 13

  1. 1. CHAPTER THIRTEEN Switching and VLANs
  2. 2. Objectives <ul><li>Explain the features and benefits of Fast Ethernet </li></ul><ul><li>Describe guidelines and distance limitations of Fast Ethernet </li></ul><ul><li>Define full- and half-duplex Ethernet operations </li></ul><ul><li>Distinguish between cut-through, fragment-free, and store-and-forward LAN switching </li></ul><ul><li>Define the operation of the Spanning Tree Protocol and its benefits </li></ul><ul><li>Describe the benefits of virtual LANs </li></ul><ul><li>Understand purpose of VLAN trunking protocol (VTP) </li></ul>
  3. 3. Ethernet Operations <ul><li>Ethernet </li></ul><ul><ul><li>A network access method (or media access method ) </li></ul></ul><ul><ul><li>The most pervasive network access method in use </li></ul></ul><ul><ul><li>Continues to be the most commonly implemented media access method in new LANs </li></ul></ul>
  4. 4. CSMA/CD <ul><li>Carrier Sense Multiple Access with Collision Detection (CSMA/CD) </li></ul><ul><ul><li>The contention method used by Ethernet </li></ul></ul><ul><li>Interframe gap </li></ul><ul><ul><li>Also called interpacket gap (IPG) </li></ul></ul><ul><ul><li>Time required between the transmission of data frames on the network </li></ul></ul>
  5. 5. Collisions <ul><li>Carrier signal </li></ul><ul><ul><li>Transmitted electromagnetic pulse or wave on network wire indicating transmission is in progress </li></ul></ul><ul><li>Jam signal </li></ul><ul><ul><li>32-bit signal sent by the first station to detect a collision on an Ethernet network </li></ul></ul><ul><li>Backoff period </li></ul><ul><ul><li>Random interval used by devices that have caused a collision on an Ethernet network during which the devices cannot send </li></ul></ul>
  6. 6. Collision Domain <ul><li>Collision domain </li></ul><ul><ul><li>The physical area in which a packet collision might occur </li></ul></ul><ul><li>Repeaters and hubs do not segment the network and therefore do not divide collision domains </li></ul><ul><li>Routers, switches, bridges, and gateways do not segment network and thus create collision domain </li></ul>
  7. 7. Latency <ul><li>Latency </li></ul><ul><ul><li>Also called propagation delay </li></ul></ul><ul><ul><li>Length of time required to forward, send, or otherwise or propagate a data frame </li></ul></ul><ul><li>Transmission time </li></ul><ul><ul><li>Amount of time it takes for a packet to be sent from one device and received at another device </li></ul></ul>
  8. 8. Latency Table 13-1: Propagation delay for Ethernet media and devices
  9. 9. Latency <ul><li>Slot time </li></ul><ul><ul><li>512 bit times </li></ul></ul><ul><li>5-4-3 rule </li></ul><ul><ul><li>Networking rules that stipulates that between stations on a 10-Mbps half-duplex LAN: </li></ul></ul><ul><ul><ul><li>There can be no more than 5 wire segments connected </li></ul></ul></ul><ul><ul><ul><li>Maximum number of repeaters or hubs between the segments is 4 </li></ul></ul></ul><ul><ul><ul><li>Maximum number of populated hubs is 3 </li></ul></ul></ul>
  10. 10. Ethernet Errors: Frame Size Errors <ul><li>Frame size errors that occur on Ethernet networks: </li></ul><ul><ul><li>Short frame </li></ul></ul><ul><ul><ul><li>Also known as runt </li></ul></ul></ul><ul><ul><li>Long frame </li></ul></ul><ul><ul><ul><li>Also known as a giant </li></ul></ul></ul><ul><ul><li>Jabber </li></ul></ul><ul><ul><ul><li>Longer than Ethernet standards allow and has an incorrect frame check sequence (FCS) </li></ul></ul></ul>
  11. 11. Ethernet Errors: Frame Size Errors <ul><li>Frame check sequence (FCS) error </li></ul><ul><ul><li>Indicates that bits of a frame were corrupted during transmission </li></ul></ul><ul><ul><li>Detected when the calculation at the end of a packet doesn’t conform correctly to the number and sequence of bits in the frame </li></ul></ul><ul><li>If a frame with an FCS error also has an octet missing, it is also called an alignment error </li></ul>
  12. 12. Ethernet Errors: Collision Errors <ul><li>A large number of devices on a collision domain means a higher chance that there will be a significant number of collisions </li></ul><ul><li>A transmitting station will attempt to send its packet 16 times before discarding it as a NIC error </li></ul><ul><li>Late collision </li></ul><ul><ul><li>Occurs when two stations transmit more than 64-bytes of their data frame before detecting a collision </li></ul></ul>
  13. 13. Ethernet Errors: Broadcasts <ul><li>Broadcast </li></ul><ul><ul><li>Frame addressed to all stations on broadcast domain </li></ul></ul><ul><li>Broadcast storm </li></ul><ul><ul><li>Logical or physical group devices that will receive broadcast traffic from each other on a LAN </li></ul></ul><ul><li>Protocol analyzer </li></ul><ul><ul><li>Can be used to locate the device causing the broadcast storm </li></ul></ul>
  14. 14. Fast Ethernet <ul><li>Defined under the IEEE 802.3u standard </li></ul><ul><li>Has three defined implementations: </li></ul><ul><ul><li>100Base-TX </li></ul></ul><ul><ul><li>100Base-T4 </li></ul></ul><ul><ul><li>100Base-FX </li></ul></ul><ul><li>Uses multimode fiber-optic (MMF) cable with one TX and one RX strand per link </li></ul>
  15. 15. Half- and Full-Duplex Communications <ul><li>Half-duplex </li></ul><ul><ul><li>Connection that allows communication in two directions, but not simultaneously </li></ul></ul><ul><li>Full-duplex </li></ul><ul><ul><li>Connection that allows communication in two directions at once </li></ul></ul>
  16. 16. Half- and Full-Duplex Communications Figure 13-1: Half-duplex Ethernet communications
  17. 17. Half- and Full-Duplex Communications <ul><li>Benefits of using full duplex: </li></ul><ul><ul><li>Time is not wasted retransmitting frames because there are no collisions </li></ul></ul><ul><ul><li>Full bandwidth is available in both directions because the send and receive functions are separate </li></ul></ul><ul><ul><li>Stations do not have to wait to until other stations complete their transmission because there is only one transmitter for each twisted pair </li></ul></ul>
  18. 18. Half- and Full-Duplex Communications <ul><li>The four different duplex options: </li></ul><ul><ul><li>Auto </li></ul></ul><ul><ul><li>Full </li></ul></ul><ul><ul><li>Full-flow-control </li></ul></ul><ul><ul><li>Half </li></ul></ul>
  19. 19. LAN Segmentation: Segmenting with Bridges <ul><li>Bridge </li></ul><ul><ul><li>Segments a network by filtering traffic at the Data Link layer </li></ul></ul><ul><li>Bridging table </li></ul><ul><ul><li>Maintained on the bridge </li></ul></ul><ul><ul><li>Maps MAC addresses to the bridge port through which they can be accessed </li></ul></ul>
  20. 20. When Segmenting a LAN with One or More Bridges, Note the Following <ul><li>Bridges reduce collisions by segmenting LAN and filtering traffic based on MAC addresses </li></ul><ul><li>A bridge does not reduce broadcast or multicast traffic </li></ul><ul><li>A bridge can extend the useful distance of the Ethernet LAN </li></ul><ul><li>The bandwidth for individual segment is increased </li></ul><ul><li>Bridges can be used to limit traffic for security purposes by keeping traffic segregated </li></ul>
  21. 21. LAN Segmentation: Segmenting with Routers <ul><li>Router </li></ul><ul><ul><li>Operates at layer 3 of the OSI reference model </li></ul></ul><ul><ul><li>Interprets the Network layer protocol and makes forwarding decisions based on the layer 3 address </li></ul></ul><ul><ul><li>Typically do not propagate broadcast traffic </li></ul></ul><ul><ul><li>Maintain routing tables that include Network layer addresses of different segments </li></ul></ul>
  22. 22. When Segmenting a LAN with Routers, Routers Do the Following <ul><li>Decrease collisions by filtering traffic </li></ul><ul><li>Reduce broadcast and multicast traffic by blocking or selectively filtering packets </li></ul><ul><li>Support multiple paths and routes between them </li></ul><ul><li>Provide increased bandwidth for the newly created segments </li></ul><ul><li>Increase security by preventing packets between hosts on one side of the router from propagating to the other side of the router </li></ul>
  23. 23. When Segmenting a LAN with Routers, Routers Do the Following <ul><li>Increase the effective distance of the network by creating new collision domains </li></ul><ul><li>Provide layer 3 routing, packet fragmentation and reassembly, and traffic flow control </li></ul><ul><li>Provide communications between different technologies such as Ethernet and Token Ring </li></ul><ul><li>Have a higher latency than bridges because routers have more to process </li></ul>
  24. 24. LAN Switching <ul><li>Switches </li></ul><ul><ul><li>Device that connects devices on a LAN </li></ul></ul><ul><ul><li>Segments collision domain by port </li></ul></ul><ul><ul><li>Similar to bridges in several ways </li></ul></ul><ul><ul><ul><li>Using a switch on a LAN has a different effect on the way network traffic is propagated </li></ul></ul></ul>
  25. 25. Segmentation with Switches <ul><li>Switches are so similar to bridges, they are often called multiport bridges </li></ul><ul><li>Switches are hardware-controlled </li></ul><ul><li>By connecting each port to an individual workstation, switches microsegment the network </li></ul><ul><li>The bandwidth is not shred as long as each workstation connects to its own port </li></ul><ul><ul><li>This is called switched bandwidth </li></ul></ul>
  26. 26. Segmentation with Switches Figure 13-2: Switch packet forwarding actions
  27. 27. Segmentation with Switches Figure 13-3: Packet forwarding decisions made by a switch
  28. 28. Segmentation with Switches <ul><li>Benefits provided by switches: </li></ul><ul><ul><li>Reduction in network traffic and collisions </li></ul></ul><ul><ul><li>Increase in available bandwidth per station </li></ul></ul><ul><ul><li>Increase in effective distance of a LAN by dividing it into multiple collision domains </li></ul></ul><ul><ul><li>Increased security because unicast traffic is sent directly to its destination </li></ul></ul>
  29. 29. Switch Operations <ul><li>Content-addressable memory (CAM) </li></ul><ul><ul><li>Memory location on a switch that contains MAC address-to-switch port mapping information </li></ul></ul><ul><li>A switch uses one of two memory buffers to store frames as it determines to which port a frame will be forwarded </li></ul><ul><ul><li>Port-based memory buffering </li></ul></ul><ul><ul><li>Shared memory buffering </li></ul></ul>
  30. 30. Switch Operations <ul><li>Asymmetric switching </li></ul><ul><ul><li>Type of LAN switching that allows for multiple speeds of network communication </li></ul></ul><ul><li>Symmetric switching </li></ul><ul><ul><li>Type of LAN switching that requires all devices to be operating at the same speed </li></ul></ul>
  31. 31. Securing Switch Ports <ul><li>There are several different degrees of security that can be employed on a switch: </li></ul><ul><ul><li>Configure a permanent MAC address for a specific port on your switch </li></ul></ul><ul><ul><li>Define a static a MAC address entry into the switching table </li></ul></ul><ul><ul><li>You can configure port security </li></ul></ul>
  32. 32. Switching Methods Figure 13-4: Catalyst 2820 switching menu
  33. 33. Cut-Through <ul><li>Switching technique in which Ethernet frame is forwarded immediately after destination address is deciphered </li></ul><ul><li>Cisco routers us the term fast forward to indicate that a switch is in cut-through mode </li></ul><ul><li>Any errors occurring in the following fields will not be propagated by the switch: </li></ul><ul><ul><li>The preamble </li></ul></ul><ul><ul><li>The start frame delimiter (SFD) </li></ul></ul><ul><ul><li>The destination address </li></ul></ul>
  34. 34. Cut-Through Figure 13-5: Portion of packet read into buffer by a cut-through switch
  35. 35. Store-and-Forward Switches <ul><li>Read the entire into their buffers before forwarding them </li></ul>Figure 13-6: Entire packet read into buffer by a store-and-forward switch
  36. 36. Fragment-Free Switching <ul><li>Tries to provide more error-reducing benefits than cut-through switching, while keeping latency lower than store-and-forward switching </li></ul><ul><li>Fragment-free switches are also called modified cut-through switches </li></ul>13-7: Amount of packet read into buffer by fragment-free switch
  37. 37. Adaptive Cut-Through and Changing the Switch Mode <ul><li>Adaptive cut-through </li></ul><ul><ul><li>Also known as error sensing </li></ul></ul><ul><ul><li>Mostly, these switches act like cut-through switches </li></ul></ul><ul><ul><li>If a certain level of errors is detected, switch will change forwarding techniques and act more as store-and-forward switch </li></ul></ul><ul><li>Changing the Switching mode </li></ul><ul><ul><li>When you change the switching type, you change it for all ports on the switch </li></ul></ul>
  38. 38. Spanning Tree Protocol <ul><li>Physical path loops </li></ul><ul><ul><li>Occur when network devices are connected to one another by two or more physical media links </li></ul></ul><ul><li>Logical loop </li></ul><ul><ul><li>Occurs when a packet can be routed in an endless loop around a network because bridging tables and/or routing tables reference each other as the destination for a given address </li></ul></ul>
  39. 39. Spanning Tree Protocol Figure 13-8: Physical loop created on LAN by switches and bridges
  40. 40. Spanning Tree Protocol (STP) <ul><li>Invented by Radia Perlman while she was Digital Equipment Corporation (now Compaq) in the 1980s </li></ul><ul><li>Layer 2 link management protocol designed to prevent looping on bridges and switches </li></ul><ul><li>Specification for STP is IEEE802.1d </li></ul><ul><li>Uses the Spanning Tree Algorithm to interrupt the logical loop created by a physical loop in a bridged/switched environment </li></ul>
  41. 41. Building a Logical Path <ul><li>With STP enabled, switches and bridges on a network use an election process to configure a single logical path </li></ul><ul><li>Root bridge </li></ul><ul><ul><li>Also called root device </li></ul></ul><ul><ul><li>Bridge or switch that is designated the point of reference in STP operations </li></ul></ul>
  42. 42. Building a Logical Path <ul><li>Bridges use STP to transfer information about each bridge’s MAC address and priority number </li></ul><ul><ul><li>The messages the devices send to one another are called: </li></ul></ul><ul><ul><ul><li>Bridge protocol data units (BPDU) </li></ul></ul></ul><ul><ul><ul><ul><li>BPDU messages are sent between root bridge and best ports on other devices, which are called root ports </li></ul></ul></ul></ul><ul><ul><ul><li>Configuration bridge protocol data units (CBPDU) </li></ul></ul></ul>
  43. 43. Port States <ul><li>The stable states are as follows: </li></ul><ul><ul><li>Blocking </li></ul></ul><ul><ul><li>Forwarding </li></ul></ul><ul><ul><li>Disabled </li></ul></ul><ul><li>The transitory states are as follows: </li></ul><ul><ul><li>Listening </li></ul></ul><ul><ul><li>Learning </li></ul></ul>
  44. 44. Port States <ul><li>Ports on STP-enabled devices move through the different states as indicated below: </li></ul><ul><ul><li>From bridge/switch bootup or blocking </li></ul></ul><ul><ul><li>From blocking to listening (or to disabled) </li></ul></ul><ul><ul><li>From listening to learning (or to disabled) </li></ul></ul><ul><ul><li>From learning to forwarding (or to disabled) </li></ul></ul><ul><ul><li>From forwarding to disabled </li></ul></ul>
  45. 45. Virtual LAN <ul><li>Virtual LAN (VLAN) </li></ul><ul><ul><li>Grouping of network devices that is not restricted to a physical segment or switch </li></ul></ul><ul><li>Broadcast domain </li></ul><ul><ul><li>Group of network devices that will receive LAN broadcast traffic from each other </li></ul></ul><ul><li>By default, every port on a switch is in VLAN1 </li></ul><ul><ul><li>This is the management (or default ) VLAN </li></ul></ul>
  46. 46. Virtual LAN Figure 13-9: Broadcast domains on a LAN
  47. 47. Virtual LAN Figure 13-10: Broadcast domains using VLANs
  48. 48. Benefit of VLANs <ul><li>Administrators can divide LANs logically without changing actual physical configuration </li></ul><ul><ul><li>This provides administrator with several benefits: </li></ul></ul><ul><ul><ul><li>It is easier to add and move stations on the LAN </li></ul></ul></ul><ul><ul><ul><li>It is easier to reconfigure the LAN </li></ul></ul></ul><ul><ul><ul><li>There is better traffic control </li></ul></ul></ul><ul><ul><ul><li>There is increased security </li></ul></ul></ul>
  49. 49. Benefit of VLANs Figure 13-11: Securing servers with VLANs
  50. 50. Dynamic Versus Static VLANs <ul><li>Static VLANs are configured port-by-port </li></ul><ul><li>In static VLANs, the administrator manually types in mapping for each port and VLAN </li></ul><ul><li>Dynamic VLAN ports can automatically determine their VLAN configuration </li></ul><ul><li>Dynamic VLAN uses a software database of MAC address-to-VLAN mappings that is created manually </li></ul>
  51. 51. VLAN Standardization <ul><li>Frame filtering </li></ul><ul><ul><li>Technique used on early VLAN implementations that employed the use of multiple switching tables </li></ul></ul><ul><li>When creating its VLAN standards, the IEEE did not choose the frame filtering method </li></ul><ul><li>Frame tagging </li></ul><ul><ul><li>Also known as frame identification </li></ul></ul><ul><ul><li>Method of VLAN identification endorsed by IEEE 802.1q specification </li></ul></ul>
  52. 52. VLAN Standardization <ul><li>Two most common types of frame tagging: </li></ul><ul><ul><li>802.1q </li></ul></ul><ul><ul><li>Inter-Switch Link (ISL) protocol </li></ul></ul><ul><li>Other types of frame tagging include: </li></ul><ul><ul><li>LAN emulation (LANE) </li></ul></ul><ul><ul><li>IEEE 802.10 (FDDI) </li></ul></ul>
  53. 53. Creating VLANs <ul><li>Creating VLANs on the Cisco Catalyst 1900 switch using the command line interface is straightforward </li></ul><ul><li>You name the VLANs individually via global configuration mode </li></ul><ul><li>Assigning dynamic VLANs is more complicated than assigning static VLANs, and is not a requirement of CCNA candidates </li></ul>
  54. 54. Link Types and Configuration <ul><li>There are two types of links on Cisco switches: </li></ul><ul><ul><li>Trunk </li></ul></ul><ul><ul><ul><li>Switch-to-switch or switch-to-router links that can carry traffic from multiple VLANs </li></ul></ul></ul><ul><ul><li>Access lists </li></ul></ul><ul><ul><ul><li>Links going to non-VLAN-aware devices such as hubs and individual workstations </li></ul></ul></ul>
  55. 55. Link Types and Configuration <ul><li>Five different states you can set for a trunk link: </li></ul><ul><ul><li>Auto </li></ul></ul><ul><ul><li>Desirable </li></ul></ul><ul><ul><li>Nonegotiate </li></ul></ul><ul><ul><li>Off </li></ul></ul><ul><ul><li>On </li></ul></ul>
  56. 56. Trunking Protocol <ul><li>VLAN trunking protocol (VTP) </li></ul><ul><ul><li>Layer 2 messaging protocol </li></ul></ul><ul><ul><li>Manages all changes to the VLANs across networks </li></ul></ul><ul><li>Any changes made to a VLAN by an administrator are automatically propagated by VTP to all VTP-enabled devices </li></ul>
  57. 57. VTP Domains <ul><li>VTP domain </li></ul><ul><ul><li>Group of VTP-enabled devices configured under one name to share VLAN information </li></ul></ul><ul><li>When you make changes to the VTP configuration, you should verify them with the show vtp command from enable mode </li></ul><ul><li>If all switches are in the same VLAN, there is no need to configure a VTP domain </li></ul>
  58. 58. VTP Device Modes <ul><li>There are three different modes for VTP-enabled devices: </li></ul><ul><ul><li>Server </li></ul></ul><ul><ul><li>Client </li></ul></ul><ul><ul><li>Transparent </li></ul></ul>
  59. 59. VTP Pruning <ul><li>Reduces the number of VTP updates that traverse a link </li></ul><ul><li>Off by default on all switches </li></ul><ul><ul><li>If turned on, VTP message broadcasts are only sent through trunk links that must have the information </li></ul></ul><ul><li>When enabled on a server, it is enabled on every device in the entire domain </li></ul>
  60. 60. Switch Interface Descriptions <ul><li>You can configure a name for each port on a switch </li></ul><ul><li>This is useful when defining roles for a switch port on a global basis </li></ul><ul><ul><li>Such as when you configure VLANs </li></ul></ul>
  61. 61. Nonswitching Hubs and VLANs <ul><li>Considerations to keep in mind when implementing hubs on a network that employs VLANs: </li></ul><ul><ul><li>If you insert a hub into a port on a switch and then connect several devices to the hub, all the system attached to that hub will be in the same VLAN </li></ul></ul><ul><ul><li>If you must move a single workstation that is attached to a hub with several workstations, you will have to physically attach the device to another hub or switch ports in order to change its VLAN assignment </li></ul></ul><ul><ul><li>The more hosts attached at individual switch ports, the greater the microsegmentation and flexibility the VLAN can offer </li></ul></ul>
  62. 62. Routers and VLANs Figure 13-12: Router implemented in a VLAN configuration
  63. 63. Chapter Summary <ul><li>Ethernet (CSMA/CD) is a media access method developed in the 1960s </li></ul><ul><li>Stations on an Ethernet LAN must listen to the network media before transmitting to ensure that no other station is currently transmitting </li></ul><ul><li>If two stations transmit simultaneously on the same collision domain, there will be a collision </li></ul><ul><li>You can segment a network with bridges, switches, or routers to reduce the number of collisions occurring on a network </li></ul><ul><li>Switches do the most to divide collision domains and reduce traffic without dividing broadcast domains </li></ul>
  64. 64. Chapter Summary <ul><li>Switches do the most to divide collision domains and reduce traffic without dividing broadcast domains </li></ul><ul><li>Another way to increase the speed at which a LAN operates is to upgrade from Ethernet to Fast Ethernet </li></ul><ul><li>Full duplex can also improve Ethernet performance over half-duplex operations </li></ul><ul><li>STP allows administrators to create physical loops between bridges and switches without creating logical loops that would create a problem for packet delivery </li></ul>
  65. 65. Chapter Summary <ul><li>Another way to increase performance, flexibility, and security of a network is to implement VLANs via switches </li></ul><ul><li>VLANs are separate broadcast domains that are not limited by physical configurations </li></ul><ul><li>VLAN information is communicated to switches using the VLAN trunking </li></ul>
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