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Ethernet Switching

Ethernet Switching. CCNA 1. Need for Switching. When the number of devices trying to access the network is low, the number of collisions stays well within acceptable limits. Bridging was developed to help ease performance problems that arose from increased collisions.

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Ethernet Switching

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  1. Ethernet Switching CCNA 1

  2. Need for Switching • When the number of devices trying to access the network is low, the number of collisions stays well within acceptable limits. • Bridging was developed to help ease performance problems that arose from increased collisions. • Switching evolved from bridging

  3. Layer 2 Bridging/Switching • A Bridge keeps a table of MAC addresses and the associated ports. The bridge then forwards or discards frames based on the table entries.

  4. Bridge Operation • The bridge has just been started so the bridge table is empty. The bridge just waits for traffic on the segment.

  5. Bridge Operation • Host A pings Host B. Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the packet. • The bridge adds the source address of the frame to its bridge table.

  6. Bridge Operation • The destination address of the frame is checked against the bridge table. The address is not in the table • Therefore the frame is forwarded to the other segment. • The address of Host B has not been recorded yet.

  7. Bridge Operation • Host B processes the ping request and transmits a ping reply back to Host A. The data is transmitted over the whole collision domain (Host A and the bridge).

  8. Bridge Operation • The bridge adds the frame’s source address to its table. • Since it received the frame on port 1, the bridge associates this address with port 1. • As the destination address is already in the table, and is associated with port 1, the frame is not forwarded out port 2.

  9. Bridge Operation • Host A pings Host C. Host B receives but discards the frame since it was not the intended destination. • The bridge renews the source address of the frame to its bridge table.

  10. Bridge Operation • Since the address is not in the table, the frame is forwarded to the other segment. • The address of Host C has not been recorded yet.

  11. Bridge Operation • Host C processes the ping request and replies back to Host A. • The data is transmitted over the whole collision domain. Host D discards the frame since it is not the intended destination. • The bridge adds the source address of the frame to its bridge table. • The destination address is associated with Port 1, and is forwarded.

  12. Bridge Operation • Host D pings Host C. • The bridge adds the source address to its bridge table. • As the destination address is on the same segment, the frame is not forwarded out port 1.

  13. Bridge Operation MAC 2 • Bridges make decisions based on _____ addresses, or layer __ addresses. • Bridges create _________ domains, which reduce traffic, but do not affect ________ , or layer __ addresses. • The entire network will be part of the same __________ domain. COLLISION 3 LOGICAL BROADCAST

  14. Layer 2 Switching • A switch is a multi-port bridge. • Each port on a switch is in its own collision domain. A 20 port switch will have 20 collision domains. • Increasing the number of collision domains is known as micro-segmentation. • A switch builds and maintains a Content-Addressable Memory (CAM) table, which holds the MAC information for each port.

  15. Layer 2 Switching • A switch dynamically builds and maintains a Content-Addressable Memory (CAM) table, holding all of the necessary MAC information for each port, using ASICs. • An Application-Specific Integrated Circuit (ASIC) is a device that can be programmed to perform functions at high speeds. • Operations that were done in software can now be done in hardware using an ASIC. • The use of these technologies greatly reduced the delays caused by software processing.

  16. Full Duplex • Most switches and most network interface cards (NICs) are capable of supporting full duplex. • In full duplex mode, there is no contention for the media. • Thus, with switched full-duplex transmission there are no collision domains . • Theoretically, the bandwidth is doubled when using full duplex. • A Switch: • Doubles bandwidth between nodes • Collision-free transmission • Two 10 or 100 Mbps data paths A switch uses full-duplex mode to provide full bandwidth between two nodes on a network.

  17. Switch Benefits

  18. Latency • Latency is the delay between the time a frame first starts to leave the source device and the time the first part of the frame reaches its destination. • Caused by: • Media - the time it takes for a signal to move through the media • Electronic processing of signal (circuit delay) • Software switching decisions • Content of frame – eg reading destination address

  19. Switch Modes • Time is the issue. Users want their computers to work as fast as possible. • How a frame is switched to the destination port is a trade off between latency and reliability. • A switch has three different methods it can used in processing frames.

  20. Switch Modes • Cut-through switching— A switch can start to transfer the frame as soon as the destination MAC address is received. This has the lowest latency. • There is NO error checking. • Store-and-forward switching—The switch can receive the entire frame before sending it out the destination port. This gives the switch software an opportunity to verify the frame check sum (FCS). • Store-and-forward mode must be used for asynchronous switching.

  21. Symmetric vs. Asymmetric switching • Symmetric – the source and destination ports must have the same bit rate to keep the frame intact. • Asymmetric – the bit rates are not the same, so the frame must be stored at one bit rate, before it is sent out at the other bit rate. • Used with ports of different bandwidths, such as with a server in a client/server network. (needed to prevent bottlenecks)

  22. Symmetric vs. Asymmetric switching

  23. Symmetric vs. Asymmetric switching

  24. Switch Modes • Fragment-free mode – a compromise between cut-through and store-and-forward packet switching • The first 64 bytes are read before it starts to send the frame out the destination port. • The CRC and the frame length are verified before forwarding the frame.

  25. Switch Modes Start of frame

  26. Collision Domains Broadcast Domains

  27. Shared Media Environments • It is important to be able to identify a shared media environment, because collisions only occur in a shared environment. Multiple hosts have access to the same medium; eg copper, fiber, air Networking devices extend the network to accommodate multiple access or longer cable distances. One device is connected to only one other device; eg dialup connection

  28. Collision Domains • Collision domains are the connected physical network segments where collisions can occur. • Collisions cause network inefficiency.

  29. The Four Repeater Rule • The 5-4-3-2-1 rule requires that the following guidelines should not be exceeded: • Five segments of network media • Four repeaters or hubs • Three host segments of the network • Two link sections with no hosts • One large collision domain

  30. Collisions and Collision Domains • When two bits are propagated at the same time on the same network, a collision will occur. • They are not inherently bad. • They are a normal function of Legacy Ethernet. • Data on the network during a collision is lost and usually must be retransmitted. • Increased collisions indicate congestion. • Collisions affect ALL nodes in a collision domain. • Networks with only Layer 1 components are a single collision domain.

  31. Created by a hub Extended by a huband repeater Shared Media, Repeaters, Hubs, and Collision Domains Shared access Extended by a repeater Extended by a repeater Each is a single collision domain!

  32. How many collision domains? ONE

  33. How do you solve the problem? Add a Bridge

  34. Add a bridge = 2 collision domains

  35. Segmentation • Layer 2 devices segment or divide collision domains. • Segmentation makes networks more efficient and allow data to be transmitted on different segments of the LAN at the same time without the frames colliding.

  36. Segmenting with Bridges

  37. Segmenting with Switches

  38. Segmenting with Routers

  39. Layer 2 Broadcasts = FFF-FFF-FFF-FFF • The three sources of broadcasts and multicasts in IP networks are workstations, routers, and multicast applications. • ARP requests • Routing table updates • Multicasts to specified IP addresses • Layer 2 devices must flood all broadcast and multicast traffic. • The accumulation of broadcast and multicast traffic from each device in the network is referred to as broadcast radiation.

  40. Broadcast Domains? The only devices that can segment collision domains are bridges, switches (both Layer 2), and routers (Layer 3).

  41. 2 collision domains1 broadcast domain

  42. 2 collision domains1 broadcast domain

  43. Summary • Bridges and Switches are layer two devices. Forward frames after examining destination MAC address • Each port of the switch has a separate collision domain (microsegmentation). • Small collision domain => less collisions. • STP is used to prevent loops created by redundant links.

  44. Collision and Broadcast Domains • All decisions made by a bridge are based on the ____ or ______ addressing and do not affect the _______ or ___________. • A bridge will create more ____________ but will not add _____________. MAC Layer 2 Layer 3 Logical address Collision domains Broadcast domains

  45. Switching/Bridging Table E0 0260.8c01.1111 E0 0260.8c01.2222 E1 0260.ec01.3333 0260.8c01.4444 E1

  46. Fill in the blanks 2 Multiple 2 Multiple 1 1 Content-Addressable memory (CAM) Bridging Table

  47. How many collision/broadcast domains? Collision = 6 Broadcast = 2

  48. What impact does a bridge have on a network? • Breaks up collision domains • What happens with switched full-duplex transmissions? • No collisions domains • What is STP and what 2 tasks does it perform? • Spanning Tree Protocol • elect root bridge & decide designated ports

  49. Broadcast Domains Layer 2 • A broadcast domain is a grouping of collision domains that are connected by ________ devices. • Smaller collision domains increase the opportunity for each host in the network to ____________________. • _________ are forwarded by Layer 2 devices and if excessive, can reduce the efficiency of the entire LAN. • Broadcasts have to be controlled at Layer __. • _______ do not forward broadcasts.  • Layer 3 forwarding is based on the destination __________ and not the MAC address. gain access to the media Broadcasts 3 Routers IP address

  50. Segmentation 16 6 How many collision domains are there? How many broadcast domains are there?

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