Ch. 4 – Switching Concepts

1. Ch. 4 – Switching Concepts CCNA 3 version 3.0

2. Overview • Ethernet networks used to be built using repeaters. • When the performance of these networks began to suffer because too many devices shared the same segment, network engineers added bridges to create multiple collision domains. • As networks grew in size and complexity, the bridge evolved into the modern switch, allowing microsegmentation of the network. • Today’s networks typically are built using switches and routers, often with the routing and switching function in the same device. Routers Switches, Bridges Hub, Repeaters

3. Ethernet/802.3 LAN development • Distance limitations • Ethernet is fundamentally a shared technology where all users on a given LAN segment compete for the same available bandwidth. • This situation is analogous to a number of cars all trying to access a one-lane road at the same time. • Because the road has only one lane, only one car can access it at a time. • The introduction of hubs into a network resulted in more users competing for the same bandwidth. • Collisions are a by-product of Ethernet networks.

4. Bridges • A bridge is a Layer 2 device used to divide, or segment, a network. • A bridge is capable of collecting and selectively passing data frames between two network segments. • Bridges do this by learning the MAC address of all devices on each connected segment. Using this information, the bridge builds a bridging table and forwards or blocks traffic based on that table. • This results in smaller collision domains and greater network efficiency. • Bridges do NOT restrict broadcast traffic.

5. Switches • Switches create a virtual circuit between two connected devices, establishing a dedicated communication path between two devices. • Switches on the network provide microsegmentation. • This allows maximum utilization of the available bandwidth. • A switch is also able to facilitate multiple, simultaneous virtual circuit connections. • Broadcast frames to all connected devices on the network.

6. Router • A router is a Layer 3 device. • Used to “route” traffic between two or more Layer 3 networks. • Routers make decisions based on groups of network addresses, or classes, as opposed to individual Layer 2 MAC addresses. • Routers use routing tables to record the Layer 3 addresses of the networks that are directly connected to the local interfaces and network paths learned from neighboring routers. • Routers are not compelled to forward broadcasts.

7. Factors that impact network performance

8. Elements of Ethernet/802.3 networks • Broadcast data frame delivery of Ethernet/802.3 • The carrier sense multiple access/collision detect (CSMA/CD) method allows only one station to transmit at a time. • Multimedia applications with higher bandwidth demand such as video and the Internet, coupled with the broadcast nature of Ethernet, can create network congestion. • Normal latency as the frames travel across the layers • Extending the distances and increasing latency of the Ethernet/802.3 LANs by using Layer 1 repeaters.

9. Half-Duplex • Originally Ethernet was a half-duplex technology. • Using half-duplex, a host could either transmit or receive at one time, but not both. • If the network is already in use, the transmission is delayed. • When a collision occurs, the host that first detects the collision will send out a jam signal to the other hosts. • Upon receiving the jam signal, each host will stop sending data, then wait for a random period of time before attempting to retransmit. • The back-off algorithm generates this random delay. • As more hosts are added to the network and begin transmitting, collisions are more likely to occur.

10. Duplex Transmissions • Simplex Transmission: One way and one way only. • One way street • Half-duplex Transmission: Either way, but only one way at a time. • Two way street, but only one way at a time (land slide). • Full-duplex Transmission: Both ways at the same time. • Two way street

11. Network Congestion • Today's networks are experiencing an increase in the transmission of many forms of media: • Large graphics files • Images • Full-motion video • Multimedia applications

12. Network Latency • Latency, or delay, is the time a frame or a packet takes to travel from the source station to the final destination. • It is important to quantify the total latency of the path between the source and the destination for LANs and WANs. • Latency has at least three sources: • First, there is the time it takes the source NIC to place voltage pulses on the wire and the time it takes the receiving NIC to interpret these pulses. This is sometimes called NIC delay. • Second, there is the actual propagation delay as the signal takes time to travel along the cable. • Third, latency is added according to which networking devices, whether they are Layer 1, Layer 2, or Layer 3, are added to the path between the two communicating computers.

13. Ethernet 10 BASE-T transmission time • Transmission time equals the number of bits being sent times the bit time for a given technology. • Another way to think about transmission time is the time it takes a frame to be transmitted. • Small frames take a shorter amount of time. Large frames take a longer amount of time. • Each 10 Mbps Ethernet bit has a 100 ns transmission window. • Therefore, 1 byte takes a minimum of 800 ns to transmit. • A 64-byte frame, the smallest 10BASE-T frame allowing CSMA/CD to function properly, takes 51,200 ns ( 51.2 microseconds). • Transmission of an entire 1000-byte frame from the source station requires 800 microseconds.

14. The benefits of using repeaters • The distance that a LAN can cover is limited due to attenuation. • Attenuation means that the signal weakens as it travels through the network. • The resistance in the cable or medium through which the signal travels causes the loss of signal strength. • An Ethernet repeater is a physical layer device on the network that boosts or regenerates the signal on an Ethernet LAN.

15. Full-duplex transmitting • Full-duplex Ethernet allows the transmission of a packet and the reception of a different packet at the same time. • To transmit and receive simultaneously, a dedicated switch port is required for each node. • The full-duplex Ethernet switch takes advantage of the two pairs of wires in the cable by creating a direct connection between the transmit (TX) at one end of the circuit and the receive (RX) at the other end. • Ethernet usually can only use 50%-60% of the available 10 Mbps of bandwidth because of collisions and latency. • Full-duplex Ethernet offers 100% of the bandwidth in both directions. • This produces a potential 20 Mbps throughput, which results from 10 Mbps TX and 10 Mbps RX. 

16. Duplex Transmissions • Simplex Transmission: One way and one way only. • One way street • Half-duplex Transmission: Either way, but only one way at a time. • Two way street, but only one way at a time (land slide). • Full-duplex Transmission: Both ways at the same time. • Two way street

17. LAN segmentation • Not the best diagram, let’s look at some examples…

18. Sending and receiving Ethernet frames on a bus • When an Ethernet frame is sent out on the “bus” all devices on the bus receive it. • What do they do with it? Abbreviated MAC Addresses 1111 2222 3333 nnnn 3333 1111

19. Sending and receiving Ethernet frames on a bus • Each NIC card compares its own MAC address with the Destination MAC Address. • If it matches, it copies in the rest of the frame. • If it does NOT match, it ignores the rest of the frame. • Unless you are running a Sniffer program Hey, that’s me! Nope Nope Abbreviated MAC Addresses 1111 2222 3333 nnnn 3333 1111

20. Sending and receiving Ethernet frames on a bus • So, what happens when multiple computers try to transmit at the same time? Abbreviated MAC Addresses 1111 2222 3333 nnnn

21. Sending and receiving Ethernet frames on a bus Collision! Abbreviated MAC Addresses 1111 2222 3333 nnnn X

22. Access Methods Two common types of access methods for LANs include • Non-Deterministic: Contention methods (Ethernet, IEEE 802.3) • Only one signal can be on a network segment at one time. • Collisions are a normal occurrence on an Ethernet/802.3 LAN • Deterministic: Token Passing (Token Ring)

23. • CSMA/CD CSMA/CD (Carrier Sense Multiple Access with Collision Detection) • Common contention method used with Ethernet and IEEE 802.3 • “Let everyone have access whenever they want and we will work it out somehow.”

24. • CSMA/CD and Collisions CSMA/CD (Carrier Sense Multiple Access with Collision Detection) • Listens to the network’s shared media to see if any other users on “on the line” by trying to sense a neutral electrical signal or carrier. • If no transmission is sensed, then multiple accessallows anyone onto the media without any further permission required. • If two PCs detect a neutral signal and access the shared media at the exact same time, a collision occurs and is detected. • The PCs sense the collision by being unable to deliver the entire frame (coming soon) onto the network. (This is why there are minimum frame lengths along with cable distance and speed limitations. This includes the 5-4-3 rule.) • When a collision occurs, a jamming signal is sent out by the first PC to detect the collision. • Using either a priority or random backoff scheme, the PCs wait certain amount of time before retransmitting. • If collisions continue to occur, the PCs random interval is doubled, lessening the chances of a collision.

25. • CSMA/CD and Collisions And as we said, • When information (frame) is transmitted, every PC/NIC on the shared media copies part of the transmitted frame to see if the destination address matches the address of the NIC. • If there is a match, the rest of the frame is copied • If there is NOT a match the rest of the frame is ignored. Hey, that’s me! Nope Nope Abbreviated MAC Addresses 1111 2222 3333 nnnn Notice the location of the DA! 3333 1111

26. • Sending and receiving Ethernet frames via a hub • So, what does a hub do when it receives information? • Remember, a hub is nothing more than a multiport repeater. 3333 1111 1111 ? 2222 5555 3333 4444

27. • Sending and receiving Ethernet frames via a hub Hub or

28. • Sending and receiving Ethernet frames via a hub • The hub will flood it out all ports except for the incoming port. • Hub is a layer 1 device. • A hub does NOT look at layer 2 addresses, so it is fast in transmitting data. • Disadvantage with hubs: A hub or series of hubs is a single collision domain. • A collision will occur if any two or more devices transmit at the same time within the collision domain. • More on this later. 3333 1111 1111 2222 Nope 5555 Nope 3333 For me! 4444 Nope

29. • Sending and receiving Ethernet frames via a hub • Another disadvantage with hubs is that is take up unnecessary bandwidth on other links. 2222 1111 1111 2222 For me! 5555 Wasted bandwidth Nope 3333 Nope 4444 Nope

30. • Sending and receiving Ethernet frames via a switch

31. • Sending and receiving Ethernet frames via a switch Source Address Table PortSource MAC Add.PortSource MAC Add. • Switches are also known as learning bridges or learning switches. • A switch has a source address table in cache (RAM) where it stores source MAC address after it learns about them. • A switch receives an Ethernet frame it searches the source address table for the Destination MAC address. • If it finds a match, it filters the frame by only sending it out that port. • If there is not a match if floods it out all ports. 3333 1111 switch 1111 3333 Abbreviated MAC addresses 2222 4444

32. • No Destination Address in table, Flood Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 • How does it learn source MAC addresses? • First, the switch will see if the SA (1111) is in it’s table. • If it is, it resets the timer (more in a moment). • If it is NOT in the table it adds it, with the port number. • Next, in our scenario, the switch will flood the frame out all other ports, because the DA is not in the source address table. 3333 1111 switch 1111 3333 Abbreviated MAC addresses 2222 4444

33. • Destination Address in table, Filter Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 • Most communications involve some sort of client-server relationship or exchange of information. (You will understand this more as you learn about TCP/IP.) • Now 3333 sends data back to 1111. • The switch sees if it has the SA stored. • It does NOT so it adds it. (This will help next time 1111 sends to 3333.) • Next, it checks the DA and in our case it can filter the frame, by sending it only out port 1. 1111 3333 switch 1111 3333 Abbreviated MAC addresses 2222 4444

34. • Destination Address in table, Filter Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 • Now, because both MAC addresses are in the switch’s table, any information exchanged between 1111 and 3333 can be sent (filtered) out the appropriate port. • What happens when two devices send to same destination? • What if this was a hub? • Where is (are) the collision domain(s) in this example? 3333 1111 switch 1111 3333 1111 3333 Abbreviated MAC addresses 2222 4444

35. • No Collisions in Switch, Buffering Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 9 4444 • Unlike a hub, a collision does NOT occur, which would cause the two PCs to have to retransmit the frames. • Instead the switch buffers the frames and sends them out port #6 one at a time. • The sending PCs have no idea that their was another PC wanting to send to the same destination. 3333 1111 switch 3333 4444 1111 3333 Abbreviated MAC addresses 2222 4444

36. • Collision Domains Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 9 4444 • When there is only one device on a switch port, the collision domain is only between the PC and the switch. (Cisco curriculum is inaccurate on this point.) • With a full-duplex PC and switch port, there will be no collision, since the devices and the medium can send and receive at the same time. 3333 1111 Collision Domains switch 3333 4444 1111 3333 Abbreviated MAC addresses 2222 4444

37. • Other Information Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 9 4444 • How long are addresses kept in the Source Address Table? • 5 minutes is common on most vendor switches. • How do computers know the Destination MAC address? • ARP Caches and ARP Requests • How many addresses can be kept in the table? • Depends on the size of the cache, but 1,024 addresses is common. • What about Layer 2 broadcasts? • Layer 2 broadcasts (DA = all 1’s) is flooded out all ports. switch 1111 3333 Abbreviated MAC addresses 2222 4444

38. Side Note - Transparent Bridging • Transparent bridging (normal switching process) is defined in IEEE 802.1d describing the five bridging processes of: • learning • flooding filtering • forwarding • aging • These will be discussed further in STP (Spanning Tree Protocol)

39. Transparent Bridge Process - Jeff Doyle Receive Packet Learn source address or refresh aging timer Is the destination a broadcast, multicast or unknown unicast? Yes No Flood Packet Are the source and destination on the same interface? No Yes Filter Packet Forward unicast to correct port

40. • What happens here? Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 1 2222 1 3333 • Notice the Source Address Table has multiple entries for port #1. 1111 3333 3333 1111 2222 5555

41. • What happens here? Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 1 2222 1 5555 • The switch filters the frame out port #1. • But the hub is only a layer 1 device, so it floods it out all ports. • Where is the collision domain? 1111 3333 3333 1111 2222 5555

42. • What happens here? Source Address Table PortSource MAC Add.PortSource MAC Add. 1 1111 6 3333 1 2222 1 5555 1111 3333 Collision Domain 3333 1111 2222 5555

43. • LAN segmentation with routers • Routers provide segmentation of networks, adding a latency factor of 20% to 30% over a switched network. • This increased latency is because a router operates at the network layer and uses the IP address to determine the best path to the destination node. • Bridges and switches provide segmentation within a single network or subnetwork. • Routers provide connectivity between networks and subnetworks. • Routers also do not forward broadcasts while switches and bridges must forward broadcast frames.

44. • Layer 2 and layer 3 switching • A layer 3 switch is typically a layer 2 switch that includes a routing process, I.e. does routing. (Oh yea, also known as routing. Got to love those people in Marketing.) • Layer 3 switching has many meanings and in many cases is just a marketing term. • Layer 3 switching is a function of the network layer. • The Layer 3 header information is examined and the packet is forwarded based on the IP address. (routing)

45. • Symmetric and asymmetric switching Note: Most switches are now 10/100, which allow you to use them symmetrically or asymmetrically.

46. Ethernet switch latency • Latency is the period of time from when the beginning of a frame enters to when the end of the frame exits the switch. • Latency is directly related to the configured switching process and volume of traffic.

47. • Memory buffering • An Ethernet switch may use a buffering technique to store and forward frames. • Buffering may also be used when the destination port is busy. • The area of memory where the switch stores the data is called the memory buffer. • This memory buffer can use two methods for forwarding frame: • port-based memory buffering • shared memory buffering • In port-based memory buffering frames are stored in queues that are linked to specific incoming ports. • Shared memory buffering deposits all frames into a common memory buffer which all the ports on the switch share. switch 1111 3333 Abbreviated MAC addresses 2222 4444

48. • Two switching methods • Store-and-forward – The entire frame is received before any forwarding takes place. • The destination and source addresses are read and filters are applied before the frame is forwarded. • CRC Check done • Cut-through – The frame is forwarded through the switch before the entire frame is received. • This mode decreases the latency of the transmission, but also reduces error detection. • 1900 and 2800 series switches this is configurable, otherwise depends on the model of the switch.

49. • Cut-through Cut-through • Fast-forward – Offers the lowest level of latency. • Fast-forward switching immediately forwards a packet after reading the destination address. • There may be times when packets are relayed with errors. • Although this occurs infrequently and the destination network adapter will discard the faulty packet upon receipt.

50. • Cut-through Cut-through • Fragment-free – Fragment-free switching filters out collision fragments before forwarding begins. • Collision fragments are the majority of packet errors. • In a properly functioning network, collision fragments must be smaller than 64 bytes. • Anything greater than 64 bytes is a valid packet and is usually received without error. • Fragment-free switching waits until the packet is determined not to be a collision fragment before forwarding.