1. Module 4 – Switching Concepts
CCNA 3
Cabrillo College

2. Overview – Review of CCNA 1
The first part of this presentation should be mostly a review from CCNA 1:
Describe the history and function of shared, half-duplex Ethernet
Define collision as it relates to Ethernet networks
Define micro-segmentation
Define CSMA/CD
Describe some of the key elements affecting network performance
Describe the function of repeaters
Define network latency
Define transmission time
Describe the basic function of Fast Ethernet

3. Overview – New Concepts
Define network segmentation using routers, switches, and bridges
Describe the basic operations of a switch
Define Ethernet switch latency
Explain the differences between Layer 2 and Layer 3 switching
Define symmetric and asymmetric switching
Define memory buffering
Compare and contrast store-and-forward and cut-through switching
Understand the differences between hubs, bridges, and switches
Describe the main functions of switches
List the major switch frame transmission modes
Describe the process by which switches learn addresses
Identify and define forwarding modes
Define LAN segmentation
Define microsegmentation using switching
Describe the frame-filtering process
Compare and contrast collision and broadcast domains
Identify the cables needed to connect switches to workstations
Identify the cables needed to connect switches to switches

4. Overview Routers Switches, Bridges Hub, Repeaters
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 micro-segmentation of the network.
Today’s networks typically are built using switches and routers, often with the routing and switching function in the same device.

5. Ethernet Limitations

6. 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.

7. Bridges

8. 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.

9. Switches
Switches create a virtual circuit between two connected devices, establishing a dedicated communication path between two devices.
Switches on the network provide micro-segmentation.
This allows maximum utilization of the available bandwidth.
Broadcast frames to all connected devices on the network.

10. 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.

11. Factors that impact performance

12. 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.

13. 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.

14. 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

15. Network Congestion

16. 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:
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.
the actual propagation delay as the signal takes time to travel along the cable.
the latency 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.

17. 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.

18. 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.

19. 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. 

20. 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

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

22. Sending and receiving Ethernet frames on a bus
Hey, that’s me!
Nope
Nope
Abbreviated MAC Addresses
1111
2222
3333
nnnn
3333
1111
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

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

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

25. •
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.”

26. CSMA/CD and Collisions•
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 access allows 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 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.

27. CSMA/CD and Collisions•
CSMA/CD and Collisions
Hey, that’s me!
Nope
Nope
Abbreviated MAC Addresses
1111
2222
3333
nnnn
Notice the location of the DA!
3333
1111
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.

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

29. •
Hubs
Hub or

30. •
Hubs
3333
1111
1111
2222
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.
Nope
5555
Nope
3333
For me!
4444
Nope

31. Hubs • Wasted bandwidth
2222
1111
1111
2222
Another disadvantage with hubs is that is take up unnecessary bandwidth on other links.
For me!
5555
Wasted bandwidth
Nope
3333
Nope
4444
Nope

32. Sending and receiving Ethernet frames via a switch•
Sending and receiving Ethernet frames via a switch

33. •
Switches
Source Address Table
Port Source MAC Add. Port Source MAC Add.
3333
1111
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 addresses after it learns about them.
A switch receives an Ethernet frame and 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.
switch
1111
3333
Abbreviated MAC addresses
2222
4444

34. No Destination Address in table, Flood•
No Destination Address in table, Flood
Source Address Table
Port Source MAC Add. Port Source MAC Add.
3333
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.
switch
1111
3333
Abbreviated MAC addresses
2222
4444

35. Destination Address in table, Filter•
Destination Address in table, Filter
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1111
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.
switch
1111
3333
Abbreviated MAC addresses
2222
4444

36. Destination Address in table, Filter•
Destination Address in table, Filter
Source Address Table
Port Source MAC Add. Port Source MAC Add.
3333
1111
switch
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?
1111
3333
1111
3333
Abbreviated MAC addresses
2222
4444

37. No Collisions in Switch, Buffering•
No Collisions in Switch, Buffering
Source Address Table
Port Source MAC Add. Port Source MAC Add.
3333
1111
switch
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 there was another PC that wanted to send to the same destination.
3333
4444
1111
3333
Abbreviated MAC addresses
2222
4444

38. Collision Domains • Collision Domains switch
Source Address Table
Port Source MAC Add. Port Source MAC Add.
3333
1111
Collision Domains
switch
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
4444
1111
3333
Abbreviated MAC addresses
2222
4444

39. •
Other Information
Source Address Table
Port Source MAC Add. Port Source MAC Add.
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

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

41. What happens here? • The switch filters the frame out port #1.
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1111
3333
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?
3333
1111
2222
5555

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

43. LAN segmentation with routers•
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•
Layer 2 and layer 3 switching
(routing)
A layer 3 switch is typically a layer 2 switch that includes a routing process, I.e. does routing. 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.

45. Symmetric and Asymmetric•
Symmetric and Asymmetric
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 • switch 1111 3333 2222 4444
Abbreviated MAC addresses
2222
4444

48. 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.

49. 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.
Depends on the model of the switch.

50. 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.

51. Cut-through • Cut-through
Fragment-free – Fragment-free switching filters out collision fragments before forwarding begins.
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.

52. Two switching methods Adaptive cut-through •
In this mode, the switch uses cut-through until it detects a given number of errors.
Once the error threshold is reached, the switch changes to store-and-forward mode.

53. Functions of a switch The main features of Ethernet switches are:
Isolate traffic among segments
Achieve greater amount of bandwidth per user by creating smaller collision domains

54. Learning Addresses “Learning bridges” or Learning switches”
Bridges and switches learn in the following ways:
Reading the source MAC address of each received frame or datagram
Recording the port on which the MAC address was received.
The bridge or switch learns which addresses belong to the devices connected to each port.
The learned addresses and associated port or interface are stored in the addressing table.
The bridge examines the destination address of all received frames.
The bridge then scans the address table searching for the destination address.

55. Filter or Flood
If a switch has the frame’s destination address in its CAM table (or Source Address Table) it will only send the frame out the appropriate port.
If a switch does not have the frame’s destination MAC address in its CAM table, it floods (sends) it out all ports except for the incoming port (the port that the frame came in on) known as an Unknown Unicast, or if the destination MAC address is a broadcast.
Note: A CAM table may contain multiple entries per port, if a hub or a switch is attached to that port.
Most Ethernet bridges can filter broadcast and multicast frames.

56. Filter or Flood Switches flood frames that are: Unknown unicasts
Layer 2 broadcasts
Multicasts (unless running multicast snooping or IGMP)
Multicast are special layer 2 and layer 3 addresses that are sent to devices that belong to that “group”.

57. Why segment LANs? (Layer 2 segments)
Hub
Switch
to isolate traffic between segments.
to achieve more bandwidth per user by creating smaller collision domains.

58. Why segment LANs? (Layer 2 segments)•
Why segment LANs? (Layer 2 segments)
switch
Collision Domains
A switch employs “microsegmentation” to reduce the collision domain on a LAN.
The switch does this by creating dedicated network segments, or point-to-point connections.
1111
3333
Abbreviated MAC addresses
2222
4444

59. Broadcast domains • ARP Request
Even though the LAN switch reduces the size of collision domains, all hosts connected to the switch are still in the same broadcast domain.
Therefore, a broadcast from one node will still be seen by all the other nodes connected through the LAN switch.

60. Broadcast domains
When a device wants to send out a Layer 2 broadcast, the destination MAC address in the frame is set to all ones.
A MAC address of all ones is FF:FF:FF:FF:FF:FF in hexadecimal.
By setting the destination to this value, all the devices will accept and process the broadcasted frame.

61. Switches and broadcast domains

62. Using Switches Layer 2 devices•
Using Switches
Layer 2 devices
Layer 2 filtering based on Destination MAC addresses and Source Address Table
One collision domain per port
One broadcast domain across all switches

63. Other Switching Features•
Other Switching Features
Review
Asymmetric ports: 10 Mbps and 100 Mbps
Full-duplex ports
Cut-through versus Store-and-Forward switching

64. Other Switching Features•
Other Switching Features
Ports between switches and server ports are good candidates for higher bandwidth ports (100 Mbps) and full-duplex ports.
Most switch ports today are full-duplex.

65. Introducing Multiple Subnets/Networks without Routers•
Introducing Multiple Subnets/Networks without Routers
Switches are Layer 2 devices
Router are Layer 3 devices
Data between subnets/networks must pass through a router.

66. Switched Network with Multiple Subnets•
Switched Network with Multiple Subnets
ARP Request
What are the issues?
Can data travel within the subnet? Yes
Can data travel between subnets? No, need a router!
What is the impact of a layer 2 broadcast, like an ARP Request?

67. Switched Network with Multiple Subnets•
Switched Network with Multiple Subnets
ARP Request
All devices see the ARP Request, even those on the other subnets that do not need to see it.
One broadcast domain means the switches flood all broadcast out all ports, except the incoming port.
Switches have no idea of the layer 3 information contained in the ARP Request.This consumes bandwidth on the network and processing cycles on the hosts.

68. One Solution: Physically separate the subnets•
One Solution: Physically separate the subnets
But still no data can travel between the subnets.
How can we get the data to travel between the two subnets?

69. Another Solution: Use a Router•
Another Solution: Use a Router
Two separate broadcast domains, because the router will not forward the layer 2 broadcasts such as ARP Requests.

70. Switches with multiple subnets•
Switches with multiple subnets
So far this should have been a review.
Lets see what happens when we have two subnets on a single switch and we want to route between the two subnets.

71. Router-on-a-stick or One-Arm-Router (OAR)•
Router-on-a-stick or One-Arm-Router (OAR)
interface e 0
ip address
ip address secondary
ARP Request
Secondary addresses can be used when the router does not support sub-interfaces which will be discussed later.
When a single interface is used to route between subnets or networks, this is know as a router-on-a-stick.
To assign multiple ip addresses to the same interface, secondary addresses or subinterfaces are used.

72. Router-on-a-stick or One-Arm-Router (OAR)•
Router-on-a-stick or One-Arm-Router (OAR)
interface e 0
ip address
ip address secondary
Advantages
Useful when there are limited Ethernet interfaces on the router.
Disadvantage
Because a single link is used to connect multiple subnets, one link is having to carry the traffic for multiple subnets.
Be sure this is link can handle the traffic.

73. Router-on-a-stick or One-Arm-Router (OAR)•
Router-on-a-stick or One-Arm-Router (OAR)
interface e 0
ip address
ip address secondary
ARP Request
Still the same problem of the switch forwarding broadcast traffic to all devices on all subnets.

74. Router-on-a-stick or One-Arm-Router (OAR)•
Router-on-a-stick or One-Arm-Router (OAR)
interface e 0
ip address
ip address secondary
Remember to have the proper default gateway set for each host.
hosts - default gateway is
hosts - default gateway is

75. Interface for each subnet•
Interface for each subnet E0 E1
An Ethernet router interface per subnet may be used instead of one.
However this may be difficult if you do not have enough Ethernet ports on your router.

76. Still one broadcast domain•
Still one broadcast domain
ARP Request
Still the same problem of the switch forwarding broadcast traffic to all devices on all subnets.

77. Introducing VLANs VLAN = Subnet•
Introducing VLANs
VLAN = Subnet
VLANs create separate broadcast domains within the switch.
Routers are needed to pass information between different VLANs
This is only an introduction, as we will discuss VLANs in later chapters.

78. Layer 2 Broadcast Segmentation•
Layer 2 Broadcast Segmentation
Switch Port: VLAN ID
ARP Request
An ARP Request from for will only be seen by hosts on that VLAN.
The switch will flood broadcast traffic out only those ports belonging to that particular VLAN, in this case VLAN 1.

79. Layer 2 Broadcast Segmentation•
Layer 2 Broadcast Segmentation
Port-centric VLAN Switches
As the Network Administrator, it is your job to assign switch ports to the proper VLAN.
This assignment is only done at the switch and not at the host.
Note: The following diagrams show the VLAN below the host, but it is actually assigned on the switch.

80. Without VLANs – No Broadcast Control•
Without VLANs – No Broadcast Control
ARP Request
Without VLANs, the ARP Request would be seen by all hosts.
Again, consuming unnecessary network bandwidth and host processing cycles.

81. With VLANs – Broadcast Control•
With VLANs – Broadcast Control
Switch Port: VLAN ID
ARP Request

82. •
Inter-VLAN Traffic
Switch Port: VLAN ID
1. Remember that VLAN IDs (numbers) are assigned to the switch port and not to the host. (Port-centric VLAN switches)
2. Be sure to have all of the hosts on the same subnet belong to the same VLAN, or you will have problems.
Hosts on subnet / VLAN 1
Hosts on subnet / VLAN 2
etc.

83. •
Inter-VLAN Traffic
Switch Port: VLAN ID To
A switch cannot route data between different VLANs.
Note: The host will not even send the Packet unless it has a default gateway to forward it to.

84. Inter-VLAN Routing needs a Router•
Inter-VLAN Routing needs a Router
A router is need to route traffic between VLANs (VLAN = Subnet).
There are various methods of doing this including Router-on-a-stick with trunking (more than one VLAN on the link).
This will be discussed later when we get to the chapter on VLANs and Inter-VLAN Routing.

85. Module 4 – Switching Concepts
CCNA 3
Cabrillo College