1. Networking Basics CCNA 1 Chapter 8

2. Ethernet Switch Operations
Layer 2 Bridging and Switching Operations
Earliest networking devices were repeaters and hubs
Multiple LAN segments could be connected to make larger LANs, within design rules
As it became apparent that reducing size of collision domains was important, bridges were created
Bridges are aware of Ethernet framing and Layer 2 MAC addressing (IEEE 802.3)

3. Ethernet Switch Operations
Layer 2 Bridging and Switching Operations
Bridges extend LAN distances, without some of the negative effects of repeaters and hubs
Bridges were typically much more expensive than repeaters and hubs (were usually a PC running software to perform the bridging function)
Bridges usually had only two interfaces, where hubs had multiple ports

4. Ethernet Switch Operations
Layer 2 Bridging and Switching Operations
Next major step in LAN devices was the LAN switch
Does the same thing as a bridge
Instead of using software, process could be done with a chip (sometimes called application-specific integrated circuits – ASICs)
Switches have more interfaces than bridges, are smaller, and do the same work faster
As switch prices fell, bridges disappeared from the market

5. Ethernet Switch Operations
The Forwarding and Filtering Decision
Repeaters and hubs simply react to the incoming signal
make no decisions and require no programming logic
Receive, regenerate and send signal out all ports except the one on which it was received
Bridges implemented their logic in software

6. Ethernet Switch Operations
The Forwarding and Filtering Decision
Switches implement their logic in hardware
Run much faster than bridges
Cisco makes switches that can forward hundreds of millions of Ethernet frames per second

7. Ethernet Switch Operations
The Forwarding and Filtering Decision
Filtering and forwarding logic
Examine incoming signal; interpret as 0s and 1s (OSI Layer 1 standards)
Interpret the received bits based on Ethernet framing rules; find MAC destination address in frame (OSI Layer 2 standards, IEEE MAC sublayer)

8. Ethernet Switch Operations
The Forwarding and Filtering Decision
Filtering and forwarding logic (continued)
Examine table that maps MAC addresses with corresponding interfaces
Find table entry that matches the destination MAC address of frame
If frame came in on a different interface than the one listed on the table, process is called forwarding the frame
If the frame came in on the same interface as the one it was received on, discard it (this is called filtering)

9. Ethernet Switch Operations
The Forwarding and Filtering Decision
The table a bridge or switch refers to may be called:
Bridging table
Switching table
MAC address table
Forwarding table
Content Addressable Memory (CAM) table

10. Ethernet Switch Operations
A Bridge Filtering Decision Based on the CAM

11. Ethernet Switch Operations
A Bridge Forwarding Decision Based on the CAM

12. Ethernet Switch Operations
Learning CAM Table Entries and Flooding Unknown Unicasts
Switches and bridges learn entries in the CAM dynamically
They use this logic:
Examine the source MAC address of the frame and the interface on which it was received
Add that source MAC address and corresponding interface to the table

13. Ethernet Switch Operations
Learning CAM Table Entries: One Switch

14. Ethernet Switch Operations
Learning CAM Table Entries: Two Switches

15. Ethernet Switch Operations
Handling Unknown Unicasts
Switches typically learn CAM entries for all working devices on the LAN as soon as those devices start sending data
Sometimes a switch receives a frame that does not have a CAM entry – this is an unknown unicast frame
The switch sends the unknown unicast frame out all ports, a process called flooding

16. Ethernet Switch Operations
Forwarding Broadcasts and Multicasts
Unicast frame has a destination MAC address of a single NIC or interface
Broadcast frames are sent to a destination MAC address of FFFF.FFFF.FFFF.FFFF and are delivered to all devices on the LAN
Multicast frames are sent to one of a range of MAC addresses

17. Ethernet Switch Operations
Flooding Unknown Unicasts

18. Ethernet Switch Operations
Forwarding Broadcasts and Multicasts
Multicast addresses provide a way to send certain frames to a subset of devices
Streaming video
Some low-end switches flood multicasts like broadcasts
Higher-end switches allow multicasting, making the process more efficient

19. Ethernet Switch Operations
Different Forwarding Behavior for Multicasts

20. Ethernet Switch Operations
The Cisco Switch CAM
All switches and bridges use some table that lists the MAC address and port through which each MAC address can be reached
Cisco calls this the CAM (Content Addressable Memory)
The MAC address is input into the memory and CAM instantly outputs the table entry
This process occurs quickly, every time, regardless of table size

21. Ethernet Switch Operations
Switch Internal Processing
The amount of time it takes for a frame to progress through a network from one device to another is called latency
Some factors that affect latency cannot be improved, such as propagation delay (the amount of time it takes for electricity to go from one end of the network to another)
Other types of delay vary with network conditions; frames may be waiting in a buffer (queuing delay)

22. Ethernet Switch Operations
Switch Internal Processing – Factors that Impact Latency
The finite speed that signals can travel (propagation delay)
Circuit delays caused by electronics
Software delays caused by software decisions being made
Delays caused by frame contents and location of the frame switching decisions

23. Ethernet Switch Operations
Store-and-Forward Switching
Switch receives entire frame before forwarding it
Advantages of store-and forward switching
FCS field is at end of frame; frame can be checked for an error
Can check for rare error in which the Length field does not match the Data field length
Can forward between ports running at different speeds (asymmetric switching)
Disadvantage
More latency than other switching types

24. Ethernet Switch Operations
Cut-Through Switching
Destination MAC address is located at beginning of Ethernet frame
Advantage of cut-through switching
Once destination MAC address is read, switch can begin forwarding frame
Less latency than store-and-forward
Disadvantages of cut-through switching
Cannot check FCS; may forward frames with errors
Forwards before some legitimate collisions have occurred
Only works with symmetric switching

25. Ethernet Switch Operations
Fragment-Free Switching
Overcomes a problem that cut-through switching has: cut-through is too fast
Collisions should occur while a frame’s first 64 bytes are being transmitted
Cut-through switching often begins transmitting before 64 bytes are received
Cut-through switching can forward collision fragments

26. Ethernet Switch Operations
Fragment-Free Switching
Fragment-free switching waits until it has received first 64 bytes to begin transmitting
Ensures switch does not forward frames that have collided

27. Ethernet Switch Operations
Cisco Enterprise Switch – Internal Processing Paths

28. Ethernet Switch Operations
Spanning Tree Protocol
Most LAN design include redundant physical paths
A trunk is a link between two switches; sometimes called a backbone link
Spanning tree protocol (STP) prevents switching loops from the logic used to forward unknown unicast and broadcast frames

29. Ethernet Switch Operations
Typical Enterprise Campus Building Block Design, with Redundancy

30. Ethernet Switch Operations
The Problem That STP Solves: Switching Loops

31. Ethernet Switch Operations
The Problem That STP Solves: Switching Loops
In previous slide, if PC1 sends a broadcast, it goes around LAN in both directions
Each switch broadcast the frame(s) out every port (except the one on which it was received)
This process continues for a long time, continuing until no other traffic can be sent over the LAN: a “broadcast storm”

32. Ethernet Switch Operations
STP Protocol: STP Blocking
STP makes some ports quit forwarding or receiving frames
An interface that is not allowed to process traffic by STP is considered to be in an STP blocking state
In the figure that follows, SW3’s port 1 is in a blocking state – it receives the broadcast frame but ignores it
STP causes the LAN to use particular paths and leaves others idle and unused

33. Ethernet Switch Operations
IEEE 802.1D STP Interface States

34. Ethernet Switch Operations
IEEE 802.1D STP Interface States
The forwarding and blocking states are the most common, because a working network interface stabilizes into one of these states
Failed interfaces stabilize into a disabled state
Listening and learning states are used to solve problems with CAM tables

35. Ethernet Switch Operations
Stable STP Topology and Switch CAMs in a Three-Switch Network

36. Ethernet Switch Operations
Changing the CAM with the Listening and Learning States
The topology can fail when a trunk fails or when a new trunk comes up
STP determines the topology by having switches send bridge protocol data units (BPDUs) to each other
BPDUs and the Spanning Tree Algorithm (STA) are part of the IEEE 802.1D standard
Information learned allows the switches to determine the topology and decide which interfaces should forward and which should block frames

37. Ethernet Switch Operations
Changing the CAM with the Listening and Learning States
The listening and learning states are used by STP when it needs to transition to a new topology
An STP topology refers to the topology of the network when each interface is in one of three stable states
STP remains in the stable topology until something happens
A trunk goes down (perhaps cut)
The network engineer shuts down a trunk
A new switch is added
An interface fails

38. Ethernet Switch Operations
Changing the CAM with the Listening and Learning States
Switches use listening and learning states as interim states when transitioning an interface for two reasons:
For the switches’ CAM table entries to time out (during the listening state)
For the switches to relearn the MAC addresses and (possibly different) interfaces used to reach the MAC addresses

39. Ethernet Switch Operations
A New STP Topology After a Failure

40. LAN Design: Collision Domains and Broadcast Domains
A collision domain is a set of LAN interfaces for which a frame sent out any two of these interfaces, at the same time, would cause a collision
Hubs repeat signals out interfaces and do not consider CSMA/CD logic, so any frames sent simultaneously will collide
The terms shared bandwidth and shared media refer to the fact that the devices in a hubbed network share the same media and bandwidth

41. LAN Design: Collision Domains and Broadcast Domains
One Collision Domain with One 10BASE-T Hub

42. LAN Design: Collision Domains and Broadcast Domains
Large/Long Collision Domains
The (or ) Rule for 10BASE-T networks
5 segments of network media
4 repeaters or hubs at most
3 links at most, between two end-user devices
If 5 segments exist between two end-user devices, 2 segments must not have any end-user devices connected to them
It’s all 1 large collision domain

43. LAN Design: Collision Domains and Broadcast Domains
One Collision Domain with Multiple 10BASE-T Hubs

44. LAN Design: Collision Domains and Broadcast Domains
Large/Long Collision Domains
The rule for 10BASE-T restrictions are required due to the round-trip time of the collision domain
Within one collision domain, all the devices share the 10 Mbps of bandwidth
Within one collision domain, a (practically) simultaneous transmission of a frame by two or more PCs results in a collision

45. LAN Design: Collision Domains and Broadcast Domains
Large/Long Collision Domains
The more PCs in a collision domain, the less efficient it is
The more frames, the more collisions
The more collisions, the more time sent waiting to resend frames
Once a LAN reaches about 30-40% of bandwidth utilization, the number of collisions increases dramatically

46. LAN Design: Collision Domains and Broadcast Domains
High LAN Utilization Resulting in Much Higher Percentage of Collisions

47. LAN Design: Collision Domains and Broadcast Domains
Large/Long Collision Domains
Large collision domains should not be used for the following reasons:
Shared bandwidth – as the size of the collision domain grows, each device has less available bandwidth
Higher utilization – the more devices in a single collision domain, the better the chance of a collision and of driving the utilization rate higher

48. LAN Design: Collision Domains and Broadcast Domains
Creating Many Small Collision Domains
Significantly reduces the negative effects of a large collision domain
Process of breaking a LAN into multiple collision domains is called segmentation
Switches, bridges, and routers can segment LANs into multiple collision domains

49. LAN Design: Collision Domains and Broadcast Domains
Two LANs with Many Small Collision Domains

50. LAN Design: Collision Domains and Broadcast Domains
Creating Many Small Collision Domains
Benefits of segmenting 10BASE-T LANs:
Design rules ( ) apply to each individual collision domain
With smaller collision domains, reaching the point of utilization where performance is degraded is less likely
Each domain gets its own bandwidth, so fewer devices are sharing the available bandwidth

51. LAN Design: Collision Domains and Broadcast Domains
Creating Many Small Collision Domains
When switches are used on the LAN, the terms switched LAN and switched bandwidth are used
Each switch port connects to a separate collision domain
Connecting a single end-user device to each switch port is a process called microsegmentation

52. LAN Design: Collision Domains and Broadcast Domains
Creating Many Small Collision Domains
Microsegments meet the requirements to allow full duplex
Full duplex gives twice the bandwidth
A 24 port 10BASE-T hub shares 10 Mbps of bandwidth among 24 ports
A 24 port 10BASE-T switch gives each port 20 Mbps of bandwidth

53. LAN Design: Collision Domains and Broadcast Domains
Main Benefits of Using Many Small Collision Domains
Collision domain design rules are easier to achieve
Smaller collision domains reduce the probability of LAN overutilization
Each collision domain gets its own separate switched bandwidth
With a collision domain consisting of only two interfaces/NICs, full duplex can be used

54. LAN Design: Collision Domains and Broadcast Domains
How Switches and Bridges Prevent Collisions
Switches reduce or prevent collisions by buffering or queuing frames
Repeaters and hubs do not perform buffering
Bridges, switches and routers follow CSMA/CD rules if not using full duplex

55. LAN Design: Collision Domains and Broadcast Domains
Switch Buffering Example

56. LAN Design: Collision Domains and Broadcast Domains
Layer 2 Broadcast Domains
A broadcast domain is:
The set of LAN interfaces (including NICs and network device interfaces) for which a broadcast frame sent by one device with be forwarded to all other interfaces in that same broadcast domain
Bridges and switches forward broadcasts
Routers do not forward broadcasts

57. LAN Design: Collision Domains and Broadcast Domains
One Router Creating Two Broadcast Domains

58. LAN Design: Collision Domains and Broadcast Domains
Performance Impact of Multicast and Broadcast Domains
PC NICs see all frames on the LAN
PC NICs can ignore unicast frames not for them
PC NICs must send multicast and broadcast frames to their CPU for processing, which affects PC performance
This is less of an issue today with fewer proprietary network protocols doing broadcasts and with more powerful processors

59. LAN Design: Collision Domains and Broadcast Domains
NIC Giving Broadcasts and Multicasts to the CPU

60. LAN Design: Collision Domains and Broadcast Domains
More Broadcasts, Less CPU Capacity for End-User Work

61. LAN Design: Collision Domains and Broadcast Domains
The Impact of Broadcasts and Multicasts Today
Biggest risk is in wasting CPU cycles from multicasts
Switches flood multicasts just like broadcasts
LAN engineers must enable multicast optimization tools in switches to prevent switches from flooding multicasts to every device in the LAN

62. LAN Design: Collision Domains and Broadcast Domains
The Impact of Broadcasts and Multicasts Today
Broadcasts such as RIP and ARP don’t cause problems in today’s networks, but did in the past when networks were slower
ARP remembers the info it learns, so an individual PC might not send one ARP per minute
RIP broadcasts may be sent by routers and UNIX workstations; now most UNIX workstations have it turned off by default so these are no longer an issue

63. LAN Design: Collision Domains and Broadcast Domains
Identifying Networking Devices by OSI Layer
Repeaters and hubs are Layer 1 devices
Bridges and switches are Layer 2 devices
Routers are Layer 3 devices

64. LAN Design: Collision Domains and Broadcast Domains
Sample Network with Collision Domains and Broadcast Domains Shown

65. LAN Design: Collision Domains and Broadcast Domains
Data Flow with Layer 1, Layer 2, and Layer 3 Devices

66. LAN Design: Collision Domains and Broadcast Domains
The Ambiguous Term Segment
Three main uses of the term segment
LAN concepts – a segment is a collision domain
LAN (physical) – in a LAN using a bus topology, a segment is a continuous electrical circuit, often connected to other segments with repeaters
TCP – the process of taking a large piece of data and breaking it into smaller pieces; one of those pieces

67. Summary
Bridges and switches work the same way regarding basic forwarding, learning, flooding and STP
They build forwarding tables by examining the source MAC addresses of incoming frames
They make filtering and forwarding decisions by looking at the destination MAC address of the frame and comparing it to the table
They flood broadcast frames and also flood multicast frames, unless optimization features have been enabled

68. Summary Switches differ from bridges
They have much more powerful hardware
They use content addressable memory (CAM) to hold the switching table
The CAM allows the switch to find a MAC address and its associated port very quickly every time
Latency is the time that passes as a frame or packet is sent through the network
Propagation delay is the time it takes for electrical or optical energy to pass over the cable, and contributes to latency

69. Summary Three internal switch processing options:
Cut-through switching begins forwarding the frame as soon as the destination MAC address is read; does not check FCS to determine if frame is good; low latency
Store-and-forward switching receives the entire frame; does error-checking; necessary for asymmetrical switching
Fragment-free switching waits for the first 64 bytes to be received before beginning forwarding; enables it to detect normal collisions

70. Summary
Switches and bridges use Spanning Tree Protocol (STP) to identify and block redundant paths through the network; gives a logical path with no loops
A collision domain with a single device connected to a switch port is called a microsegment
Microsegments use UTP cabling, allow the use of full duplex
With no collisions possible, CSMA/CD can be disabled

71. Summary
Placing a large number of PCs in a collision domain increases demand for bandwidth
This increases possibility of collisions
Breaking large collision domains into multiple smaller collision domains reduces the chance of collisions while adding bandwidth
Separating LANs into more segments by using bridges and switches creates additional collision domains, one per bridge and switch port
Broadcast domains are a set of devices in which if one device sends a broadcast, all other devices receive the broadcast; Layer 3 devices (routers) separate broadcast domains