Ethernet Switching

1
Ethernet Switching

• CCNA 1
• Module 8

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 frames 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

• 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.

MAC
2
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
Switches and CAM

• Most memory devices store and retrieve data by
  addressing specific memory locations.
• In CAM stored data item can be identified by the
  content of the data itself rather than by its
  address.

16
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.

17
ASIC
18
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.
19
Switch Benefits
20
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

21
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.

22
Switch Modes fragment free

• 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. 

23
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)

24
Symmetric switching
25
Asymmetric switching
26
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.

27
Switch Modes
Start of frame
Latency delay
28
Switched LANs
A
Broadcast frame
A wants to contact B but doesnt know Bs
address. So A sends a broadcast.
B
29
Switched LANs
A
Broadcast frame
Link is down
How can you allow for reliability, so A can
still contact B before the link is repaired?
B
While this might enable A to contact B, what
problem could this solution cause ?
30
Switched LANs
A
Broadcast frame
This loop can result in a broadcast storm!
B
How can you have the desired redundant paths and
eliminate the routing loops?
31
Switched LANs

• Answer Spanning Tree Protocol
• This allows for redundant paths without routing
  loops.

32
Spanning Tree Protocol

• Loops can occur when extra switches and bridges
  are added to provide redundant paths for
  reliability and fault tolerance.
• A switch sends special messages called bridge
  protocol data units (BPDUs) out all its ports to
  let other switches know of its existence.
• The switches use a spanning-tree algorithm (STA)
  to resolve and shut down the redundant paths
• The protocol used to resolve and eliminate loops
  is known as the Spanning Tree Protocol (STP).

33
STP States
B-L-L-F-D Bluffed
STP is used to create a logical hierarchical tree
with no loops. The alternate paths are still
available if necessary.
34
http//www.nikmakris.com/cisco_spanning_tree.asp
35
(No Transcript)
36
STP States
Bridge Protocol Data Units (BPDUs)
STP STA BPDUs
Root Bridge
Forwarding
Forwarding
Forwarding
Blocking
Blocking
Root port
Designated port
37
Collision Domains Broadcast Domains
38
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
39
Collision Domains

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

40
The Four Repeater Rule

• No more than four repeaters or repeating hubs can
  be between any two computers on a network
• If exceeded, the RTD (round Trip Delay) is too
  high. (caused by repeater latency, propagation
  delay, and NIC latency)
• This means that all nodes may not hear a
  collision.
• A late collision is a collision after the first
  64 bytes are transmitted.
• Retransmission is not required

41
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

42
Four-Repeater Rule Example

• The 5-story building shown violates the
  four-repeater rule because host A and B are 5
  repeaters apart.
• Hubs would cause the same result.
• Even if all servers were on the third floor, and
  A and B would never communicate directly they
  are too far to hear each other transmit and can
  cause data collisions.
• What are implications for taller buildings?

43
Four-Repeater Rule Example Layer 1 Solution

• The hub added, which could be on any floor,
  allows us to comply with the four-repeater rule.
• No 2 hosts are more than 3 repeaters apart.
• What are implications for taller buildings? It
  really wouldnt matter if each floor connects to
  the hub.
• How many collision domains do we have? Still only
  one and getting bigger with each floor.

44
Data Collisions
When two bits are propagated at the same time on
the same network, a collision will occur.
45
Collisions and Collision Domains

• When two bits are propagated at the same time on
  the same network, a collision will occur.
• 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.

46
Shared Media, Repeaters, Hubs, and Collision
Domains
Shared access
Extended by a repeater
Extended by a repeater
Each is a single collision domain!
47
How many collision domains?
ONE
48
How do you solve the problem?
Add a Bridge
49
Add a bridge 2 collision domains
50
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.

51
Ethernet LAN Segmentation
52
Segmenting with Bridges
53
Segmenting with Switches
54
Segmenting with Routers
55
Microsegmentation
Hubs
56
Layer 2 Broadcasts Destination MAC address of
0xFFFFFFFFFFFF.

• 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.

57
Broadcast Domains?
The only devices that can segment collision
domains are bridges, switches (both Layer 2), and
routers (Layer 3).
58
2 collision domains1 broadcast domain
59
2 collision domains1 broadcast domain
60
Data Flow
Data flow refers to the movement of data through
Layer 1, 2 and 3 devices and how data must be
encapsulated to effectively make that journey.
61
Network Segment, what segment?

• If the segment is used in TCP, it would be
  defined as a separate piece of the data.
• If segment is being used in the context of
  physical networking media in a routed network, it
  would be seen as one of the parts or sections of
  the total network.

62
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 gt less collisions.
• STP is used to prevent loops created by redundant
  links.

63
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
64
Switching/Bridging Table
E0
0260.8c01.1111
E0
0260.8c01.2222
E1
0260.ec01.3333
0260.8c01.4444
E1
65
Fill in the blanks
2
Multiple
2
Multiple
1
1
Content-Addressable memory (CAM)
Bridging Table
66

• 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

67
Broadcast Domains

• 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.

Layer 2
gain access to the media
Broadcasts
3
Routers
IP address
68
Collision Domains
collision domains

• Layer 1 devices do not break up _______________.
• _______ and _______ devices do break up collision
  domains.
• Breaking up, or increasing the number of
  collision domains with Layer 2 and 3 devices is
  also known as _____________. 
• Layer 1 devices, such as repeaters and hubs
  extend cable segments so __________ can be added.
• More hosts ? __________. More traffic ?
  _____________.
• The final result is diminished ___________________
  __.
• Repeater latency, propagation delay, and NIC
  latency all contribute to the ________________.
• Exceeding the four repeater rule can lead to
  violating the _____________________.
• When this delay limit is exceeded, the number of
  ____________ dramatically increases.
• Late collision frames add delay that is referred
  to as ____________ delay.

Layer 2
Layer 3
segmentation
more hosts
more traffic
more collisions
network performance
four repeater rule
maximum delay limit
late collisions
consumption
69
Fill in the blanks
Receives BPDUs only
Building active topology
Create bridge table
Sending receiving
Administrator decided
70
What are the 3 types of shared media environment?

Shared media
Multiple hosts, same medium
Extended shared media
Network devices extended the environment
Point-to-point media
Dialup network connections
71

• Breaking up collision domains is called
  _____________
• What would relieve the problems of excessive
  collisions?
• Reduce collisions domains replace hubs with
  switches or bridges
• What is the area that controls frame propagation
  within that area?
• Collision domain
• What is the only device that breaks up broadcast
  domains? Describe how it works at each layer?
• Routersdo not forward broadcasts
• What does a router use to make forwarding
  decisions?
• Destination IP address

segmentation
72
Fill in the blanks
Always forwards frames
Physical
Will not forward unless it must
Network
73

• Name the four devices above, the layer of the OSI
  model they operate at, and the method they base
  their addressing on to forward packets on a
  network.

Bridge layer 2 Frame
Router layer 3 Packet
Repeater layer 1 Bits
Switch layer 2 Frame
74
How many collision/broadcast domains?
Collision
6
Broadcast
2
75
Segmentation
14

• How many collision domains are there?
• How many broadcast domains are there?

4
76
THATS ALL FOLKS!

• Now you can either
• Answer the revision questions provided on hard
  copy
• Revise for the exam
• Take the exam and previous exams if you are
  behind
• Configure the router / switch lab from previous
  weeks
• Explore the CD in the CCNA book
• Video on Bridging
• Test Bank use Study Mode to revise topics so
  far
• Install Packet Tracer and use tutorials / demos