LAN Switching

1
LAN Switching

• Semester 3, Chapter 2
• Instructor Mehran Dowlatshahi

2
Table of Contents

• LAN Communication Problems

Go There!

• Full-Duplex, Fast Ethernet, and Segmentation

Go There!

• Switching and VLANs

Go There!

• The Spanning-Tree Protocol

Go There!
3
LAN Communication Problems
Table of Contents
4
Network Performance

• Network congestion has increased significantly
  since the mid-90s due to
• Multitasking Operating Systems
• multiple simultaneous network transactions (e.g.,
  ftp download browsing)
• Faster Processing Power
• 1980s 1 MIPS Today over 75 MIPS
• Network-intensive Applications
• accessing network servers to use applications,
  files, etc.

5
Elements of Ethernet/802.3

• Characteristics
• Most common LAN architecture
• Used to transport data between devices connected
  to the same delivery medium
• Uses a data frame broadcast method
• Negative effects of a shared LAN
• broadcast delivery of all frames
• CSMA/CD collisions are inherent
• distance limitation requires using repeaters to
  extend

6
Half-Duplex Ethernet

• Properties
• Only one host can transmit at a time because the
  NIC needs to listen for collisions
• The NIC provides several circuits. Most
  important are
• receive (RX), transmit (TX), and collision
  detection
• bandwidth usage 50 to 60

7
CSMA/CD

• Operation
• Devices on shared media listen for a carrier
  before transmitting
• If no carrier is sensed for a specific period of
  time, a device can transmit
• If two devices transmit simultaneously, a
  collision occurs. The NIC senses this because it
  is transmitting and receiving at the same time
• The first device to detect the collision will
  generate a jam signal (colliding devices continue
  to transmit so that all devices will hear the
  collision)
• All devices calculate a backoff algorithm which
  will delay transmission for a random length of
  time.
• First device whos delay time expires can attempt
  to transmit data.

8
Network Congestion

• Occurs as more people utilize a network to...
• Share large files (e.g. databases, applications,
  etc.)
• Access file servers
• connect to the Internet
• Relieving congestion requires
• Increasing the amount of bandwidth and/or
• Using available bandwidth more efficiently

9
Network Latency

• Latency Definitions
• The time it takes a frame to travel from its
  source device to its final destination on the
  network ( propagation delay)
• The delay between the time a device requests
  access to a network and the time it is granted
  permission to transmit
• For switches and routers, latency is defined as
  the amount of delay between the time when the
  device receives the frame on one interface and
  forwards that frame out another interface
• Routers have more inherent latency than a switch.
  Why?

10
Ethernet Transmission Time

• Defined
• Transmission time is the time necessary to move a
  packet from the data link layer to the physical
  layer
• 10BaseT Transmission Time
• Each bit has a 100ns window for transmission
• ns-nanosecond (1 billionth of a second)
• So each byte has what size window?
• A 64 byte frame (the smallest allowed frame)
  requires 51,200 ns or 51.2 microseconds
• Just to frame a 1000 byte packet requires 800
  microseconds
• Additional latency will be added propagating the
  frame down the wire and by any additional devices
  the frame has to go through before reaching the
  destination

11
Using Repeaters

• What is attenuation?
• Loss of signal strength as it travels through the
  network caused by resistance inherent in the
  medium
• Benefits of Using a Repeater
• a layer 1 device that cleans up and boosts the
  signal
• extends the coverage area of a LAN segment
• Negative Effects of Using a Repeater
• increases the collision domain size
• increases the broadcast domain size
• cant filter traffic based on Layer 2 or 3
  addressing

12
Full-Duplex, Fast Ethernet,and Segmentation
Table of Contents
13
Full-Duplex Ethernet

• Simultaneous TX and RX
• allows the transmission of a packet and the
  reception of a different packet at the same time.
• requires the use of two pairs of wires in the
  cable and a switched connection between each
  node.
• this connection is considered point-to-point and
  is collision free.
• because both nodes can transmit and receive at
  the same time, there are no negotiations for
  bandwidth.
• 100 of bandwidth is available 10 Mbps increases
  to 20 Mbps of potential throughput (10 Mbps TX
  10 Mbps RX)

14
LAN Segmentation

• Benefits of Segmenting the Network

• By segmenting a LAN fewer devices are sharing the
  same bandwidth, improving performance of a shared
  media LAN
• Each segment is considered its own collision
  domain
• How many broadcast domains in graphic?

15
Segmenting with Bridges

• Bridge Operation
• Bridges learn a networks segmentation by
  building address tables that contain
• Bridge interface that will reach that device
• Each devices MAC address

16
Segmenting with Bridges

• Generic Frame Format
• Frame can be any length depending on technology
• Ethernet frame can be up to 1522 bytes long
• Address section is 12 bytes (6 bytes for each
  MAC)
• FCS contain the CRC to check frame for errors

17
Segmenting with Bridges

• Bridge Performance
• adds 10 to 30 latency due to decision-making
  process
• considered a store-and-forward device because it
  must calculate the CRC at the end of the frame to
  check it for errors before forwarding
• if the bridge does not have an entry for the
  destination MAC, it...
• adds the Source MAC to its bridging table
• forwards the frame out all interfaces except the
  one it was received on
• when a reply returns, it adds the destination MAC
  to the table

18
Segmenting with Routers

• Router Operation
• Routers...
• use layer 3 addressing (IP, IPX) and routing
  protocols (RIP, IGRP) to determine the path and
• switch the packet out the correct interface to
  the destination
• because a router must open the packet to read
  Layer 3 addressing, it adds latency
• In addition, protocols like TCP which require
  acknowledgments of every packet can increase
  latency, reducing throughput from 20 to 40

19
Segmenting with Routers

• Router Benefits
• Like switches, routers segment collision domains.
• However, since a router will not forward
  broadcasts, it also segments broadcast domains.
• Each router interface represents its own
  broadcast domain.

20
Segmenting with Switches

• Switching Benefits
• a switch is simply a multi-port bridge, making
  forwarding decisions based on MAC addresses
• so, like a bridge, segmenting a LAN with a switch
  creates more collision domains
• replacing hubs with switches therefore decreases
  congestion and increases available bandwidth.
• a switch can microsegment a LAN creating
  collision-free domains but still be in the same
  broadcast domain.
• switch creates a virtual circuits, allowing many
  users to communicate in parallel.

21
Switching and VLANs
Table of Contents
22
Switch Operation

• Switches perform two basic functions
• Building and maintaining switching tables
  (similar to a bridge table) based on MAC
  addresses
• Switching frames out the interface to the
  destination
• Differences between switches bridges
• Switches operate at higher speeds
• Switches are capable of creating virtual LANs
  (VLANs) through microsegmentation
• Bridges switch using software switches typically
  switch using hardware (called the switch fabric)

23
Switch Latency

• A switch adds 21 microseconds of latency.
• This can be reduced by using a different
  switching method
• As opposed to store-and-forward, the switch can
  use cut-through switching which switches the
  packet as soon as the destination MAC is read.

24
How a LAN Switch Learns Addresses

• MAC addresses are learned dynamically and are
  stored in CAM (content-addressable memory)
• Each time a switch stores an address entry in the
  table, it is time-stamped.
• The time-stamp is updated each time a frame is
  received
• Addresses whose time-stamp expires are deleted
  from the table
• This keeps switching tables small

25
Benefits of LAN Switching

• Cost-effective switches only cost 3 to 5 times
  that of a hub
• Allows the creation of virtual circuits
• More flexibility in managing the network
• Reduces number of collisions
• Works with existing 802.3 cabling

26
Symmetric Switching

• symmetric switching provides switched connections
  between ports with the same bandwidth (10/10 Mbps
  or 100/100 Mbps)
• can cause bottlenecks as users try to access
  servers on other segments.

27
Asymmetric Switching

• asymmetric switching reduces the likelihood of a
  potential bottleneck at the server by attaching
  the segment with the server to a higher bandwidth
  port (100 Mbps)
• asymmetric switching requires memory buffering in
  the switch

28
Memory Buffering

• Defined
• Area of memory in a switch where destination and
  transmission data are stored until it can be
  switched out the correct port.
• Two types
• Port-based memory buffering
• packets are stored in a queue on each port
• possible for one packet to delay transmission of
  other packets because of a busy destination port
• Shared memory buffering
• common memory buffering shared by all ports
• allows packets to be RX on one port and TX out
  another port without changing it to a different
  queue.

29
Two Switching Methods

• Store-and-Forward
• The switch receives the entire frame, calculating
  the CRC at the end, before sending it to the
  destination
• Cut-through
• Fast forward switching--only checks the
  destination MAC before immediately forwarding
  the frame
• Fragment Free--reads the first 64 bytes to reduce
  errors before forwarding the frame

30
VLANs (IEEE 802.1q)

• Characteristics
• A logical grouping of network devices or users
  that are not restricted to a physical switch
  segment.
• The devices or users in a VLAN can be grouped by
  function, department, application, and so on,
  regardless of their physical segment location.
• A VLAN creates a single broadcast domain that is
  not restricted to a physical segment and is
  treated like a subnet.
• VLAN setup is done in the switch by the network
  administrator using the vendors software.

31
The Spanning-Tree Protocol
Table of Contents
32
Overview of STP

• Elements of the Spanning Tree Protocol
• Main function of STP is to allow redundant paths
  in a switched/bridged network without incurring
  latency from the effects of loops.
• STP prevents loops by calculating a stable
  spanning-tree network topology (similar to OSPF
  operation)
• Spanning-tree frames (called bridge protocol data
  units--BPDUs) are sent and received by all
  switches in the network and are used to determine
  the spanning-tree topology
• STP operation is covered in detail in Semester 7
  of the CCNP curriculum.

33
Five STP States

• States are established by configuring each port
  according to policy
• Then the STP modifies the states based on traffic
  patterns and potential loops
• The default order of STP states are
• Blocking--no frames forwarded, BPDUs heard
• Listening--no frames forwarded, listening for
  data frames
• Learning--no frames forwarded, learning addresses
• Forwarding--frames forwarded, learning addresses
• Disabled--no frames forwarded, no BPDUs heard

34
Required Labs for this Chapter

• Spend your lab time completing all four labs in
  this Chapter
• Lab 2.3.7--Switching Characteristics
• Lab 2.3.10.1--Switch Management Console
• Lab 2.3.10.2--Switch Port Options
• Lab 2.4.2--Switch browser configuration

35
Ping for Troubleshooting

• Ping 127.0.0.1
• Ping own Address
• Ping Host on different Network
• Find what happens if Default GW not set

36
Table of Contents
End Slide Show