COS 338

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COS 338

• Day 7

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DAY 7 Agenda

• Questions?
• Capstone Proposal must be approved by OCT 6
• Submit at any time (prior to Oct 6) using format
  specified in Capstone guidelines
• Today is Lecture on Ethernet LANS and Exam 1
  (again)
• Chap 1-3, open book, open notes, 60 min, 25 M/c
  questions

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Figure 4-8 Multiswitch Ethernet LAN
The Situation A1 Sends to E5
Switch 2
Port 5 on Switch 1 to Port 3 on Switch 2
Port 7 on Switch 2 to Port 4 on Switch 3
Switch 1
Switch 3
C3-2D-55-3B-A9-4F Switch 2, Port 5
B2-CD-13-5B-E4-65 Switch 1, Port 7
E5-BB-47-21-D3-56 Switch 3, Port 6
A1-44-D5-1F-AA-4C Switch 1, Port 2
D4-47-55-C4-B6-9F Switch 3, Port 2
4
Figure 4-8 Multi-Switch Ethernet LAN, Continued
On Switch 1
Switch 2

• Switching Table Switch 1
• Port Station
• 2 A1-44-D5-1F-AA-4C
• 7 B2-CD-13-5B-E4-65
• C3-2D-55-3B-A9-4F
• 5 D4-47-55-C4-B6-9F
• 5 E5-BB-47-21-D3-56

Port 5 on Switch 1 to Port 3 on Switch 2
Switch 1
B2-CD-13-5B-E4-65 Switch 1, Port 7
A1-44-D5-1F-AA-4C Switch 1, Port 2
E5-BB-47-21-D3-56 Switch 3, Port 6
5
Figure 4-8 Multi-Switch Ethernet LAN, Continued
On Switch 2
Switch 2
Port 5 on Switch 1 to Port 3 on Switch 2
Port 7 on Switch 2 to Port 4 on Switch 3
C3-2D-55-3B-A9-4F Switch 2, Port 5
Switch 1
Switch 3

• Switching Table Switch 2
• Port Station
• A1-44-D5-1F-AA-4C
• 3 B2-CD-13-5B-E4-65
• C3-2D-55-3B-A9-4F
• D4-47-55-C4-B6-9F
• 7 E5-BB-47-21-D3-56

E5-BB-47-21-D3-56 Switch 3, Port 6
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Figure 4-8 Multi-Switch Ethernet LAN, Continued
On Switch 3
Switch 2
Port 7 on Switch 2 to Port 4 on Switch 3

• Switching Table Switch 3
• Port Station
• 4 A1-44-D5-1F-AA-4C
• B2-CD-13-5B-E4-65
• 4 C3-2D-55-3B-A9-4F
• 2 D4-47-55-C4-B6-9F
• 6 E5-BB-47-21-D3-56

Switch 3
A1-44-D5-1F-AA-4C Switch 1, Port 2
D4-55-C4-B6-9F Switch 3, Port 2
E5-BB-47-21-D3-56 Switch 3, Port 6
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Figure 4-9 Hub Versus Switch Operation
Ethernet Hub
Hub Broadcasts Each Bit If A Is Transmitting to
C, B Must Wait to Transmit
X
C
D
A
B
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Figure 4-9 Hub Versus Switch Operation, Continued
Ethernet Switch
Switch Sends Frame Out One Port. If A Is
Transmitting to C, Frame Only Goes Out Cs Port.
C
D
A
B
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Figure 4-9 Hub Versus Switch Operation, Continued
Ethernet Switch
Switch Sends Frame Out One Port If A Is
Transmitting to C, B Can Transmit to
D Simultaneously
C
D
A
B
10
Advanced Ethernet Considerations

• STP and RSTP
• VLANs
• Momentary Traffic Peaks

11
Figure 4-10 Hierarchical Ethernet LAN
Single Possible Path Between Client PC 1 and
Server Y
Ethernet Switch A
Ethernet Switch C
Ethernet Switch B
Ethernet Switch F
Ethernet Switch D
Ethernet Switch E
Server X
Server Y
Client PC1
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Figure 4-10 Hierarchical Ethernet LAN, Continued

• Only one possible path between stations
• Therefore only one entry per MAC address in
  switching table
• The switch can find the one address quickly, with
  little effort
• This makes Ethernet switches inexpensive per
  frame handled
• Low cost has ledto EthernetsLAN dominance

Port Station
2 A1-44-D5-1F-AA-4C 7 B2-CD-13-5B-E4-65 5 E5-BB-4
7-21-D3-56
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Figure 4-10 Hierarchical Ethernet LAN, Continued
Core and Workgroup Switches
Core
Core Ethernet Switch A
Core Ethernet Switch C
Core Ethernet Switch B
Workgroup Ethernet Switch F
Workgroup Ethernet Switch D
Workgroup Ethernet Switch E
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Figure 4-10 Hierarchical Ethernet LAN, Continued

• Workgroup switches connect to stations via access
  lines
• Core switches higher in the hierarchy connect
  switches to other switches via trunk lines
• The core is the collection of all core switches
• Core switches need more capacity than workgroup
  switches because they have to handle the traffic
  of many conversations instead of just a few

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Figure 4-11 Single Point of Failure in a Switch
Hierarchy
Switch Fails
Switch 2
No Communication
No Communication
C3-2D-55-3B-A9-4F
Switch 1
Switch 3
B2-CD-13-5B-E4-65
D4-47-55-C4-B6-9F
E5-BB-47-21-D3-56
A1-44-D5-1F-AA-4C
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Figure 4-12 802.1D Spanning Tree Protocol
Normal Operation
Loop, but Spanning Tree Protocol Deactivates One
Link
Activated
Switch 2
Activated
Deactivated
C3-2D-55-3B-A9-4F
Switch 1
Switch 3
B2-CD-13-5B-E4-65
D4-47-55-C4-B6-9F
E5-BB-47-21-D3-56
A1-44-D5-1F-AA-4C
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Figure 4-12 802.1D Spanning Tree Protocol,
Continued
Switch 2 Fails
Deactivated
Switch 2
Deactivated
Reactivated
Switch 1
Switch 3
C3-2D-55-3B-A9-4F
B2-CD-13-5B-E4-65
D4-47-55-C4-B6-9F
A1-44-D5-1F-AA-4C
E5-BB-47-21-D3-56
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Figure 4-13 Virtual LAN (VLAN) with Ethernet
Switches
Server Broadcasting without VLANS
Servers Sometimes Broadcast Goes To All
Stations Latency Results
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Figure 4-13 Virtual LAN (VLAN) with Ethernet
Switches, Continued
Server Broadcasting with VLANS
With VLANs, Broadcasts Only Go To a Servers
VLAN Clients Less Latency
Server Broadcast
No
No
Client C on VLAN1
Client B on VLAN2
Client A on VLAN1
Server D on VLAN2
Server E on VLAN1
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Figure 4-14 Tagged Ethernet Frame (Governed By
802.1Q)
By looking at the value in the 2 octets
after the addresses, the switch can tell if this
frame is a basic frame (value less than 1,500) or
a tagged (value is 33,024).
Basic 802.3 MAC Frame
Tagged 802.3 MAC Frame
Preamble (7 octets)
Preamble (7 octets)
Start-of-Frame Delimiter (1 Octet)
Start-of-Frame Delimiter (1 Octet)
Destination Address (6 Octets)
Destination Address (6 Octets)
Source Address (6 Octets)
Source Address (6 Octets)
Length (2 Octets) Length of Data Field in
Octets 1,500 (Decimal) Maximum
Tag Protocol ID (2 Octets) 1000000100000000 81-00
hex 33,024 decimal. Larger than 1,500, So not a
Length Field
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Figure 4-14 Tagged Ethernet Frame (Governed By
802.1Q), Continued
Basic 802.3 MAC Frame
Tagged 802.3 MAC Frame
Tag Control Information (2 Octets) Priority Level
(0-7) (3 bits) VLAN ID (12 bits) 1 other bit
Data Field (variable)
PAD (If Needed)
Length (2 Octets)
Data Field (variable)
Frame Check Sequence (4 Octets)
PAD (If Needed)
Frame Check Sequence (4 Octets)
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Figure 4-15 Handling Momentary Traffic Peaks
with Overprovisioning and Priority
Congestion and Latency
Traffic
Momentary Traffic Peak Congestion and Latency
Network Capacity
Time
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Figure 4-15 Handling Momentary Traffic Peaks
with Overprovisioning and Priority, Continued
Overprovisioned Traffic Capacity in Ethernet
Traffic
Overprovisioned Network Capacity
Momentary Peak No Congestion
Time
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Figure 4-15 Handling Momentary Traffic Peaks
with Overprovisioning and Priority, Continued
Priority in Ethernet
Traffic
Momentary Peak
High-Priority Traffic Goes Low-Priority Waits
Network Capacity
Time
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Purchasing Switches
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Figure 4-16 Switch Purchasing Considerations

• Number and Speeds of Ports
• Decide on the number of ports needed and the
  speed of each
• Often can buy a prebuilt switch with the right
  configuration
• Modular switches can be configured with
  appropriate port modules before or after purchase

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Figure 4-16 Switch Purchasing Considerations,
Continued

• Switching Matrix Throughput (Figure 4-17)
• Aggregate throughput total speed of switching
  matrix
• Nonblocking capacity switching matrix sufficient
  even if there is maximum input on all ports
• Less than nonblocking capacity is workable
• For core switches, at least 80
• For workgroup switches, at least 20

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Figure 4-17 Switching Matrix
100 Mbps
1
Port 1 to Port 3
100 Mbps
400 Mbps Aggregate Capacity to Be Nonblocking
2
Any-to-Any Switching Matrix
100 Mbps
3
100 Mbps
4
Input Queue(s)
100Base-TX Input Ports
100Base-TX Output Ports
1
2
3
4
Note Input Port 1 and Output Port 1 are the same
port
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Figure 4-16 Switch Purchasing Considerations,
Continued

• Store-and-Forward Versus Cut-Through Switching
  (Figure 4-18)
• Store-and-forward Ethernet switches read whole
  frame before passing it on
• Cut-through Ethernet switches read only some
  fields before passing it on
• Perspective Cut-through switches have less
  latency, but this is rarely important

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Figure 4-18 Store-and-Forward Versus Cut-Through
Switching
Cut-Through Based On MAC Destination Address (14
Octets)
Preamble
Start-of-Frame Delimiter
Destination Address
Source Address
Cut-Through for Priority or VLANs (24 Octets)
Tag Fields if Present
Store-and- Forward Processing Ends
Here (Often Hundreds Of Bytes)
Length
Data (and Perhaps PAD)
Cut-Through at 64 Bytes (Not a Runt)
Cyclical Redundancy Check
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Figure 4-19 Jitter

• Jitter
• Variability in latency from cell to cell. Makes
  voice sound jittery

High Jitter (High Variability in Latency)
Low Jitter (Low Variability in Latency)
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Figure 4-16 Switch Purchasing Considerations,
Continued

• Manageability
• Manager controls many managed switches (Figure
  4-20 Managed Switches)
• Polling to collect data and problem diagnosis
• Fixing switches remotely by changing their
  configurations
• Providing network administrator with summary
  performance data

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Figure 4-20 Managed Switches
Get Data
Data Requested
Managed Switch
Manager
Command to Change Configuration
Managed Switch
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Figure 4-16 Switch Purchasing Considerations,
Continued

• Manageability
• Managed switches are substantially more expensive
  than unmanageable switches
• To purchase and even more to operate
• However, in large networks, the savings in labor
  costs and rapid response are worth it

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Figure 4-21 Physical and Electrical Features

• Form Factor
• Switches fit into standard 19 in (48 cm) wide
  equipment racks
• Sometimes, racks are built into enclosed
  equipment cabinets
• Switch heights usually are multiples of 1U (1.75
  inches or 4.4 cm)

19 inches (48 cm)
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Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Fixed-port switches
• No flexibility number of ports is fixed
• 1U or 2U tall
• Most workgroup switches are fixed-port switches

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Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Stackable Switches
• Fixed number of ports
• 1U or 2U tall
• High-speed interconnect bus connects stacked
  switches
• Ports can be added in increments as few as 12

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Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Modular Switches
• 1U or 2U tall
• Contain one or a few slots
• Each slot module contains 1 to 4 ports

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Figure 4-21 Physical and Electrical Features,
Continued

• Port Flexibility
• Chassis switches
• Several U tall
• Contain several expansion slots
• Each expansion board contains 6 to 12 slots
• Most core switches are chassis switches

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Figure 4-21 Physical and Electrical Features,
Continued

• UTP Uplink Ports
• Normal Ethernet RJ-45 switch ports transmit on
  Pins 3 and 6 and listen on Pins 1 and 2 (NICs do
  the reverse)
• If you connect two normal ports on different
  switches, they will not be able to communicate
• Most switches have an uplink port, which
  transmits on Pins 1 and 2. You can connect a UTP
  uplink port on one switch to any normal port on a
  parent switch

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Figure 4-21 Physical and Electrical Features,
Continued

• 802.3af brings electrical power over the
  stations ordinary UTP cord
• Limited to 12.95 watts (at 48 volts)
• Sufficient for wireless access points (Chapter 5)
• Sufficient for IP telephones (Chapter 6)
• Not sufficient for computers
• Automatic detection of compatible devices will
  not send power to incompatible devices

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Topics Covered

• Who develops Ethernet standards?
• Many physical layer standards (100Base-TX,
  1000Base-SX, etc.)
• Baseband versus broadband transmission
• Link aggregation
• Switch signal regeneration allows maximum
  distances spanning several UTP and fiber links

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Topics Covered

• MAC and LLC layers
• Ethernet Frame
• Preamble and Start of Frame Delimiter fields
• 48-bit Source and Destination Address fields
• Length field (length of data field)
• Data field
• LLC subheader
• Packet
• PAD if needed to make data field PAD 64 bits
  long

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Topics Covered

• Ethernet Frame
• Frame check sequence field
• Discard if detect error unreliable
• Hexadecimal Notation
• For humans, not computers
• Multi-Switch LAN Operation with Switching Tables

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Topics Covered

• Hubs versus Switches
• Hierarchical Topology
• Only one possible path between any two end
  stations
• Makes switching decisions easy and fast
• This makes the cost per frame handled low
• Key to Ethernets LAN dominance
• Core and Workgroup Switches

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Topics Covered

• VLANs to reduce congestion due to server
  broadcasting
• Handling Momentary Traffic Peaks
• Overprovisioningleast expensive Ethernet choice
  today
• Priority is more efficient but more expensive to
  do
• Tagged Frames for VLANs and Priority

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Topics Covered

• Switch Purchasing Decisions
• Number and speeds of ports
• Switch matrix capacity and nonblocking switches
• Store-and-forward versus cut-though switches
• Jitter
• Manageability
• Form factor (U)
• Port flexibility
• UTP uplink ports
• 802.3af for electrical power