More on Switched Ethernet

1
More on Switched Ethernet

• Switch Learning
• Purchase Considerations
• VLANs
• Intelligent Switched Network Design

2
Switch Learning

• Switch Forwarding Table has Address-Port Pairs
• Manual Entry is Too Time Consuming
• Many addresses
• Addresses change
• Solution Learn addresses automatically

Address A3.. B2..
Port 3 5
3
Switch Learning

• Situation Switch with
• NIC A1-33-B6-47-DD-65 (A1) on Port 1
• NIC BF-78-C1-34-17-F4 (BF) on Port 2
• NIC C9-34-78-AB-DF-96 (C9) on Port 5
• Switch Forwarding Table is Initially Empty

Ethernet Switch
Address
Port
At Start
A1
BF
C9
4
Switch Learning

• A1 on Port 1 Sends to C9 on Port 5
• Switch does not know port for C9
• Broadcasts the frame, acting as a hub
• Notes from source address that A1 is on Port 1
• Adds this information to switch forwarding table

Ethernet Switch
Address A1
Port 1
After Transmission
A1
BF
C9
5
Switch Learning

• C9 on Port 5 Sends to A1 on Port 1
• Table shows that A1 is on Port 1
• Switch only sends out Port 1 Acts like a switch!
• Source address shows that C9 is on Port 5
• Switch adds this information to forwarding table

Ethernet Switch
Address A1 C9
Port 1 5
After Transmission
A1
BF
C9
6
Switch Learning

• Every Few Minutes, Switch Erases Switch
  Forwarding Table
• To eliminate obsolete information
• Relearning is very fast

Ethernet Switch
Address
Port
Erased
A1
BF
C9
7
Switch Learning

• Switches Can be in Hierarchy
• Switches only learn that stations are out certain
  ports
• Do not Learn of switch in Between

Switch A
Address A1 BF C9
Port 1 1 1
Port 1
Switch B
A1
BF
C9
8
Switch Purchasing Decisions

• Hub Purchases are Simple
• Number of Ports and Port Speeds
• Switch Purchases are More Complex

9
Switch Purchasing Decisions

• Maximum Number of MAC address-port entries
• Small switches may not be able to store many MAC
  addresses
• For addresses that cannot be stored, switch must
  act like a hub, broadcasting and so creating
  latency

Address A1 C9
Port 1 5
10
Switch Purchasing Decisions

• Queue Size
• Incoming frames are placed in queues if they
  cannot be processed immediately
• May have several queues
• If queues are too small, frames will be lost
  during brief peak loads

Frames
Switch Matrix
Queues
Output Ports
Input Ports
11
Switch Purchasing Decisions

• Switching Matrix
• Receives input from multiple input ports, via
  queues
• Switches each frame to the correct output port

Switch Matrix
Frames
Queues
Output Ports
Input Ports
12
Switch Purchasing Decisions

• Switching Matrix Aggregate Throughput
• The number of bits it can switch per second
• Nonblocking if aggregate throughput equals the
  number of ports times the speed of the ports.
• Can handle the load even if all ports are
  receiving input simultaneously

Frames
Switch Matrix
Queues
Output Ports
Input Ports
13
Switch Purchasing Decisions

• Nonblocking Calculation
• 12 input ports
• 100 Mbps each
• Maximum possible input 1,200 Mbps (1.2 Gbps)
• Nonblocking switch needs 1.2 Gbps of aggregate
  switching capacity

Frames
Switch Matrix
Queues
Output Ports
Input Ports
14
Switch Purchasing Decisions

• Does Not Have to Be Completely Nonblocking
• Because not all input ports will send at once
• However, for switches high in the hierarchy, must
  be close to nonblocking (Tolly at least 75)
• Even for workgroup switches, should never be less
  than 25 nonblocking (Tolly)

Frames
Switch Matrix
Queues
Output Ports
Input Ports
15
Switch Purchasing Decisions

• Reliability through Redundancy
• Redundant power supplies and cooling fans
• May even have redundant switch matrix for backup

Frames
Switch Matrix
Queues
Output Ports
Input Ports
16
Switch Purchasing Decisions

• Manageability
• Can be managed remotely from the network
  administrators desk
• Network administrator can check on status of
  switch
• Network administrator can modify how the switch
  functions
• We will see remote management in Chapter 12
• Remote management greatly reduces labor

Frames
Switch Matrix
Queues
Output Ports
Input Ports
17
Switch Purchasing Decision

• Recap
• Port speed
• Number of ports
• Maximum number of MAC-Port pairs in forwarding
  table
• Queue sizes
• Switching matrix aggregate throughput
• Blocking or nonblocking
• Reliability
• Manageability