Download - Larger Site Networks
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Larger Site Networks
Part 1
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• Small Site– Single-hub or Single-
Switch Ethernet LANs
• Large Site– Multi-hub Ethernet
LANs
– Ethernet Switched Site Networks
– Congestion, Latency, and solutions
– ATM LANs
– Routers, Layer 3, and Layer 4 Switches
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Multi-hub LANs•Multiple hubs
•Multiple hubs in 10Base-T
•Multiple hubs in 100Base-TX
•Multiple hubs in Gigabit Ethernet
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Hubs
• Small LANs– Single-hub or single-switch LAN– 200 meter maximum distance span between
farthest two stations with UTP
100 m
100 m
X
Y
200 m
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Hubs
• Large LANs– Multiple-hub LANs– To increase maximum
distance span
100 m
100 m
100 m
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Two Hubs in 802.3 10Base-T
• 1. Station X transmits bit to Hub A
– Hub operates at the physical layer (one bit at a time)
• 2. Hub A broadcasts bit out all ports
A
B
X
Y
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Two Hubs in 802.3 10Base-T
• 3. Uplink Port sends bit to Hub B
– Uplink ports aremarked by an “X”
• 4. Hub B broadcasts bit to all attached stations, including Y
• Note that all stations on both hubs receive the bit broadcast almost simultaneously
A
B
X
Y3
UplinkPort
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Multiple Hubs in 10Base-T
• Farthest stations in 10Base-T can be five segments (500 meters apart)
– 100 meters per segment
– Separated by four hubs
100m
100m
100m
100m
100m
500m, 4 hubs
10Base-T hubs
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Multiple Hubs in 10Base-T
• No loops allowed– Only one possible path between any two
stations
No LoopsA
B
C1
2
3 4
5
6AB=1,2,3,4,5AC=1,2,3,4,6BC=5,4,6First two havetoo many hubs
No!
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Multiple Hubs in 10Base-T
• No loops allowed– If hub or link fails, network is divided
No LoopsA
B
C1
2
3 4
5
6
No!
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Multiple Hubs in 10Base-T
• Practical Limit in 10Base-T is Number of Stations
– Degradation of service beyond 100 stations
– Unacceptable service beyond 200 stations
– Maximum possible span normally embraces more than 200 stations
– In 10Base-T, the number of stations is the real limit to distance spans
– Still, it is possible to have a LAN with more than a 200 meter maximum span
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Multiple Hubs in 100Base-TX
• Limit of Two Hubs in 100Base-TX– Must be within a few meters of each other– Maximum span is 200 meters– Shorter maximum span than 10Base-T
100m
100m2 CollocatedHubs
100Base-TXHubs
~200 m
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Multiple Hubs with 1000Base-T
• Limit of One Hub in Gigabit Ethernet
– Maximum span is 200 meters
– Same limit as 100Base-TX
– Shorter maximum span than 10Base-T
100m
100m
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Multiple Hubs in Perspective
• 10Base-T Hubs– 500 meter maximum distance span with UTP– Farther with some optical fiber links– However 10Base-T is limited by the number of
stations it can support– So the maximum practical distance span is really
much smaller
• 100Base-TX Hubs and Gigabit Ethernet Hubs– 200 meter maximum distance span
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Switched Ethernet Site Networks
•No Maximum Distance Spans
•Hierarchies and Single Possible Paths
•High Speeds and Low Prices
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Ethernet Switched Networks
• There are Distance Limits Between Pairs of Switches– 100 meters with UTP– Longer with optical fiber
• But There is No Limit on the Number of Switches Between the Farthest Stations– So there is no maximum distance span
Maximum Separation100 m with UTP
Longer with optical fiberEthernetSwitch
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Hierarchies
• Ethernet Switches Must be Arranged in a Hierarchy– Root is the top-level
• Usually, Fastest Switches are at the Top (Root)– Sizes given are only examples
GigabitEthernet
CampusSwitch
100Base-XBuilding Switch
10Base-TWorkgroup
Switch
Root
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Hierarchies
• Only a Single Possible Path (2,1,3,4) Between Any Two Stations
Single PossiblePathEthernet
Switch
A
13 4
5
B
2
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Hierarchies
• Vulnerable to Single Points of Failure– Switch or Link (trunk line between switches)– Divide the network into pieces
X XEthernetSwitch
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Hierarchies
• 802.1D Spanning Tree Allows Redundant Links– Automatically deactivated to prevent loops
DeactivatedRedundant
Link
EthernetSwitch
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X
Hierarchies
• 802.1D Spanning Tree Allows Redundant Links– Automatically reactivated in case of failure– Slow and not completely effective
ReactivatedRedundant
Link
EthernetSwitch
Failure
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Hierarchies
• Link Aggregation Protocol Allows Multiple Links Between Stations– If one link fails, others continue– Switch failures or cuts of all links still fatal
MultipleLinks
EthernetSwitch
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Hierarchies
• Single Possible Path Simplifies Switch Forwarding Decisions– When frame arrives, only one possible output
port (no multiple alternative routes to select among)
– Switch sends frame out that port
SimpleForwarding
Decision EthernetSwitch
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Hierarchies
• Switches allow only a single path for each MAC destination address– Associated with a single port on each switch– So switch forwarding table has one and only
one row for each MAC address
EthernetSwitch
AddressA3..B2..
Port35
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Hierarchies
• Ethernet switch only has to find the single row that matches the destination MAC address
– Only has to examine half the rows on average; less if the table is alphabetized
– Comparison at each row is a simple match of the frame and row MAC addresses; much less work that row comparison in routers
– Overall, this is much less work than routers must do
AddressA3..B2..
Port35
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Hierarchies
• Overall, then, Ethernet switch forwarding is much simpler than router forwarding – So Ethernet switches are both cheaper and
faster than routers
SimpleForwarding
DecisionEthernetSwitch
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Hierarchies
• Router networks are meshes, allowing multiple alternative routes to the destination host
– Each alternative route is represented by a row in the router forwarding table
– Router must evaluate each row for each packet
– For each row, may have to compute match length, and metric
– After looking at all rows, must choose the best alternative route
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More on Switched Ethernet•Switch Learning
•Purchase Considerations
•VLANs
•Intelligent Switched Network Design
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Switch Learning
• Switch Forwarding Table has Address-Port Pairs
• Manual Entry is Too Time Consuming– Many addresses– Addresses change
• Solution: Learn addresses automatically
AddressA3..B2..
Port35
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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
Address Port
A1 BF C9
EthernetSwitch
At Start
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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
AddressA1
Port1
A1 BF C9
EthernetSwitch
After Transmission
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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
AddressA1C9
Port15
A1 BF C9
EthernetSwitch
After Transmission
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Switch Learning
• Every Few Minutes, Switch Erases Switch Forwarding Table– To eliminate obsolete information– Relearning is very fast
Address Port
A1 BF C9
EthernetSwitch
Erased
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Switch Learning
• Switches Can be in Hierarchy– Switches only learn that stations are out certain ports– Do not Learn of switch in Between
A1 BF C9
AddressA1BFC9
Port111
Port1
Switch A
Switch B
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Switch Purchasing Decision
• Hub Purchases are Simple– Number of Ports and Port Speeds
• Switch Purchases are More Complex– 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