Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 1
Computernetze 1 (CN1)
3 LAN Design Basics
Prof. Dr. Andreas Steffen
Institute for Internet Technologies and Applications
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 2
Lesestoff im Ethernet Buch
• Kapitel 6 Ethernet Internals, Seiten 189-2146.1 Längenbeschränkung im Ethernet
• Kapitel 8 Ethernet-Komponenten, Seiten 275-3298.1 Netzwerkkarten8.2 Repeater und Hubs8.3 Switches8.4 Medienwandler
• SelbststudiumErarbeiten Sie als Vorbereitung für die Übung 4selbständig das Thema “Spanning Tree Protocol”mit Hilfe des Cisco Tutorials von Dan DiNicolo unddes Kapitels 4 des CN1 Foliensatzes.
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 3
LAN and LAN Devices
• LANs make it possible for businesses that use computer technology to efficiently share such items as files and printers and to make possible communications such as e-mail.
• LANs are designed to do the following: • operate within a limited geographic area or building• allow many users to access high-bandwidth media • provide full time connectivity to local services • connect physically adjacent devices
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 4
The Beginning of LAN Design
• Initial idea: shared media LAN• Bus structure; CSMA/CD was access method• Coax cable; transmission rate up to 10 Mbit/s, half-
duplex
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Repeater
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Repeater
• The purpose of a repeater is to amplify and retime network signals at the bit level.
• Repeaters can be single-port "in" and single-port "out" devices, though more often now, they are stackable (modular), or multi-port repeaters, better known as hubs.
• Repeaters are classified as layer 1 devices, in the OSI model, because they act only on the bit level and look at no other information.
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 7
Hub
• Used in 10BASE-T and 100BASE-T networks
• Is equivalent to a multi-port repeater
• Hubs come in three basic types:• passive – no traffic and signal manipulation, used only
toshare the media
• active – amplifies the incoming signal• intelligent – (smart hub) same function as an active
hub,additionally includes a controller chip and diagnostic capabilities
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 8
Ethernet StrukturierungCollision Domain
Hub
Collision Domain
Multiport Repeater (Concentrator, Hub)
simple Repeater Collision Domain
Collision Domain
single Segment
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Bridge
• The purpose of a bridge is to filter traffic on a LAN, to keep local traffic local, yet allows connectivity to other parts (segments) of the LAN for traffic that has been directed there.
• The bridge makes its decisions based on MAC addresses.
• The bridge is a layer 2 device.
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 10
MAC Layer Bridge (Transparent Bridge)
HigherLayers
LLC Sublayer
MAC Sublayer
PhysicalLayer
LAN-Segment “1”
PhysicalLayer
PhysicalLayer
MAC MAC
MAC Layer Bridge
BC
A
ZY
X
HigherLayers
LLC Sublayer
MAC Sublayer
PhysicalLayer
SA
DA
FC
S DATA
Port1 Port2
Table
LAN-Segment “2”
(ForwardingDatabase)
SA
DA
FC
S DATA
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 11
Bridge: Flooding or Filtering (destination address)
Error-free frame received on port x
1
Flooding Filtering Discardframe
Forward frameto port y
no yesDestinationaddress found in forwarding
database?
Is destination again on port x?
no yesForward frame
to all ports (except port x)
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 12
Bridge: Learning (source address)
Learning
1
end
Add address with portand age value to theforwarding database
no yesSource addressfound in forwarding
database?
Update port and age value
Typical address aging value: 300 s
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AA
Learning / Flooding
table of bridge 1
table of bridge 2p1 p2
SA DAA F L2data
MAC A
MAC FMAC C
MAC B MAC Gbridge
1bridge
2
p1
p2
p1
p2
LAN 1
LAN 2
LAN 3learn A
flood Flearn A
flood F
A F L2data
A F L2data
p1 p2
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F
Learning / Filtering
FDA SAAL2 data
MAC FMAC C
MAC B MAC Gbridge
1bridge
2
p1
p2
p1
p2
LAN 1
LAN 2
LAN 3learn F
filter A
MAC A
A
table of bridge 1 p1 p2
A
table of bridge 2p1 p2
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F
Learning / Flooding
MAC FMAC C
MAC B MAC Gbridge
1bridge
2
p1
p2
p1
p2
LAN 1
LAN 2
LAN 3
flood G
FGL2 dataDA SA
learn F
flood GMAC
A
FA
table of bridge 1 p1 p2
A
table of bridge 2p1 p2
F G L2data
F G L2data
Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 16
G G
Learning / Forwarding
MAC FMAC C
MAC B MAC Gbridge
1bridge
2
p1
p2
p1
p2
LAN 1
LAN 2
LAN 3 learn G
forward Flearn G
forward F
GFL2 dataDA SA
MAC A
FA
table of bridge 1 p1 p2
FA
table of bridge 2p1 p2
GFL2 data
G F L2data
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• Bridges split a LAN into several workgroups with smaller collision domains -> reduces whole LAN traffic
• One "broadcast domain"
half-duplex collision domains
Segmentation with Bridges
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Switch
• The purpose of a switch is to concentrate connectivity, while guaranteeing bandwidth.
• It switches packets from incoming ports (interfaces) to outgoing ports, while providing each port with full bandwidth.
• The switch uses the MAC address to make its switching decisions. You might think of each port on a switch as a micro-bridge, which makes it a layer 2 device.
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Ethernet Strukturierung Broadcast Domain (IP-Subnet)
Broadcast Domain
(simple) Bridge
Broadcast DomainMultiport Bridge
Broadcast Domain
Bridges and Backbone
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Switch Forwarding Principles
6B
DestinationAddress
6B
SourceAddress
2B
Length(E-Type)
4BFrameCheck
Sequence
0 (46)…1500BLLC PDU
Early-Cut
Cut-Through
Fast-Forward
Store-and-Forward
6B 4,8s=12B 9,6s=
64B 51,2s=
1518B 1,2144ms=
* delays computed for 10 Mbit Ethernet
*
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Switch Forwarding Principles
• Early-Cut• only usable when finished
learning• forwards nearly all
faulty frames
• Cut-Through• reads destination and
source address• forwards many
faulty frames
• Fast-Forward (Fragment Free)• detects runts and
collisions
• can filter specific Ethertypes
• Store-and-Forward• detects CRC-errors
• can do filtering based onhigher-layer Information
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Switching Fabric
Port 0
Port 3
Port 2
Port 1
cross-connect any two switch ports
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Non-Blocking Switching Fabric
10 Gbps
10 Gbps
10 Gbps
10 Gbps
60 Gbps Fabric
Bandwidth of Fabric > Ingress + Egress
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Blocking Switching Fabric
10 Gbps
10 Gbps
10 Gbps
10 Gbps
20 Gbps Fabric
Bandwidth of Fabric < Ingress + Egress
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CPU Memory
Data/Address/Control Buses
Packet Memory
BuffersQueuesPointersHeader
s
IOS Image/Files
System Buffers
Forwarding Tables
Processor Queues
Inte
rface
Inte
rface
Inte
rface
Inte
rface
CPU
General Purpose CPU (CISC older or RISC newer)
Physical Media Interfaces
(Fixed or Modular)
Shared Memory Architecture
Inte
rface
Inte
rface
Inte
rface
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Tx
Rx
Tx
Rx
Tx
Rx
CPU Memory(DRAM)
(C) ForwardingTable
CPU
CPU
InterfaceCard(D) FT
Packet Memory
CPU
InterfaceCard(D) FT
Packet Memory
CPU
InterfaceCard(D) FT
Packet Memory
CPU
InterfaceCard(D) FT
Packet Memory
Cross Bar ASIC
• Multiple conflict free paths• Typically higher bandwidth• Signaling and scheduling
is more complex
Cross Bar Switch Architecture
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Router
• The purpose of a router is to examine incoming packets, choose the best path for them through the network, and then switch them to the proper port.
• Routers make their path selection decisions based on layer 3 information - the network addresses therefore they are consideredlayer 3 devices.
• Routers can connect different layer 2 tech-nologies, such as Ethernet, Token-ring, or FDDI.