1. 3 LAN Design Basics Computernetze 1 (CN1) Prof. Dr. Andreas Steffen
Institute for Internet Technologies and Applications

2. Lesestoff im Ethernet Buch
Kapitel 6 Ethernet Internals, Seiten
6.1 Längenbeschränkung im Ethernet
Kapitel 8 Ethernet-Komponenten, Seiten
8.1 Netzwerkkarten
8.2 Repeater und Hubs
8.3 Switches
8.4 Medienwandler
Selbststudium
Erarbeiten Sie als Vorbereitung für die Übung 4
selbständig das Thema “Spanning Tree Protocol”
mit Hilfe des Cisco Tutorials von Dan DiNicolo und
des Kapitels 4 des CN1 Foliensatzes.

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 .
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

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

5. Repeater

6. 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.

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 to share the media
active – amplifies the incoming signal
intelligent – (smart hub) same function as an active hub, additionally includes a controller chip and diagnostic capabilities

8. Ethernet Strukturierung Collision Domain

9. 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.

10. MAC Layer Bridge (Transparent Bridge)

11. Bridge: Flooding or Filtering (destination address)
Error-free frame received on port x no
yes
Destination
address found in forwarding
database?
Forward frame to all ports (except port x)
Is destination again on port x? no
yes
Forward frame
to port y
Discard frame
Flooding
Filtering 1

12. Bridge: Learning (source address)1 no
yes
Source address
found in forwarding database?
Add address with port
and age value to the
forwarding database
Update port
and age value
Learning
end
Typical address aging value: 300 s

13. Learning / Flooding LAN 1 LAN 3 p1 p1 MAC A MAC F MAC B MAC G bridge 1
SA DA A F L2
data A F L2
data
LAN 1
LAN 3
learn A
flood F p1 p1
MAC A
MAC F
MAC B
MAC G
bridge 1
bridge 2
MAC C p2 p2
flood F
learn A
LAN 2 A F L2
data
table of bridge 1
table of bridge 2 p1 p2 p1 p2 A A

14. Learning / Filtering LAN 1 LAN 3 p1 p1 MAC A MAC F MAC B MAC G
DA SA A L2
data
LAN 1
LAN 3
learn F p1 p1
MAC A
MAC F
MAC B
MAC G
bridge 1
bridge 2
MAC C p2 p2
filter A
LAN 2
table of bridge 1
table of bridge 2 p1 p2 p1 p2 A A F

15. Learning / Flooding LAN 1 LAN 3 p1 p1 MAC A MAC F MAC B MAC G bridge 1
data
DA SA F G L2
data
LAN 1
LAN 3
flood G p1 p1
MAC A
MAC F
MAC B
MAC G
bridge 1
bridge 2
MAC C p2 p2
flood G
learn F
LAN 2 F G L2
data
table of bridge 1
table of bridge 2 p1 p2 p1 p2 A A F F

16. Learning / Forwarding LAN 1 LAN 3 p1 p1 MAC A MAC F MAC B MAC G
data
DA SA G F L2
data
LAN 1
LAN 3
forward F
learn G p1 p1
MAC A
MAC F
MAC B
MAC G
bridge 1
bridge 2
MAC C p2 p2
learn G
forward F
LAN 2 G F L2
data
table of bridge 1
table of bridge 2 p1 p2 p1 p2 A G G A F F

17. Segmentation with Bridges
Bridges split a LAN into several workgroups with smaller collision domains -> reduces whole LAN traffic
One "broadcast domain"
half-duplex collision domains

18. 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.

19. Ethernet Strukturierung Broadcast Domain (IP-Subnet)

20. Switch Forwarding Principles*
* delays computed for 10 Mbit Ethernet

21. 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 on higher-layer Information

22. Switching Fabric cross-connect any two switch ports Port 1 Port 0

23. Non-Blocking Switching Fabric
10 Gbps
10 Gbps
10 Gbps
60 Gbps Fabric
Brief overview on basic concept of non-blocking switches
Anatomy of a switch, good or bad?
Importance of buffers & queues
Will learn more about how this is implemented per Cisco platform in our product sessions
10 Gbps
Bandwidth of Fabric > Ingress + Egress

24. Blocking Switching Fabric
10 Gbps
10 Gbps
10 Gbps
20 Gbps Fabric
Heart of a switch or router, must work or nothing will
10 Gbps
Bandwidth of Fabric < Ingress + Egress

25. Shared Memory Architecture
General Purpose CPU
(CISC older or RISC newer)
CPU
Forwarding Tables
Buffers
Queues
Pointers
Headers
IOS Image/Files
Packet Memory
CPU Memory
System Buffers
Processor Queues
Data/Address/ Control Buses
Interface
Interface
Interface
Interface
Interface
Interface
Interface
Physical Media Interfaces
(Fixed or Modular)

26. Cross Bar Switch Architecture
CPU Memory
(DRAM)
CPU
(C) Forwarding
Table
Multiple conflict free paths
Typically higher bandwidth
Signaling and scheduling is more complex
CPU
Interface
Card
(D) FT
Packet Memory Tx Rx
CPU
Interface
Card
(D) FT
Packet Memory Tx Rx
CPU
Interface
Card
(D) FT
Packet Memory Tx
CPU
Interface
Card
(D) FT
Packet Memory Rx
Cross Bar ASIC

27. 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 considered layer 3 devices.
Routers can connect different layer 2 tech-nologies, such as Ethernet, Token-ring, or FDDI.