Chapter 10 – Analyzing Ethernet LAN Designs

Analyzing collision domains and broadcast domains

  • Collision domain 
  • Broadcast domain 

Ethernet collision domains

  • Term that has been used in the past, but nowadays with modern LANs, properly done LAN can prevent collisions entirely. 

10BASE-T with hub

  • Hubs don’t use CSMA/CD 
  • All devices connected to the hub, sit on the same collision domain 
  • Hub acts as a multiport repeater 
  • It blindly regenerates and repeats any incoming electrical signal out all other ports, ignoring CSMA/CD rules 
  • When two or more devices send at the same time, the hub makes electrical collision, both signals are corrupt 
  • Connected devices use CSMA/CD logic so the devices share the bandwidth 
  • Hubs create a physical star topology 

Ethernet transparent bridges

  • Bridges sat between hubs and divided the network into multiple collision domains 
  • Bridges increase the capacity of the entire Ethernet 
1 Collision Domain, Sharing 10 Mbps 
red 
Figure 10-3 
000000000 
Hub 
Bridge 
1 Collision Domain, Sharing 10 Mbps 
Wilma 
000000000 
Hub 
Bridge Creates Two Collision Domains and Two Shared Ethernets

Ethernet switches and collision domains

  • Each of those collision domains may also never have a collision.  
  • Any link that uses full-duplex does not have collisions 
  • No collisions occur between switch and end device so we can turn off CSMA/CD by running full duplex 
Four Possible Collision Domains 
Fred 
Barney 
ps 
00 Mbps 
Full Du lex 
FO/2 
0/4 
100 Mbps 
Full Du lex 
Wilma 
Betty 
Figure 10-4 Switch Creates Four Collision Domains and Four Ethernet Segments

The impact of collisions on LAN design

  • In modern LAN, collision domains still happen 
  • In misconfigured port, instead of using full-duplex, the port can use half-duplex, therefore creating collision domain issue that must be T-SHOOTED 
  • LAN switches place each separate interface into a separate collision domain 
  • LAN bridges placed each interface into a separate collision domain 
  • Routers place each LAN interface into a separate collision domain 
  • LAN hubs do not place each interface into a separate collision domain 
  • A modern LAN, with switches and routers, with full-duplex on each link, would not have collisions at all 
  • A modern LAN with switches and routers, think of each Ethernet link as a separate collision domain 
Five Collision Domains 
Hub 
Figure 10-5 
Bridge 
Hub 
Router 
Switch 
Example of a Hub Not Creating Multiple Collision Domains, While Others Do

Ethernet broadcast domains

  • An ethernet broadcast domain is the set of devices to which that broadcast is delivered 
  • Broadcast does not travel to another VLAN 
  • Only a router does not forward a broadcasts 
  • Routers separate network into separate broadcast domains 
DI 
10/100/1000 10/100/1000 
DI 
A39 
A40 
10/100/1000 10/100/1000 
Figure 10-6 A Single Large Broadcast Domain
Two Broadcast Domains 
ooaaaaaaa 
Hub 
Figure 10-7 
Bridge 
Hub 
Router 
Switch 
Broadcast Domains Separated by a Router

Virtual LANs

  •  LAN consists of the all devices in the same broadcast domain!!! 
VLAN 1 
Dino 
Left Broadcast Domain 
SWI 
VLAN 2 
DI Wilma 
Right Broadcast Domain 
Figure 10-9 Sample Network with Two VLANs Using One Switch

Impact of broadcast domains on LAN design

  • Broadcasts exist, so be ready to analyze a design to define each broadcast domain 
  • VLANs are broadcast domains created by configurations 
  • Routers create separate broadcast domains off their separate Ethernet interfaces 

Analyzing campus LAN topologies

  • Term campus LAN refers to devices in a building or multiple buildings in close proximity to one another. 

Two tier campus design (collapsed core): 

  • Access switches are connected to the end-devices, providing user device access to the LAN 
  • Access switches send traffic to and from the end-user devices to which they are connected. 
  • They sit at the edge of the LAN 
  • Distribution switches provide a path through which the access switches can forward traffic to each other. 
  • Each access switch connects to at least one distribution switch, typically two distribution switches for redundancy 
DI 
GigE 
10/100/1000 10/100/1000 
To WAN 
2 x 10 GbE 
DI 
Uplinks 
GigE 
A40 
10/100/1000 10/100/1000 
Distribution 
2 Distribution 
Switches 
40 Access 
Switches 
1000 PCs 
Layer 
Access 
Layer 
Figure 10-10 Campus LAN with Design Terminology Listed

Two tier design solves two major design needs: 

  1. provides a place to connect end-user devices 
  2. connects the switches with a reasonable number of cables and switch ports 

Topology terminology seen within a two-tier design 

  • star: one central device connects to several others 
  • full mesh: design that connects a link between each pair of nodes 
  • partial mesh: design that connects a link between some pairs of nodes, but not all (not full mesh) 
  • hybrid: design that combines larger and more complex topology link concepts 
Figure 10-11 The Star Topology Design Concept in Networking

Three-tier campus design (Core) 

  • third core saves money, cable lengths, not so expensive, saves ports… 

Summary of terms for campus switches: 

  • Access: provides connection point for end-user devices 
  • Distribution: provides aggregation for access switches, providing connectivity to the rest of the devices in the LAN, forwarding frames between switches but not connecting directly to end-user devices. 
  • Core: aggregates distribution switches in very large campus LANs and it provides very high forwarding rates for the large volume of traffic dues to the size of network 
Building 1 
Building 2 
All 
A12 
A13 
A14 
Figure 10-14 
Dil 
D12 
A31 
Corel 
D31 
A32 
D21 
D22 
A21 
A22 
A23 
A24 
Core2 
D32 
A33 
Building 3 
Three-Tier Building Design (Core Design), Three Buildings

Analyzing LAN physical standard choices

KEY TOPIC: 

  • the IEEE has developed many additional 802.3 standards for different type of cabling, cabling lengths, and faster speeds 
  • all standards rely on same consistent data-link layer details, with the same standard frame formats 

Ethernet standards: 

Thicknet 
(DIX) 
IOM 
1980 
Thinnet 
(IEEE) 
IOM 
1985 
Ethernet 
IOBase-T 
IOM 
1990 
Fast 
Ethernet 
100M 
1995 
Gigabit 
Ethernet 
2000 
10 
Gig E 
40 
Gig E 
2005 
100 
Gig E 
IOOG 
2010 
Figure 10-16 Ethernet Standards Timeline
Table 10-2 IEEE Physical Layer Standards 
Original IEEE Shorthand 
Standard 
802.3i 
802.311 
802.3z 
802.3ab 
802.3ae 
802.3m 
802.3ba 
802.3ba 
Name 
IOBASE-T 
IOOBASE-T 
IOOOBASE-X 
IOOOBASE-T 
IOGBASE-X 
IOCJBASE-T 
40GBASE-X 
IOOGBASE-X 
Informal Names 
Ethernet 
Fast Ethernet 
Gigabit Ethernet, 
Gigabit Ethernet, 
10 GigE 
10 GigE 
40 GigE 
100 GigE 
GigE 
GigE 
Speed 
10 Mbps 
100 Mbps 
1000 Mbps (1 Gbps) 
1000 Mbps (1 Gbps) 
10 Gbps 
10 Gbps 
40 Gbps 
100 Gbps 
Typical 
Cabling 
UTP 
UTP 
Fiber 
UTP 
Fiber 
UTP 
Fiber 
Fiber

Choosing the right Ethernet standard for each link

  • We need to pick Ethernet standard based on the following kinds of facts about each physical standard: 
  • The speed 
  • The maximum distance allowed between devices  
  • The cost of the cabling and switch hardware 
  • Availability of that type of cabling already installed at buildings 
Table 10-3 Ethernet Types, Media, and Segment Lengths (Per IEEE) 
Ethernet Type 
IOBASE-T 
IOOBASE-T 
IOOOBASE-T 
IOCJBASE-T 
IOCJBASE-TI 
IOOOBASE-SX 
IOOOBASE-LX 
IOOOBASE-LX 
Media 
TIA CAT3 or better, 2 pairs 
TIA CAT5 UTP or better, 2 pairs 
TIA CAT5e UTP or better, 4 pairs 
TIA CAT6a UTP or better, 4 pairs 
TIA CAT6 UTP or better, 4 pairs 
Multimode fiber 
Multimode fiber 
9-micron single-mode fiber 
Maximum Segment Length 
100 m (328 feet) 
100 m (328 feet) 
100 m (328 feet) 
100 m (328 feet) 
38-55 m (127-180 feet) 
550 m (1800 feet) 
550 m (1800 feet) 
5 km (3.1 miles) 
1 The option for IOGBASE-T with slightly less quality CAT6 cabling, but at shorter distances, is an attempt 
to support 10Gig Ethernet for some installations with CAT6 installed cabling.

Wireless LANs combined with wired Ethernet: 

  • Home office wireless LANs 
  • Enterprise wireless LANs and wireless LAN controllers 

Wireless LAN controller:  

  • Controls and manages all AP functions (for example, roaming, defining WLANs, authentication) 

Lightweight AP (LWAP):  

  • Forwards data between the wired and wireless LAN, and specifically forwarding data through the WLC using a protocol like Control And Provisioning of Wireless Access Points (CAPWAP) 
DI 
WLC 
Figure 10-20 
A3 
Campus LAN, Multiple Lightweight APS, with Roaming

Leave a Reply