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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 

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 

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 

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 

Virtual LANs

•  LAN consists of the all devices in the same broadcast domain!!! 

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 

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 

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 

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: 

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 

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)