08 Ethernet

Protocol for connecting multiple computer systems to form a LAN, with protocols to

• control passing of information
• avoid simultaneous transmission by multiple systems

IEEE 802.3 Data Link Layer Sublayers¶

Diagram¶

Domains¶

Topology¶

	Bus	Star
		Active switch in center
Collision domain	All nodes in same collision domain	Each spoke runs separate Ethernet protocol
Collisions Prevented?	❌	✅

Normal Ethernet Operation¶

Types¶

Standard Ethernet Implementations¶

Steps of Routing to another LAN¶

Assuming A has all the required addresses already, and wants to send a message to B via R.

1. Create packet in Network Layer with
2. Source address = A’s IP address
3. Destination address = B’s IP address
4. Create frame in Datalink Layer with
5. Source address = A’s MAC address
6. Destination address = R’s receiving terminal MAC address
7. A sends message to R
8. R receives message
9. R processes and removes frame in Datalink Layer
10. R processes packet in Network Layer
11. R forwards packet with the same source and destination as before in the Network layer
12. R creates frame in the Network layer with
13. Source address = R’s sending terminal MAC address
14. Destination address = B’s MAC address

Ethernet Switch¶

• Examines incoming frame’s MAC address
• Selectively forwards frame to one/more outgoing links when frame is to be forwarded on segment
• Uses CSMA/CD to access segment
• Buffers packets

Every host has dedicated & direction connection to switch

Each link connected to switch is its own collision domain; hosts transmitting simultaneously does not affect other transmissions if they are on different link.

Characteristics¶

• Transparent: Hosts are unaware of presence of switches
• Plug-and-Play device: No configuration required by network admin
• Self-Learning mechanism

Types¶

Switch Table¶

Helps switch data from source to destination

Self-Learning¶

1. Check if receiver exists in switch table

2. If yes, go to step 5

3. ‘Flood’ (broadcast) message to all stations

4. Update switch table with receiver’s entry

5. Send to receiver

Interconnected Switches¶

Works using the same self-learning process

Switches vs Router¶

VLAN¶

Virtual Local Area Network

Allows us to divide a LAN without any additional switches

VLAN can be defined using one of the following techniques

• Switch port
• MAC addresses of endpoints

Advantages¶

VLAN helps overcome the following

• Improve traffic isolation: frames by default can only travel within their own VLAN
• Dynamic membership: ports can be dynamically assigned among VLANs
• Efficient use of switches
• Management of users
• Forwarding between VLANS
• Address Security, privacy and efficiency issues. Data link layer broadcast traffic (ARP, DHCP, unknown location of destination MAC address) need not cross entire LAN.

Port-Based VLAN¶

Details	Trunk Port connected to router Traffic isolation something else
No of usable ports	\(n-1\) (Trunk port unusable)
Actual connections
Behaves as

Trunk Port¶

Carries frames between VLANS defined over multiple physical switches. Frames forwarded over multiple switches must carry VLAN ID info as well, and hence uses IEEE 802.1Q Frame.

Ethernet/IEEE 802.1 Frame¶

All sizes shown in Bytes

Example of multiple frames¶

Receiver Address Type¶

LSB = Least Significant Bit

IEEE 802.1Q Frame¶

Adds/removes additional header fields for frames forwarded between trunk ports

(Empty cells of the following table means that they are the same as regular Ethernet frame)

VLAN Frame¶

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