Develop understanding of Data link layer (Switched LANs: Link-Layer Addressing and ARP, Ethernet, Link-Layer Switches, VLANs)

MAC Addresses

Every device on a local area network has a unique MAC (Media Access Control) address, which is used for communication within the same network. This MAC address is a 48-bit identifier usually represented in hexadecimal format. For example: 00:1A:2B:3C:4D:5E.

The MAC address is burned into the network interface card (NIC) during manufacturing and stays with the device throughout its lifetime. When devices communicate on the same local network, they use MAC addresses to identify each other at the data link layer.

Address Resolution Protocol (ARP)

ARP (Address Resolution Protocol) maps a known IP address to an unknown MAC address. When a device wants to communicate with another device on the same network, it uses ARP to find the MAC address associated with the target IP address.

The ARP process works as follows:

1. Device A (source) wants to send data to Device B (destination) but knows only Device B's IP address
2. Device A broadcasts an ARP request packet on the network asking: "Who has IP address x.x.x.x?"
3. All devices on the network receive this request, but only the device with the matching IP address responds
4. Device B sends an ARP reply containing its MAC address
5. Device A now has both the IP address and MAC address, and can send the data

This mapping is stored in the device's ARP cache for future use.

Ethernet is the dominant technology for wired LANs, defining wiring and signaling standards for the Physical layer and frame formats and protocols for the Data Link layer. Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to manage access to the shared medium.

Ethernet Frame Structure

An Ethernet frame contains several fields that enable delivery to the correct destination:

• Preamble: 7 bytes for synchronization
• Destination MAC Address: 6 bytes (the receiver's hardware address)
• Source MAC Address: 6 bytes (the sender's hardware address)
• EtherType/Length: 2 bytes (indicates which protocol is encapsulated)
• Payload: 46-1500 bytes (the actual data being transmitted)
• Frame Check Sequence (FCS): 4 bytes (for error detection using CRC)

Ethernet Standards

Ethernet has evolved through several speed standards:

• 10BASE-T (10 Mbps) - Traditional Ethernet
• Fast Ethernet (100 Mbps) - 100BASE-TX
• Gigabit Ethernet (1 Gbps) - 1000BASE-T
• 10 Gigabit Ethernet (10 Gbps) - 10GBASE-T

Link-layer switches (also called Ethernet switches) operate at Layer 2 of the OSI model. They receive incoming frames on one port and forward them to the appropriate output port based on the destination MAC address.

How Switches Work

Switches use MAC address tables to make forwarding decisions. These tables map each MAC address to the corresponding switch port. The switch builds this table dynamically by learning the source addresses of incoming frames.

Switching mechanism:

1. When a frame arrives, the switch examines the destination MAC address
2. If the destination is in the MAC address table, the frame is forwarded only to the associated port
3. If the destination is unknown, the frame is flooded to all ports except the one it arrived on
4. The switch learns the source MAC address and associates it with the incoming port

Benefits of Layer 2 Switching

• Reduced collisions: Unlike hubs that broadcast to all ports, switches create separate collision domains for each port
• Full-duplex communication: Devices can send and receive data simultaneously, effectively doubling bandwidth
• Improved network performance: Frames are forwarded only to their intended destination

Layer 2 vs Layer 3 Switching

• Layer 2 switching forwards Ethernet frames based on MAC addresses only
• Layer 3 switching (also called multilayer switching) can also perform routing functions, handling IP addresses and enabling inter-VLAN communication

VLANs allow network administrators to segment a physical network into multiple logical networks. Devices on different VLANs cannot directly communicate without a router or Layer 3 switch.

Configuring VLANs

VLANs are configured on network switches. Each port on a switch can be assigned to a specific VLAN. VLAN tagging using the IEEE 802.1Q standard adds a VLAN identifier to Ethernet frames to indicate VLAN membership.

Benefits of VLANs

1. Enhanced security: Network administrators can isolate sensitive traffic and restrict access between groups
2. Simplified network management: Makes it easier to expand, relocate, or reorganize network devices without physical changes
3. Flexibility: Administrators can configure VLANs in a centralized environment, even for devices in different geographical locations
4. Improved performance: Reduces broadcast traffic and improves overall network efficiency by limiting broadcast domains

Disadvantages of VLANs

1. Virus propagation risk: A single infected system can potentially spread a virus throughout the entire logical network
2. Equipment limitations: Very large networks may require additional routers to manage increased workload
3. Latency: While better than WANs, VLANs are generally less efficient in latency control compared to a single physical LAN

Worked Example: School Network with VLANs

Scenario: Mtakuja Secondary School has four departments: Administration, Science, Commerce, and Humanities. The network administrator wants to separate traffic for security and performance.

Solution using VLANs:

How it works:

• Each department gets its own logical network (VLAN)
• Sales computers in VLAN 30 cannot directly communicate with Science computers in VLAN 20 without a router
• Broadcast traffic from Science stays within VLAN 20, reducing network congestion for other departments
• If the Science lab computers get infected with a virus, it cannot spread to the Commerce department directly

Inter-VLAN communication: To allow communication between VLANs (e.g., Science teacher needs to access Humanities results server), a Layer 3 switch or router is required. The router would have subinterfaces configured, for example:

• Router subinterface GigabitEthernet0/0.10 for VLAN 10 (gateway: 192.168.10.1)
• Router subinterface GigabitEthernet0/0.20 for VLAN 20 (gateway: 192.168.20.1)

In Tanzania, data link layer concepts are applied when setting up networks in schools, offices, and businesses. For example, when installing a network at a local market in Dar es Salaam, a shop owner might use a Layer 2 switch to connect their point-of-sale (POS) terminals, security cameras, and inventory computers. Using VLANs, they can separate the POS traffic (VLAN 1) from the security cameras (VLAN 2) for better performance and security, ensuring that heavy video traffic from cameras does not slow down transaction processing at the cash register.