Switched & Virtual LANs

Switched LANs and VLANs

• Ethernet collision and broadcast domains are important topics to understand.
• LAN segmentation and its benefits are key for network efficiency.
• Virtual LANs (VLANs) and understanding their usage is crucial.
• VLAN implementations is another key concept.
• Understanding switch access and trunk ports is essential.

Shared Ethernet Network

• In a shared Ethernet network, devices communicate through a central hub.
• A frame sent by one computer is received by all other computers connected to the hub.

Collision and Broadcast Domains

• A collision domain is a network segment where workstations and devices share bandwidth.
• Within a collision domain, devices can "hear" each other's transmissions.
• Sending a frame could result in a collision if another device transmits simultaneously.
• A broadcast domain is a network segment where any broadcast frame is received by all devices.

Collision Domain

• In a collision domain, multiple nodes connect to a hub, where only one device can transmit at a time.
• When multiple devices transmit simultaneously, there is a collision

Network Congestion

• Increased usage of a shared Ethernet network leads to network congestion.
• Network congestion results in a slow network response.
• Options to relieve congestion are: increasing bandwidth, segmenting the network, and implementing full-duplex Ethernet.

LAN Segmentation

• Network segmentation involves dividing a network into smaller parts or segments.
• Segmentation creates smaller collision domains, where each segment has its own collision domain
• Segmentation reduces congestion within each segment.
• Achieved using switches and routers.

LAN Segmentation with Layer 2 Switches

• Each switch port in a switched LAN exists in a different collision domain.
• LAN segmentation using a switch isolates traffic, reduces collision domain size, reduces congestion, and improves performance.

Basic Switch Operation

• Switches filter and forward packets based on destination MAC addresses, unlike hubs that forward all packets to all ports.

Layer 2 Switches Switching Modes

• Layer 2 switches support store-and-forward and cut-through switching modes.
• Store-and-forward switching stores the entire frame in a buffer.
• It checks the CRC for errors and, if none are found, filters and forwards the frame to the appropriate port.
• If there is an error, the frame is discarded.
• There are fast-forward and Fragment-free forms of cut-through switching.

Cut-Through Switching

• Fast-forward switching waits until it reads the destination MAC address, then filters and forwards it, with no error checking.
• Fragment-free switching waits for the first 64 bytes of the frame before forwarding, which helps avoid forwarding runt frames.

Layer 2 Switches Switching Modes - Packet Analysis

• Preamble (P): 8 bytes.
• Destination Address (DA): 6 bytes.
• Source Address (SA): 6 bytes.
• Length/Type (L/T): 2 bytes.
• Data: 46 to 1500 bytes.
• Cyclic Redundancy Checksum (CRC): 4 bytes.

Broadcast Traffic

• Broadcast traffic is essential for routing protocols to share internetwork topology.
• Servers use broadcasts and multicasts to advertise services.
• Workstations use broadcasts to find services, obtain addresses, and check address uniqueness.
• Excessive broadcast traffic saturates the network, uses bandwidth, slows network response, and overwhelms workstations.
• Workstations must process each broadcast packet.

Broadcast Traffic

• Layer two switches forward broadcast and multicast frames to all ports, potentially flooding the network.

Reducing the Size of a Broadcast Domain

• Using a router, networks can be divided into multiple broadcast domains to control network traffic.

Virtual LANs (VLANs)

• VLANs are logical groupings of workstations in a switched LAN, irrespective of physical location.
• VLANs segment networks based on user groups, applications, protocols, performance, and security requirements.

VLANs (Cont.)

• VLANs follow logical instead of physical connections.
• User assignment to a VLAN is irrespective of physical location.
• VLAN advantages include smaller broadcast domains.
• Each VLAN is a separate broadcast domain, which helps control the impact of broadcast frames.
• Better network performance.

Reducing the Size of a Broadcast Domain Using VLANs

• VLANs can be used to reduce the size of broadcast domains, which improves network performance.

VLAN Implementation and Assigning VLAN Memberships

• There are Port-based VLANS, Protocol-based VLANs and MAC-based VLANs.
• Port-based VLANs is port-centric.
• A switch port is manually configured to be a member of a VLAN.
• Layer 3 protocol within the frame is used to determine VLAN membership.
• VLAN membership is based on the MAC address of the workstation.

Port-Based VLANs

• Manually configure switch ports to be members of a VLAN.

Switches Access Ports

• Access ports connect end devices (PCs) to the switch.

VLAN Trunks

• A VLAN trunk is a point-to-point link that transports multiple VLAN traffic through a single switch port.
• The link between switches is often configured as a VLAN trunk link.
• A VLAN trunk link is not assigned to a specific VLAN.