2302-Ch02: LAN Switching Concepts

• Switch operation basics are crucial for effective network troubleshooting.

Switching in Networking

• Switching and forwarding frames are fundamental in networking and telecommunications.
• Switches operate in LANs, WANs, and the Public Switched Telephone Network (PSTN).
• Traffic forwarding is determined by traffic flow, involving two key terms:
• Ingress: Port where a frame enters the device.
• Egress: Port used by frames when leaving the device.
• LAN switches use a table to forward traffic, which relies on the ingress port and the Ethernet frame's destination MAC address.
• A LAN switch has one primary switching table that strictly links MAC addresses to ports.
• Ethernet frames with a specific destination address exit via the same egress port, regardless of the ingress port.
• An Ethernet frame will not be sent out the same port it was received on.

The Switch MAC Address Table

• A switch consists of integrated circuits and software that manage data paths.
• Switches use destination MAC addresses to send network communications through the switch, via the correct port, to the destination.
• Switches learn which devices are on each port to transmit frames, building a MAC address table.
• The MAC address table is stored in content addressable memory (CAM).
• CAM is a type of memory used for high-speed searching applications.
• The MAC address table is sometimes referred to as the CAM table.
• LAN switches manage incoming data frames using the MAC address table.
• Switches record the source MAC address of each device connected to its ports to populate this table.
• The switch uses the MAC address table data to direct frames destined for a specific device out of the appropriate port.

The Switch Learn and Forward Method

• A two-step process is applied to every Ethernet frame entering a switch.

Step 1. Learn - Examining the Source MAC Address

• Each frame entering a switch is analyzed for new information by examining the source MAC address and the incoming port number.
• If the source MAC address isn't in the MAC address table, it is added along with the incoming port number.
• If the source MAC address exists, the switch updates the refresh timer for that entry.
• Ethernet switches typically retain an entry for five minutes.
• If a source MAC address exists in the table but is received on a different port, this is treated as a new entry, replacing the old one with the new port number.

Step 2. Forward - Examining the Destination MAC Address

• The switch searches for a match between the destination MAC address and an entry in its MAC address table if the destination MAC address is a unicast address.
• If the destination MAC address is in the table, the frame is forwarded out of the port.
• The switch sends the frame out of all ports, except the incoming port, if the destination MAC address is not in the table.
• Called an unknown unicast.
• Broadcast or multicast destination MAC addresses are also flooded out all ports except the incoming port.

Switching Forwarding Methods

• Switches make Layer 2 forwarding decisions quickly using software on application-specific integrated circuits (ASICs).

• ASICs minimize frame-handling time, allowing the device to manage more frames without reducing performance.

• Layer 2 switches use Store-and-forward switching or Cut-through switching.

• Store-and-forward switching:

• Makes a forwarding decision after receiving and checking entire frame using a cyclic redundancy check (CRC) for errors.

• Cisco's primary LAN switching method.

• Cut-through switching:

• Begins forwarding after determining the destination MAC address and egress port of an incoming frame.

Store-and-Forward Switching

• Store-and-forward switching has two main characteristics:
• Error checking: Compares the frame check sequence (FCS) value to its own calculations after receiving the entire frame, forwarding error-free frames and dropping others.
• Automatic buffering: Supports mixed Ethernet speeds by buffering frames, computing the FCS check, forwarding it to the egress port buffer, and then sends it.

Cut-Through Switching

• Store-and-forward switching drops frames that do not pass the FCS check and does not forward invalid frames.
• Cut-through switching may forward invalid frames because no FCS check is performed.
• Can perform rapid frame switching.
• The switch can decide on forwarding as soon as the destination MAC address is found in its MAC address table.
• The switch does not need to wait for the rest of the frame to enter the ingress port before deciding to forward.
• Fragment free switching is a cut-through switching variation that only starts forwarding the frame. Fragment free switching provides better error checking but with practically no increase in latency.
• The lower latency speed makes it more appropriate for extremely demanding, high-performance computing (HPC) applications that require process-to-process latencies of 10 microseconds or less.
• Cut-through switching can cause problems if there is a high error rate, with invalid frames clogging bandwidth.

Collision Domains

• In legacy hub-based Ethernet segments, network devices competed for the shared medium.
• Network segments sharing bandwidth are known as collision domains
• A collision will occur when two or more devices communicate at the same time.
• Each segment is its own collision domain when an Ethernet switch port operates in half-duplex.
• Collision domains will not ocur in full-duplex.
• Ethernet switch ports default to auto-negotiate full-duplex when the adjacent device also supports it.
• If the switch port connects to a half-duplex device like a legacy hub, the switch port will operate in half-duplex.
• The switch port will be part of a collision domain in half-duplex.

Broadcast Domains

• Multiple interconnected switches form a single broadcast domain.
• Routers are used to segment broadcast and collision domains.
• A device sets the destination MAC address to all binary ones when sending a Layer 2 broadcast.
• The Layer 2 broadcast domain is referred to as the MAC broadcast domain.
• The MAC broadcast domain consists of all LAN devices receiving broadcast frames from a host.
• Switches forward broadcast frames out of all ports except the ingress port upon receipt.
• Each connected device processes a copy of the broadcast frame.
• Broadcasts reduce network efficiency and can cause congestion, slowing network performance.
• Connecting two switches increases the broadcast domain.

Alleviating Network Congestion

• LAN switches have features that alleviate network congestion.
• Interconnected switch ports attempt to establish a link in full-duplex by default, which eliminates collision domains.
• Full-duplex connections double bandwidth and require 1 Gbps Ethernet speeds and higher.
• Switches interconnect LAN segments, use MAC address tables to determine egress ports, and can lessen or eliminate collisions.
• Switches with these traits alleviate network congestion:
• Fast port speeds: Vary by model and purpose (100 Mbps to 100 Gbps).
• Fast internal switching: Use a fast internal bus or shared memory for high performance.
• Large frame buffers: Use large memory buffers to temporarily store more received frames without losing frames.
• High port density: Lowers overall costs by reducing the number of switches needed.

Frame Forwarding

• The decision on traffic forwarding depends on traffic flow.
• Ingress describes where frames enter, and egress describes where frames exit.
• Ethernet frames are never forwarded out the port through which they entered.
• As switches determine the relationship of ports to devices, a MAC address table is built.
• The source MAC address and port number learn MAC addresses for new information.
• The switch checks for matches in the MAC address table for unicast addresses.
• Switch forwarding methods include store-and-forward (error-checking and buffering) and cut-through (rapid frame switching).
• Cut-through performs rapid frame switching or error checks.
• switches make forwarding decisions instantly with the destination MAC address.

Switching Domains

• Each segment is its own collision domain if an Ethernet switch port operates in half-duplex.
• There are no collision domains when switch ports are operating in full-duplex.
• Ethernet switch ports automatically negotiate full-duplex when possible.
• Connecting switches creates a single broadcast domain divded by a router.
• LAN devices receive broadcast frames from a host.
• Receives a copy of the broadcast frame.
• Switches offer increased performance and reduced network congestion with fast port speeds, fast internal switching, large frame buffers, and high port density.

Test your knowledge of LAN switching concepts. Questions cover MAC address tables, forwarding decisions, and switching methods like store-and-forward.