S1 / 07 MODULE 7 / ETHERNET SWITCHING
Learn the MAC.
Forward the frame.
Ethernet switches use Layer 2 addresses to forward frames within a LAN. Understanding how a switch learns and uses its MAC address table makes forwarding behavior predictable.
Read an Ethernet frame
An Ethernet frame contains a destination MAC address, source MAC address, Type/Length field, payload, and Frame Check Sequence. The destination identifies the intended receiver on the local link. The source identifies the sender and is used by switches to learn where devices are connected. The FCS detects corruption during transmission.
IMG-0448 · IMG-0565Understand MAC addresses
An Ethernet MAC address is 48 bits, commonly written as six hexadecimal octets. The first portion identifies the organization that assigned the address (the OUI); the remaining bits identify the interface. Frames can use unicast, multicast, or broadcast destination addresses. The Ethernet broadcast address is FF:FF:FF:FF:FF:FF.
IMG-0448Learn how a switch builds its MAC table
When a switch receives a frame, it examines the source MAC address and records the address with the ingress interface and VLAN. Entries age out when they are not refreshed. This learning process lets the switch send known unicast frames only toward the port associated with the destination MAC address.
IMG-0467Forward known and unknown destinations
If the destination MAC address is in the table, the switch forwards the frame to the associated port (unless the destination is on the ingress port). If the destination is unknown, the switch floods the frame out eligible ports in the same VLAN, excluding the ingress port. Broadcast frames are replicated to eligible ports. Multicast forwarding can depend on features such as IGMP/MLD snooping, so do not assume every multicast frame is always flooded to every port.
IMG-0467 · IMG-0478Compare switching methods
Store-and-forward switching receives the entire frame and checks the FCS before forwarding. It can discard corrupted frames but adds buffering and latency. Cut-through switching begins forwarding after reading enough of the frame to identify the destination, reducing latency but potentially forwarding corrupted frames. Fragment-free switching waits for the first 64 bytes before forwarding, filtering many collision fragments while avoiding the full-frame delay. Switch speed and duplex settings also matter: full-duplex links can send and receive simultaneously and do not experience Ethernet collisions.
IMG-0478 · IMG-0573Interpret Ethernet frame sizes and buffering
A conventional untagged Ethernet frame is 64 to 1,518 bytes from destination MAC through FCS; the preamble and Start Frame Delimiter are excluded. An 802.1Q VLAN tag adds four bytes, so the conventional tagged maximum is 1,522 bytes. Jumbo frames depend on implementation support. Switches also buffer frames before egress: port-based buffering uses queues associated with individual ports, while shared-memory buffering allocates from a common pool that can be used flexibly by multiple ports.
IMG-0476 · IMG-0494 · IMG-0512Negotiate link speed and duplex
When both link partners support auto-negotiation, they advertise capabilities and select a mutually supported speed and duplex mode. Configure both ends consistently; forcing one side while the other negotiates can produce a duplex mismatch, poor throughput, and late collisions. Modern 1000BASE-T links operate full-duplex when compatible partners establish Gigabit operation. Auto-MDIX can adjust transmit/receive pair assignments on supported ports, but it cannot repair a damaged cable, bad termination, or failed transceiver.
IMG-0483 · IMG-0430 · IMG-0484 · IMG-0511Explore advanced switching enrichment
The practice bank also includes VLAN access and trunk ports, native VLAN consistency, Spanning Tree loop prevention, and router-on-a-stick inter-VLAN routing. These are useful extensions to Ethernet switching, but they go beyond the four core Module 7 source sections listed in the recovered lecture outline. Treat them as enrichment rather than evidence that the original Module 7 source taught every configuration detail.
802.1Q · VLANs · STP · Inter-VLAN routingCHECK YOUR UNDERSTANDING
Predict the switch's forwarding decision.
Practice MAC learning, flooding, unicast forwarding, and Ethernet frame behavior.