Switching
Switching is the process of forwarding Ethernet frames within a local network based on MAC addresses. Switches learn which devices connect to which ports, build forwarding tables, and deliver frames only where they need to go, creating fast, loop-free, segmented networks.
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Intro
Switching
A switch is a layer-2 device that forwards Ethernet frames based on destination MAC addresses. Unlike a hub, which floods every frame to every port, a switch learns where each device is and delivers frames only to the correct port. This creates separate collision domains per port while keeping devices in a shared broadcast domain.
Switching is the foundation of every local area network. Understanding how switches learn, forward, flood, filter, and prevent loops gives you control over LAN performance, segmentation, and resilience.
How a switch learns
A switch builds a MAC address table (also called a CAM table) by examining the source MAC address of every frame that arrives on each port.
When a frame arrives on port 3 with source MAC 00:1A:2B:3C:4D:5E, the switch records: "MAC 00:1A:2B:3C:4D:5E is reachable via port 3." This entry has a timer (typically 300 seconds). If no frame from that MAC arrives before the timer expires, the entry ages out.
This learning is automatic and requires no configuration. Within seconds of devices connecting, the switch knows which port reaches which MAC.
Forwarding decisions
When a frame arrives, the switch reads the destination MAC and checks its table:
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Sources
- https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html
Supports
- STP root bridge election and port role assignment
- RSTP proposal-agreement convergence mechanism
- MST VLAN-to-instance mapping
- Port channel interaction with STP topology
- https://www.ieee802.org/1/pages/802.1Q-2022.html
Supports
- VLAN tagging with 802.1Q (4-byte tag, 12-bit VLAN ID)
- Access and trunk port classification
- MSTP as part of the bridge standard
- LACP negotiation for link aggregation
- https://www.cisco.com/c/en/us/td/docs/switches/lan/cisco_ie3010/software/release/15-2_2_e/configuration/guide/SwCfg/swstp.html
Supports
- MAC address table learning and aging
- STP port states and transitions
- Port security and MAC limiting
- BPDU guard and root guard configuration
- Native VLAN mismatch behavior
- CAM table overflow behavior under MAC flooding attack
- https://www.rfc-editor.org/rfc/rfc7348
Supports
- VXLAN as layer-2 overlay over layer-3 infrastructure
- Extending broadcast domains across routed networks
- Modern data center switching architecture