Layer 2 protocols you should know as a network person
Spanning Tree and Loop Control
Spanning Tree Protocol prevents loops by forcing one active path through the switching fabric. The root bridge defines that path, and other links move to a blocking state so frames do not circulate. When a link drops, ports change state and traffic pauses while the tree rebuilds. Rapid Spanning Tree Protocol reduces that pause by moving ports to forwarding faster after a change.
VLANs and Trunking
IEEE 802.1Q adds a tag so switches know which VLAN a frame belongs to. Access ports take untagged frames and place them into one VLAN. Trunk ports carry many VLANs by keeping the tags on the frame between switches. If the native VLAN does not match, untagged frames map into different VLAN IDs on each side.
Link Aggregation
Link Aggregation Control Protocol bundles multiple links into one logical interface. The switch assigns each flow to a link using a hash based on header fields like source and destination IP. Each flow stays on one link, which caps that flow at a single member’s speed. LACP exchanges system and port information so both sides agree on which links forward traffic.
ARP and Neighbor Discovery
Address Resolution Protocol maps IP addresses to MAC addresses within a VLAN. A device sends a broadcast asking who owns an IP, and the owner replies with its MAC. Switches learn that mapping and update their tables so frames forward to the correct port. More hosts increase the number of ARP requests each device processes.
LLDP and Topology Discovery
Link Layer Discovery Protocol sends periodic frames with system name and port ID to directly connected neighbors. Switches store that data per interface as a neighbor entry.
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