What is microsegmentation on a switched LAN, and why does it remove the collision domain?
One device per switch port, running full-duplex — with only two endpoints on the segment and separate send and receive paths, there is nobody left to collide with.
* One shared collision domain on a hub; none at all once each port has a single full-duplex device. *
Classic Ethernet was a shared medium. Every station on a hub or a coax run transmitted into the same collision domain, so each had to listen before sending and back off when two of them started at once — CSMA/CD (Carrier Sense Multiple Access with Collision Detection). The more stations you hung off that segment, the more of the bandwidth went into collisions and retries instead of data.
Microsegmentation is the switch's answer, and it is really two conditions holding at the same time:
- One device per port — the switch gives every port its own segment instead of one shared bus, so a port's "network" has exactly two participants: the attached device and the switch itself.
- Full-duplex on that port — transmit and receive run on separate pairs simultaneously, so those two participants never contend for the same direction.
Together they leave no way for two frames to overlap on the wire, so the port has no collision domain at all and the collision-detection circuit on the network interface card is disabled outright. Each port then gets the full stated bandwidth in both directions at once — a 100 Mbps link carries 100 Mbps each way rather than 100 Mbps shared.
Why this matters when troubleshooting: because a full-duplex port is collision-free by construction, any collision counted on one is a contradiction, not a busy network. It means the far end believes the link is half-duplex — a duplex mismatch — and that is where you go looking.
Go deeper:
Collision domain (Wikipedia) — what a collision domain is, and how switching shrinks it until each port has one of its own.- Carrier-sense multiple access with collision detection (Wikipedia) — the listen-then-send-then-back-off algorithm a full-duplex port no longer needs.