STP

The switch with the lowest bridge ID becomes root. The bridge ID is a priority followed by a MAC address, and every switch ships with the same default priority, so the lowest MAC decides, which is frequently an older device.

RSTP keeps 802.1D's root election and path costs but adds alternate and backup port roles, merges disabled, blocking and listening into one discarding state, and moves ports to forwarding by an explicit neighbour handshake instead of waiting out timers.

Hello Time (IEEE default 2 s) paces the root's Configuration BPDUs, Max Age (20 s) is how long stored BPDU information lives, and Forward Delay (15 s) is each listening and learning period. Every bridge uses the root's values, carried in its BPDUs.

A direct failure shows up at once as link loss, leaving only listening and learning: about 30 s at IEEE defaults. An indirect failure is learned through BPDUs, so stored information must first reach Max Age (20 s): about 50 s.

An edge port forwards at once on an unchecked promise that no bridge is attached. BPDU Guard enforces the promise by shutting the port on the first BPDU, so a user's switch or a looped cable costs one port, not the tree.

Each switch floods the broadcast out the other cable, so two copies circle the loop indefinitely, reaching every host on each pass. Hosts receive duplicates, both switches keep relearning the sender's MAC on the looped ports, and links and CPUs saturate.

S2 and S4 use their S1-facing ports as root ports; S3, two hops either way, picks the neighbour with the lower bridge ID; each link's designated port sits on the bridge nearer the root, leaving one S3 port blocking.

802.1D holds the port in listening and then learning, each for one Forward Delay (15 s by the IEEE default), before forwarding: listening lets the tree settle so no temporary loop forms, and learning fills the MAC table first.

Every bridge starts by claiming to be root and advertising its own bridge ID. A bridge that hears a lower root ID adopts it and relays it; bridge IDs compare as one number, priority first, MAC second, lowest winning.

A detecting bridge sends a TCN out its root port; each upstream bridge acknowledges with TCA and relays it. The root then sets TC for 35 s at IEEE defaults, and bridges age MAC entries after Forward Delay so stale ones clear.

An RSTP designated port on a point-to-point link proposes; the neighbour syncs by putting its other non-edge designated ports into discarding, then agrees, and the proposing port forwards at once because no loop can pass through the neighbour.

A BPDU (bridge protocol data unit) is the spanning-tree control frame bridges exchange on the reserved address 01-80-C2-00-00-00. Configuration BPDUs flow outward from the root carrying the tree's state; TCN BPDUs climb toward the root to report a change.

Each non-root bridge has exactly one root port, its lowest-cost path to the root bridge; each segment has exactly one designated port, the one nearest the root; every other port is non-designated and held in blocking.

A Configuration BPDU carries the root ID, the sender's root path cost, bridge ID and port ID, message age, the root's three timers and the TC/TCA flags. Relaying bridges copy the root ID and timers but rewrite cost, sender IDs and age.

Switches exchange BPDUs, elect a root, and keep only each switch's least-cost path to it plus one designated port per segment; every other port blocks. A blocked port drops data frames but keeps receiving BPDUs, ready to take over.

Since IEEE 802.1t, the 16-bit priority field is split: only its top 4 bits are configurable, so priority moves in steps of 4,096, and the low 12 bits form a system ID extension carrying the VLAN or instance number.

An RSTP alternate port hears a better BPDU from another bridge, a standby root path that replaces a failed root port at once; a backup port hears its own bridge on a shared segment and never becomes root port.

Per-VLAN trees run 200 independent trees, each sending its own BPDUs on every trunk; MSTP maps the 200 VLANs onto a few instances carried in one BPDU per port, at the price of an identical region configuration everywhere.

Recurring topology changes keep every bridge on 15-second MAC ageing, flooding frames to silent hosts. Trace the reports hop by hop to the bridge whose own port keeps changing, then fix it: a flapping link, or a host port that should be edge.

Root Guard goes on designated ports facing switches that must never become root and blocks on a superior BPDU. Loop Guard goes on root and alternate ports between switches and holds them blocked when BPDUs stop, as on one-way links.

Switches still map MAC addresses to the old path. Hosts that transmit are relearned at once; frames to silent hosts go the wrong way until entries are aged out or flushed, and after a flush unlearned destinations are flooded.

RSTP promotes a precomputed alternate port at once, accepts bad news from its designated bridge immediately, and confirms forwarding by handshake on point-to-point links. Shared links, 802.1D neighbours and non-edge host ports fall back to timers.

Line-rate broadcast plus a flapping MAC means a Layer 2 loop. Confirm it from flap logs and identical repeated frames, cut a link on the loop to stop it at once, then find why spanning tree did not block it.

The downstream switch's root port is the one hearing the upstream switch's lower port ID, because sender port ID breaks the tie; so port priority must be lowered on the upstream switch's port, not the downstream one.

802.1D-1998's short table gives 16-bit costs (19 for 100 Mb/s, 4 for 1 Gb/s) that bottom out near 10 Gb/s; 802.1t's long table (200,000, 20,000) scales further. Mixed on one network, root path costs stop being comparable.

Every forwarding path radiates from the root, so traffic between the distribution switches detours through the access switch's uplinks while their direct link blocks. Fix it by giving the distribution switches the two lowest bridge priorities, as primary and secondary root.

The new switch's BPDUs announce a better root, every bridge accepts it, and the tree is rebuilt around the newcomer. Under 802.1D, ports that must start forwarding wait about 30 seconds and MAC entries age out early, so traffic drops and floods.

The 802.1D timers are coupled: the IEEE requires 2 x (Hello + 1 s) <= Max Age <= 2 x (Forward Delay - 1 s). Changing one alone can expire information too early or let ports forward before the tree settles, causing loops.

An MSTP region needs a matching name, revision level and VLAN-to-instance digest, so a differing revision level is the usual culprit; at the boundary only the CIST crosses, and boundary ports forward or discard every VLAN together.