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STP / RSTP / MSTP — Theory

Spanning Tree end to end: why Ethernet needs it, 802.1D root/port election, states & timers, RSTP (802.1w) roles & proposal/agreement fast convergence, MSTP (802.1s) regions & instances, and protection (BPDU/Root/Loop Guard, UDLD).

Ethernet has no TTL — a single L2 loop means frames circulate forever, broadcasts multiply into a storm, and MAC tables thrash. Spanning Tree exists to kill loops by pruning a physical mesh into a logical tree. The interview arc is the evolution: STP (slow) → RSTP (fast) → MSTP (fast and scalable across many VLANs).

The invariant across all three: elect one root bridge, then every other switch keeps its single best path to the root and blocks the rest — leaving a loop-free active topology.

802.1D STP

Switches exchange BPDUs and converge on a tree:

Root bridge election: lowest Bridge ID (priority + MAC). Tune priority in steps of 4096 to place the root deliberately.
Port roles: each non-root switch picks one root port (lowest cumulative path cost to root); each segment picks one designated port; everything else is blocked.
Path cost is by link speed (e.g. 10G=2, 1G=4, 100M=19) — lower is preferred.

Classic STP is slow: ports walk blocking → listening → learning → forwarding on timers (hello 2s, max-age 20s, forward-delay 15s) — 30–50 seconds to converge. A Topology Change (TCN) flushes MAC tables so traffic re-learns the new path.

RSTP (802.1w)

Rapid STP converges in sub-second by replacing timers with an active handshake.

RSTP port role	Meaning
Root	Best path to the root (same as STP).
Designated	Forwards onto a segment (same as STP).
Alternate	A backup to the root — a blocked path that can take over instantly.
Backup	A backup for a designated port on a shared segment.

States collapse to three: discarding / learning / forwarding.
Proposal/agreement handshake on point-to-point links lets two switches agree to move a port to forwarding immediately, rippling out from the root — no waiting on forward-delay.
Edge ports (PortFast) go straight to forwarding — host ports, no BPDUs expected.
Every switch now originates BPDUs (not just the root), so failures are detected fast. RSTP interoperates with legacy STP (falling back to slow behavior on that segment).

MSTP (802.1s)

Per-VLAN spanning tree (Rapid-PVST) runs one instance per VLAN — 1000 VLANs = 1000 trees = huge CPU. MSTP maps many VLANs onto a few instances (MSTIs), so you get per-instance load-balancing without the per-VLAN cost.

Switches sharing the same region — identical name + revision + VLAN-to-instance map — run the same set of MSTIs.
Inside a region, the IST (MSTI 0) handles inter-region topology; the wider network sees the whole region as one node via the CIST (Common and Internal Spanning Tree).
Typical design: map odd VLANs to instance 1 (root = switch A), even VLANs to instance 2 (root = switch B) — both uplinks forward, load-shared, with only two trees to compute.

Protecting the topology

Feature	Protects against
BPDU Guard	A switch plugged into an edge/PortFast port — err-disables the port on any BPDU.
Root Guard	A downstream switch trying to become root — blocks superior BPDUs.
Loop Guard	A blocked port going forwarding when BPDUs stop (unidirectional link) — keeps it blocking.
UDLD	Unidirectional fiber (TX/RX mismatch) that STP can't see.
BPDU Filter / Storm-control	Suppress BPDUs on edge ports / cap broadcast rates.

The modern data center: less STP

STP wastes links (blocking) and scales poorly. Modern fabrics avoid it: MLAG/vPC makes two switches look like one so both uplinks forward (STP still a backstop); routed leaf-spine + EVPN/VXLAN replaces L2 with L3 ECMP entirely — no blocked links, no spanning tree in the fabric. See VLAN & VXLAN.

Likely interview questions

Why does Ethernet need STP at all? (no TTL → loops = broadcast storms, MAC flapping)
How is the root bridge elected, and how do you influence it? (lowest Bridge ID; set priority)
STP port states & timers vs RSTP — why is RSTP so much faster? (proposal/agreement, no timers)
What are RSTP alternate vs backup ports?
What problem does MSTP solve over Rapid-PVST? (per-VLAN instance CPU; map VLANs to few MSTIs)
What must match for switches to be in the same MST region? (name, revision, VLAN-to-instance map)
BPDU Guard vs Root Guard vs Loop Guard — when each?
How do modern DCs avoid STP blocking links? (MLAG/vPC, EVPN/VXLAN L3 fabric)