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Imagine you rent office space for a three-day event. You quickly set up a few Ethernet switches and tape some cables on the floor to get everyone online. Unfortunately, Stan, your clumsiest coworker, kicks out a cable every time he gets up for coffee. You could add extra cables, but then you’d get a broadcast storm: Ethernet packets that loop and multiply until nothing else gets through.
That’s where the spanning tree protocol ( STP ) comes in. STP blocks just enough of your spare cables to leave a loop-free tree. When Stan strikes again, it rebuilds the tree in a second, leaving some time for Blobby, your one-person support crew, to reconnect the cable.1 See for yourself: the diagram below runs a real STP implementation in your browser!
1 The sprites for Stan and Blobby come from Craftpix, the coffee cups from Yanin. ❦
:demo A1 @0,0 prio=4096 A2 @0,1 A3 @0,2 A4 @0,3 B1 @1,0 prio=8192 B2 @1,1 B3 @1,2 B4 @1,3 C1 @2,0 prio=8192 C2 @2,1 C3 @2,2 C4 @2,3 A1 -- A2 hazard=0 A2 -- A3 hazard=0 A3 -- A4 hazard=0 B1 -- B2 B2 -- B3 B3 -- B4 C1 -- C2 hazard=0 C2 -- C3 hazard=0 C3 -- C4 hazard=0 A1 -- B1 cost=10 B1 -- C1 cost=10 A4 -- B4 cost=20 B4 -- C4 cost=20 Leo @-0.3,0.7 proto=none icon=👦🏻 Mia @-0.3,1.3 proto=none icon=👧🏽 Joy @0.3,0.7 proto=none icon=👱🏻♀️ Roy @0.3,1.3 proto=none icon=👨🏾 A2 -- Leo hazard=0 A2:edge A2 -- Mia hazard=0 A2:edge A2 -- Joy hazard=0 A2:edge A2 -- Roy hazard=0 A2:edge Max @-0.3,1.7 proto=none icon=👨🏽 Zoe @-0.3,2.3 proto=none icon=👩🏾 Ada @0.3,1.7 proto=none icon=👵🏾 Amy @0.3,2.3 proto=none icon=👩🏼 A3 -- Max hazard=0 A3:edge A3 -- Zoe hazard=0 A3:edge A3 -- Ada hazard=0 A3:edge A3 -- Amy hazard=0 A3:edge Eli @0.7,0.7 proto=none icon=👦🏼 Jay @0.7,1.3 proto=none icon=👨🏻 Kai @1.3,0.7 proto=none icon=🧑🏽 Ben @1.3,1.3 proto=none icon=👱🏼 B2 -- Eli hazard=0.2 B2:edge B2 -- Jay hazard=0.2 B2:edge B2 -- Kai hazard=0.2 B2:edge B2 -- Ben hazard=0.2 B2:edge Ava @0.7,1.7 proto=none icon=👩🏻 Lea @0.7,2.3 proto=none icon=🧑🏾🦱 Ivy @1.3,1.7 proto=none icon=🧕🏽 Rex @1.3,2.3 proto=none icon=👴🏿 B3 -- Ava hazard=0.2 B3:edge B3 -- Lea hazard=0.2 B3:edge B3 -- Ivy hazard=0.2 B3:edge B3 -- Rex hazard=0.2 B3:edge Ana @1.7,0.7 proto=none icon=👩🏿 Eve @1.7,1.3 proto=none icon=👧🏼 Abe @2.3,0.7 proto=none icon=🧓🏿 Ian @2.3,1.3 proto=none icon=🧔🏾 C2 -- Ana hazard=0 C2:edge C2 -- Eve hazard=0 C2:edge C2 -- Abe hazard=0 C2:edge C2 -- Ian hazard=0 C2:edge Ned @1.7,1.7 proto=none icon=👨🏼🦳 Lou @1.7,2.3 proto=none icon=🧑🏿 Fay @2.3,1.7 proto=none icon=👧🏻 Sue @2.3,2.3 proto=none icon=👩🏽🦰 C3 -- Ned hazard=0 C3:edge C3 -- Lou hazard=0 C3:edge C3 -- Fay hazard=0 C3:edge C3 -- Sue hazard=0 C3:edge
Note This article is also available as a video, but I advise you to keep reading here to try the interactive demonstrations.
The basics#
Designed in the ’80s, the spanning tree protocol has evolved into a “rapid” flavor ( RSTP ) and a “VLAN-aware” variation ( MSTP ).2 Any sound-minded network engineer knows there are better alternatives, like BGP EVPN VXLAN. Yet, because any switch speaks it, the venerable spanning tree protocol still fills a niche.
We focus on RSTP : it replaced the original protocol in 2004. To eliminate network loops, RSTP implements a complex state machine. Timers, link state changes, and the link-local control frames a bridge receives from its neighbors drive its transitions. These Ethernet frames are the Bridge Protocol Data Units ( BPDUs ). You can watch them in action below: hit the “Start” button.
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