Rapid Spanning Tree: the blocked port takes over in about a second

The idea

core, floor1 and floor2 are three switches cabled in a triangle, with alice, bob and a printer plugged in. Rapid Spanning Tree (RSTP) keeps a spare port ready, so a cut cable heals in about a second, and host ports forward at once.

Steps

1. Port roles and states: the loop, solved

   The triangle is a loop, so one cable end must not pass traffic, as in classic spanning tree. core's priority is lowest, so core is the root. Ask floor2 for its RSTP view.

   floor2:~# ovs-appctl rstp/show br0
   core:~# ovs-appctl rstp/show br0

2. Edge ports (PortFast): a host port forwards at once

   Behind bob's port there is only bob, no switch, so no loop can start there and waiting is wasted. We make it an ordinary port and bounce its cable; then we mark it edge and bounce it again.

   floor2:~# ovs-appctl rstp/show br0
   floor2:~# ovs-appctl rstp/show br0