Exercise 2: who notices a dead spine1 <-> leaf3 link?

Question: break spine1 eth2 <-> leaf3 eth0 with the agents running. Which hosts move traffic, and which don't?

Answer: only the hosts whose paths cross that link, and only for those paths:
  h1: "leaf3 plane1 DOWN"  (its plane-1 probes to h3 cross spine1 -> leaf3)
  h2: "leaf3 plane1 DOWN"  (same reason)
  h3: "leaf1 plane1 DOWN" and "leaf2 plane1 DOWN"  (its plane-1 probes' replies come back through spine1 -> leaf3)
  h1 <-> h2 traffic via spine1 never touches leaf3: 0 lost. No switch changes anything; spine1 keeps its 3
  static routes and its route to leaf3 just leads into the dead link. Each host fixes only its own view.
Compare episode 2: a BGP withdraw for 172.16.3.0/24 reached every router in the fabric.

Reproduce:  sudo ./solutions/exercise2/run.sh
