I've deliberately drawn the Ethernet L2 topologies as lines (rather than switches) because it's an L3 problem. Assume that the L2 gear is super-robust: multiple power supplies, multiple supervisors, etc... The muliaccess Ethernets might even be 10BASE5 :-)
The problem is not a typo (if there are typos, I'll fix 'em). The problem is also not related to failure of a non-redundant device like R5. If R5 goes down, all of the hosts in Area 5 will become unreachable. That's obvious. The problem I've introduced will be somewhat surprising because it's in a redundant area. It's subtle, but detailed in the diagram, and can be eliminated with a telnet client. Can you spot it?
