Lab 1 Task 3.2 IPsec VPN
The last subtask says that if R4 loses its connection to the Frame Relay cloud, the VPN should stay up. Just so you know, I wasted over 3 hours on trying to get this damn thing working only to find out the SG doesn't even test a Frame Relay outage.
But, rant aside, from looking at the diagram, the back up path will traverse the BB1 and BB3 routers. I know the BB routers are talking to each other and routes that I advertise from R6 are showing up at R1, but not vice versa. But connectivity issues, also aside, the SG didn't complete the scenario because IPSEC traffic is not being allowed in via R1 or R6 (which are treating the BB networks as "outside"). Am I right?