But are both pairs of fibers in the same cable? So if the fiber is cut then connectivity between the cores and ISP routers will be lost. So both buildings will be separated. The likelihood of a fiber cut preventing the internet routers from communicating, but the cores being OK would be pretty remote? Correct?
1- Understand about transit VLANs. The question then becomes is the infrastructure duplicated in each building? For example, redundant firewalls, HSRP addresses, etc. So you need to determine if connectivity between buildings is lost can each building operate on its own with the gateways and firewalls operating? There may be some instances that would fail, but the majority would have service until the situation was remedied.
2- If the connectivity is lost between the buildings, and provided the instance in #1 can be met and the loss of connectivity means that R1 only advertises a specific /25 to the ISP and R2 advertises another, then anything to the internet should come back through the proper router, so yes active/active.
3- Again, if R1 & 2 advertise both /25s in normal operation to to the ISP with R1 preferred, but when IBGP is lost due to loss of connectivity, then R1 would only advertise one of the /25s and R2 would advertise the other until connectivity is restored.
