Their tiers are in different subnets of one VRF and nothing is routing between them: the fabric has no LIVE L3 first-hop router on the path.
Pure-L2 access switches only bridge WITHIN a subnet — inter-subnet routing needs an L3 device. If the edge gateway (the only router) is down, cross-subnet east-west drops while same-subnet keeps working.
Fix: bring a router back (recover/add a redundant gateway) or put an L3 switch on their path so it routes subnets locally — no re-cabling needed.
