A method for communication includes partitioning local links in a subnetwork of a packet data network into at least first and second groups. For each local link that connects a first upper-tier switch to a first lower-tier switch in the subnetwork, a corresponding detour route is defined, passing through a first local link belonging to the first group from the first upper-tier switch to a second lower-tier switch, and from the second lower-tier switch over a second local link to a second upper-tier switch, and from the second upper-tier switch over a third local link belonging to the second group to the first lower-tier switch. Upon a failure of the local link connecting the first upper-tier switch to the first lower-tier switch, data packets arriving from the network at the first upper-tier switch are rerouted to pass via the corresponding detour route to the first lower-tier switch.