If a leaf switch loses all connectivity to the spine layer, by default the attached hosts continue to send traffic to that leaf without a direct path to the destination. The VLTi link to the peer leaf switch handles traffic during such a network outage, but this is not considered a best practice.
Dell Technologies recommends enabling UFD, which detects the loss of upstream connectivity. An uplink state group is configured on each leaf switch, which creates an association between the uplinks to the spines and the downlink interfaces.
If all uplinks fail on a switch, UFD automatically shuts down the downstream interfaces. This propagates to the hosts attached to the leaf switch. The host then uses its link to the remaining switch to continue sending traffic across the leaf-spine network.
