When an access switch is connected to two stacked switches through two links, is it necessary to configure LAG for those two links? (Those black links in the picture below.)
Or the stacking would handle this redundancy? (suppose no STP is enabled)
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Assuming the stacked switches behave like a single unit (not all do), connecting two links between access switch and stack creates a bridge loop, bringing down your network.
One approach for getting link redundancy is running RSTP/MSTP. xSTP disables forwarding on all but one of the redundant links, breaking the loop. It's good practice to enable and configure STP even when you're not actively using it.
To enable active use of both links they need to be aggregated (LAG trunk), preferably using LACP. Not all stacks support LAG trunks across chassis, so check the documentation before trying.
LAG requires configuration on both sides. The access switch sees the stack as a single link partner.