Organizations

L1,L2 and L3 support

What is the meaning of L1,L2 and L3 support? What is the difference in the expertise and effort needed? Asking the same for a general understanding

How organizations deal with network design for disaster recovery

I am looking for some information about how organizations deal with network design for disaster recovery. Specifically, my issue is about DNS domains, and whether to have a single network with a redundant warm site in a single domain shared by production processing, requiring unique IP addresses...