The DHCP server uses the network statement as a match so no need to specify an interface.
ip dhcp pool "vlan500-pool" ! Could be any name
network 10.2.100.0 255.255.252.0
dns-server www.xxx.yyy.zzz www.xxx.yyy.zzz ! DNS servers here
! any other options here
default-router 10.2.100.1 ! assumes you want the 3120 to be the gateway
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