Just a quick post showing a few commands that are great for troubleshooting issues with ISCSI on  ESXI host:

Netcat:

the following cmd verifies that you can reach the TCP ISCSI port (2360) on the storage array from the host.

nc -z target_ip 3260

vmkping:

To test the san and switch configuration you can use the following cmd to ping a storage array with jumbo frames from a ESXI host:

vmkping -s MTUSIZE IPADDRESS_OF_SAN -d

the -d indicates "do not fragment"

vmkping:  uses VMkernel to ping a device.

list LUN multipathing information:

esxcli storage core path list

List NMP Devices:

esxcli storage nmp device list