ip helper address in Cisco Switch

Want to clarify my understanding for DHCP relay setup on Cisco switch :

Window DHCP server has been setup for leasing of two set of IP address

VLAN 3 : 10.50.10.x
VLAN6  : 10.50.30.x

Window server has IP 10.50.10.5. Two VLAN interfaces have been created in Switch
VLAN 3: 10.50.10.1
VLAN 6: 10.50.30.1

1. Window server (10.50.10.80) can provide DHCP subnet to VLAN 3 for IP 10.50.10.x, correct ?
2. For clients in VLAN6, I need to put the following in the configuration :
      interface vlan6
      ip helper-address 10.50.10.80

In this case, vlan6 interface (with ip 10.50.30.80) will contact the DHCP server for subnet 10.50.30.x and leasing to the workstation. As the IP is from 10.50.10.80, only the subnet 10.50.30.x will be given. Am I correct ?

Suppose on the cisco configuration :
  interface Vlan3
  ip address 10.50.10.1 255.255.254.0
  ip helper-address 192.168.1.1

In case the IP is not exist, will the VLAN3 client still receive the DHCP from Window server in VLAN 3 ?

Tks
AXISHKAsked:
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Muhammad MullaSystems AdministratorCommented:
The IP address of the DHCP server should go into the ip helper-address configuration.

This helps a system find a DHCP server that is not in it's broadcast domain (VLAN in this case).
JustInCaseCommented:
It should not work if DHCP server is not present in the same VLAN or on the switch (in both cases you don't need to have ip helper-address configured).
The reason is that proper unicast packet cannot be created and sent to DHCP server if there is no IP address present on SVI.

So, DHCP server will have no idea where to return packet or from what IP DHCP pool address should be assigned. The way IP helper address works is that interface VLAN set it's address in packet sent to DHCP server, so DHCP server knows to offer IP address from the same IP address range as sender of directed broadcast packet.

More detailed explanation.
AXISHKAuthor Commented:
"It should not work if DHCP server is not present in the same VLAN or on the switch (in both cases you don't need to have ip helper-address configured).
 The reason is that proper unicast packet cannot be created and sent to DHCP server if there is no IP address present on SVI."

So, do you mean VLAN3 don't need any ip helper-address as DHCP server is in VLAN3 ?

Will it be a problem in case ip helper-address has been put in the VLAN 3 interface but pointing to a non exist DHCP ?

Tks again.
JustInCaseCommented:
So, do you mean VLAN3 don't need any ip helper-address as DHCP server is in VLAN3 ?
IP helper is needed only when server is not directly reachable - not in the same broadcast domain (VLAN).
Will it be a problem in case ip helper-address has been put in the VLAN 3 interface but pointing to a non exist DHCP ?
It should not be problem since when DHCP discovery packet is sent it will be received by DHCP server and DHCP will give DHCP offer to host etc. I would not configure ip helper-address on SVI anyway if there is no purpose for it, it could confuse you (few months, years from now) or any other network engineer - why ip helper-address is misconfigured, and you or someone else could waste time for nothing.
:)

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AXISHKAuthor Commented:
tks
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