Can anybody please explain as to why a computer in one VLAN cannot get an IP address from a DHCP server in another VLAN by default? Why is there a need for a DHCP Relay Agent or an IP helper, they all are connected and can communicate with each other anyway.

Has it got something to do with Unicast, Multicast or Broadcast? Or something to do with the way the clients send a DHCP request?


Yes, DHCP requests are broadcast requests. If the host wanting a DHCP address is on a different subnet then you need to configure an IP Helper address. The IP helper address will forward the DHCP request using unicast to the DHCP server.

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