This is the scenario, a Cisco router with 1 VLAN with 2 or more subnets and a DHCP server inside of one of those subnets.

interface Vlan1
 ip address
 ip address secondary
 ip address secondary

Let's say the DHCP server is inside the subnet

Will the DHCP server be able to broadcast to all clients inside all the subnets?

Will clients be able to do UDP broadcasts to all the subnets?

Will clients be able to communicate with clients in another subnet without problems?


  • The clients and the server will have no idea of which network a client should be. The clients will get an address in the network of the server address. Unfortunately, questions about protocols above OSI layer-4, e.g. DHCP, are explicitly off-topic here.
    – Ron Maupin
    Mar 4, 2017 at 15:35

1 Answer 1


From the DHCP server's point of view it's no different than any other VLAN and will respond accordingly. Keep in mind that - without additional information - the server has no idea to which secondary subnet a given client should belong.

UDP broadcasts map to an L2 broadcast address and so, yes, all clients on the subnet will receive the frame. That said, though, the broadcast will be ignored by the IP stacks of the clients not in the same subnet.

Clients will be able to communicate with one another, assuming they've been configured with the corresponding default gateway.

  • This is exactly what I wanted to know, I was aware of the DHCP issue. Thank you!
    – user34559
    Mar 5, 2017 at 1:34

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