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DHCP and secondary addresses

Interface E0

Ip address

ip address secondary

ip helper-address

I know DHCP will not work for the secondary address (the broadcast are not forwarded by the secondary address).

QUESTION: Will DHCP request be forwarded by the primary address?

(assume everything is functional, DHCP server and all networking gear and configurations).


New Member

Re: DHCP and secondary addresses

the router does pass the dhcp request from the primary interface. if using a server, make sure the scopes are set up for the necessary subnets. also, make sure you inlcude the helper address.


Re: DHCP and secondary addresses

"the router does pass the dhcp request from the primary interface"

Are you saying the router WILL pass DHCP request for the primary BUT NOT for the secondary?



New Member

Re: DHCP and secondary addresses

The router will send DHCP requests from the primary and not the secondary. This is possible by the IP Helper code adding the router interface address to the request and then forwarding it on to the DHCP server. The dhcp server in turn will assign the address based on the scope for the subnat based on the gateway address That the remote router provided. I use an NT 4 server for this and works great. In the case of NT 4, you need to creat a scope for the subnet of the primary address, and your done. Other server may differ from this. To provide DHCP to both subnets you will need two routers.

New Member

Re: DHCP and secondary addresses

I would add

no ip directed-broadcast

Also, I would consider limiting the forwarded protocols to just what you need. IP-Helper forwards more than just DHCP broadcasts. Personally, I would lock it back down. For example:

no ip forward-protocol udp tftp

no ip forward-protocol udp dns

no ip forward-protocol udp time

no ip forward-protocol udp netbios-ns

no ip forward-protocol udp netbios-dgm

ip forward-protocol udp bootpc

Food for thought.