Multiple ip helper-addresses

[cwainwright]

If you have more than one ip helper address on an interface to forward dhcp requests. Which entry gets used ?

The first entry is my primary DHCP server, the second entry is a secondary DHCP server located on a different subnet .

Will dhcp requests get forwarded to the first one only ?

[steve.barlow]

DHCP requests will be sent to each server, the host will select which ever server's reply was recieved back first. The router will forward the dhcp broadcast as a unicast packet to the dhcp servers and populate the giaddr field with the primary IP address on the interface (secondary addresses are ignored/not put in the giaddr field).

eg

!

interface vlan 10

ip address 10.10.10.1 255.255.255.0

ip address 10.10.11.1 255.255.255.0 secondary

ip helper-address 192.168.10.1

!

If your dhcp server is using the giaddr field to determine the scope to hand out from, you need to ensure that both IP address ranges are part of the same scope (eg. with the 10.10.10.0 range being the primary scope and 10.10.11.0 being the secondary).

You can put a sniffer on to see what is happening if you want.

Hope it helps.

Steve

[patrick.payne]

The router will forward the DHCP request braodcast, a.k.a. the DHCPDISCOVER packet, to all ip helper adresses.

All DHCP servers will respond with a DHCPOFFER. The client will choose one and reply with a DHCPREQUEST packet that informs the chosen server to complete the exchange while also informing the other servers that their offer has been declined.