multiple helper addresses

[martintonner]

When an interface responds to DHCP broadcast, how does the router select which helper address to send the request to? Does it go sequentially i.e. first helper address is selected unless it is down, or is it some other order like round robin?

Example config as follows:

interface Vlan456

ip address 7.7.x.x.255.254.0

ip helper-address 8.x.x.8

ip helper-address 9.x.x.9

Thanks,

Martin.

[pkhatri]

Martin,

In such a case, the router will forward the request to all configured helper addresses.

Hope that helps - pls do remember to rate posts that help.

Paresh

[martintonner]

Thanks, that definitely ties in with what we're seeing. So if both DHCP servers respond, how does the router know which response packet/frame to discard? Or does the host itself discard the packet/frame?

[pkhatri]

The host will pick one of the responses and respond itself with a DHCPREQUEST indicating the server it picked.

The server(s) that are not chosen will simply return the offered address back to the pool after a little while.

Hope that helps - pls do remember to rate posts that help.

Paresh

[pkhatri · Accepted solution]

Hi again,

If you think about it, this situation is no different to what you would have if you had multiple DHCP servers on the same LAN segment as the client. The client would broadcast a DHCPDISCOVER which would be seen by all the DHCP servers, all of whom would respond with a DHCPOFFER message. The client would then pick one and broadcast a DHCPREQUEST (whose 'server identifier' option indicates the server it selected). Since the DHCPREQUEST is broadcast, the DHCP servers which were not selected would then back off, and release the offered address.

Paresh

[martintonner]

Paresh,

That is absolutely spot on, I couldn't have asked for any more.

Thanks for your help,

Martin.