Cisco Support Community
Community Member

secondary ip helper address?

A secondary IP address range is created on an Interface and the hosts use DHCP to aquire their IP address, On that same interface a second IP Helper address is configured within the secondary IP range. How will the second Ip-helper address be used ? round robin ? or will the first helper address always be preffered ?


Re: secondary ip helper address?

Configuring 2 ip helper address statemenst on the interface has the efect of forwarding broadcasts, DHCP DISCOVER messages, to both addresses in the helper address statement.

In this case the DHCP client will receive two DHCP OFFER messages from the DHCP servers and will choose one of them, usually the client accepts the first offer it receives.

Community Member

Re: secondary ip helper address?

Any broadcast for DHCP will be sent to both IP helper-addresses, and it is up to the PC to decide which response to accept (usually the first reply).

The problem you are likely to encounter, though, is that any packet forwarded from the interface to the helper address will have the primary address of the interface as the source address. This would cause the DHCP server to allocate an address from the scope that would include the primary interface address, and so all addresses allocated by DHCP would be in the same subnet as the primary interface address.

An option to get around this, dependent on whether a switch is being used and the type of router / interface, is to configure the router interface with sub-interfaces in two different VLANs, each with its own primary address, and then run a trunk to the switch. The switch ports to which the PCs attached would then be placed into the two VLANs using a 50-50 split, and any traffic forwarded to the helper addresses would have the appropriate primary address as their source. This would then allow the appropriate IP address to be assigned to allow both address ranges to be used.

CreatePlease to create content