DHCP failover setup different IP offering with 2 helper address

[Gero Palacio]

Hi Alp,

We have setup a DHCP failover configuration, with Wireless LAN Controllers
> having multiple ip-helper-address statements. The controllers are Cisco and
> Aruba branded. As I understand it, Controllers relay DHCP discover messages
> to both DHCP servers, however sometimes we see different IP addresses are
> OFFERED for the same request by both DHCP servers. Is it the normal
> behavior?


It is the expected behavior when the "sec" field in the DHCP client's
message is grater than the "mclt" defined in the primary DHCP server. Or if
the DHCP server are *not* in normal state. If that's not the case (both
servers in normal state and "sec" < "mclt") both servers will receive the
DHCP DISCOVERY message but only one of them will reply back with an offer.
There's a thread describing a similar problem I had in this mail list,
subject "DHCP Relay agent not forwarding messages to the client" first
message from June 15.

Could you also point some documentation that I can read to understand how
> exactly failover works in ISC DHCP version 4.2.8? Especially how multiple
> discovery and request packets are handled from multiple ip-helper address
> statements.


You can check out this link DHCP Failover Protocol Draft IEFT
<https://tools.ietf.org/html/draft-ietf-dhc-failover-12 and> and this DHC
Load Balancing Algorithm RFC 3074 <https://tools.ietf.org/html/rfc3074>.
Always keep in mind this tip which I got from another thread in this mail
list:

The split *does not* determine the share of the pool. The pool is
>  *always* balanced 50/50. The split value determines which peer will
>  respond to the client based on the hashed value of the client
>  identifier (MAC address).



Hope this helps!
Gero.
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