DHCP failover setup different IP offering with 2 helper address

John Wobus jw354 at cornell.edu
Wed Jul 1 16:14:24 UTC 2015

On Jun 28, 2015, at 8:33 AM, Alp Eren Kose wrote:
> 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?

As far as I know, DHCP redundancy works by having the servers offer  
different addresses and the client picking one.
ISC DHCP servers reduce the obvious duplicated effort by having one of  
the two DHCP servers wait a few seconds to see if
the other server makes an offer.  But if the one server doesn't pick  
up on it's peer's offer, the address it
offers itself is always a different one.

John Wobus
Cornell IT

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