Trying to use an ASA5510 to relay DHCP requests to a windows DHCP server. Only windows clients can successfully lease addresses through the relay. Phones can not.
DHCP clients and DHCP server both attached to networks directly connected to different interfaces of the ASA.
Windows and IP Phone clients can successfully lease IPs when on the same network as the DHCP server. When on the network that is relaying through the ASA, only Windows clients can lease addresses.
Debug on the ASA shows the request going through. phone reports an invalid response from the dhcp server, and the dhcp server its self gets a corrupted lease when talking to the phones.
For testing purposes, all trafic is explicitly permitted between the two networks.
Found this post that describes an identical problem but using a linux DHCP server, though i cant really understand his fix: