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DHCP Relay on ASA, phones dont get addresses

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:


Re: DHCP Relay on ASA, phones dont get addresses

Hi Luke,

It sounds like the ASA might be affecting the DHCP packets in a way that the IP phone doesn't like.

I would start by setting up simultaneous, bi-directional packet captures on both sides of the ASA so that we can see what changes in the packet as it passes through the ASA.


Re: DHCP Relay on ASA, phones dont get addresses

Did you configure the Data + Phone scopes as two separate 'Scopes' or two separate 'Super Scopes?