In order for dhcp-snooping to function correctly, the snooping device needs to be setup as just a layer 2 device (i.e. not performing DHCP functions at all). There are a few gotcha’s from 3Com's documentation, 3Com® Switch 4500G Family Configuration Guide (p. 405), which should still be applicable to your platform.
The DHCP Snooping supports no link aggregation. If an Ethernet port is added into an aggregation group, DHCP Snooping configuration on it
will not take effect. When the port is removed from the group, DHCP
Snooping can take effect.
If you have aggregated uplink ports (802.3ax), the link won’t be snooped on.
The DHCP snooping enabled device does not work if it is between the DHCP relay agent and DHCP server, and it can work when it is between
the DHCP client and relay agent or between the DHCP client and
server.
In your test bed scenario, you basically had a client and a server connected into 2 different access ports; one a trusted DHCP port. This is the simplest way to setup DHCP-snooping. Had this of gone wrong, I would suspect there is another, underlying issue/configuration mistake.
The DHCP Snooping enabled device cannot be a DHCP server, DHCP relay agent, DHCP client, or BOOTP client. Therefore, DHCP Snooping must be
disabled on a DHCP server, relay agent, DHCP relay agent, DHCP
client, and BOOTP client.
What this final bit means is that you really can’t have your switch performing any DHCP functions, aside from DHCP-snooping.
In the comments, you stated ”it is just L2 device”. I would check over your configurations more thoroughly, because you are attempting to implement that absolute basic configurations needed for DHCP snooping to function. You tested it on your test network, and it worked fine. Now your production network, with seemingly identical configurations, isn't working.
Below are basic configuration procedures from the 3Com documentation; if these don't work, I would certainly be looking elsewhere.
1 Enable DHCP snooping.
<Sysname> system-view
[Sysname] dhcp-snooping
2 Specify GigabitEthernet1/0/1 as trusted.
[Sysname] interface GigabitEthernet1/0/1
[Sysname-GigabitEthernet1/0/1] dhcp-snooping trust
Best Answer
If the switch is providing DHCP services, then the only difference should be that you should not need to "trust" any port.
The reason why is that the trusted ports are allowed to receive (and not drop) DHCP server types of messages (ACK, NAK, OFFER, etc). Since the switch is generating them, it won't be receiving them on any port.