Cisco switches and Comcast SMC3G: DHCP works, IP doesn’t

I’m the chairman of the wiring committee for a hundred-unit condominium, and not a Cisco expert. Yesterday we got Comcast Business service, with an SMC3G router/modem, which works fine in isolation, or when I connect my laptop to it via our old HP dumb switches. But when I connect my laptop via our trio of Cisco Catalyst 3550 switches, DHCP usually works fine, but after that, the SMC3G is invisible: Pinging its IP address fails, even after I manually added its MAC to the arp access-list and ip source binding. Even traceroute mac from the Cisco switch gives “Error: Source Mac address not found.” Turning off DHCP in the SMC3G made no difference.

The interface settings for the port I’m usually connecting the SMC3G to (which works fine with our old AT&T DSL, via a Cisco 1417) are:

interface FastEthernet0/5
 description *2nd router*
 switchport access vlan 101
 switchport mode access
 ip dhcp snooping trust
end

The settings for the normal user ports are:

interface FastEthernet0/42
 switchport access vlan 101
 switchport mode access
 switchport protected
 no cdp enable
 spanning-tree portfast
 service-policy input USER_INGRESS
 service-policy output USER_EGRESS
 ip verify source
 ip dhcp snooping limit rate 10
end

The configuration for the vlan is just:

Current configuration:
!
vlan 101 
end

The Recurrent consultants who sold us the switch configured (but didn’t enable) QoS. It looks to me like they used dhcp snooping to prevent our Rogue Router problem. (This may have been what prevented my old Zyxel router from connecting, and also breaks the new router they told me I had to buy, unless I run it as a bridge instead.)