Cable Modem not passing IP over VLAN

[sjenkinsLNS]

I am trying to connect a Cisco DPC3010 Cable Modem to FastEthernet0/1 on a 2811 thru a vlan on a 2950 switch. No matter what I try, it seems that any device on the same vlan as the Modem will not receive an address. If I connect the modem directly to Fe0/1 on the router, I have no issue. I have ports Fe0/47 and Fe0/48 on the switch configured as follows:

interface FastEthernet0/47
switchport access vlan 10
switchport mode access
no ip address
end

interface FastEthernet0/48
switchport access vlan 10
switchport mode access
no ip address
end

I have tried putting one or both in trunk mode, but with no change. Am I missing something completely obvious here? Thanks.

[devils_advocate · Accepted solution]

Hmmmm, in theory that should work fine as long as Vlan 10 exists on the Cisco switch.

As both ports are access ports (i.e untagged), the VLAN ID is not even relevant really because VLAN tags are not added on access ports so as long as both ports are part of the same 'vlan', the router and the cable modem should be able to talk to each other at layer 2. 

Out of interest, why are you putting a VLAN switch between the Cable Modem and the Router?

People would usually put a switch on the LAN side of the Router, not the WAN side in a small network. I am certainly not saying you are doing anything wrong, I am just curious as to what you are trying to achieve :)

Can you provide the full configuration for both the router and the switch?

Is Fe0/1 on the Router a Layer 3 routed port or does it use Vlan tagging via subinterfaces?

Thanks
 

[sjenkinsLNS]

Thanks for the reply. Fe0/1 on the router is just a routed port with nothing vlan-related going on. Honestly, the reason I'm trying to accomplish this is a combination of "I bet I can do that" and laziness. I'm getting ready to learn a few different software firewalls in my lab at home that I'm running in VM's on my server and I don't want to have to run from my desk to my rack when I break them (which will definitely be happening :) to change ports just to have access to the internet. I will upload my configs when I get home.

[sjenkinsLNS]

Just figured out what the issue is (kind-of). I plugged my config into two different switches and it worked within seconds. I guess this is an issue with my 2950 (even after a factory reset).