Cisco ISR 1111 vs Comcast

So you've got a 1111 plugged into a Comcast router?

On the 1111 port you connect to the Comcast router, what happens if you do...

int gi0/1                        (or whatever interface you use)
ip address dhcp           (or ip address negotiated)

Thanks for the suggestion. I did try that but it picked up no IP address. :-(

Talking to Comcast tech support was like talking to the wall.

When you disconnect the laptop and connect the 1111, do you reboot the Comcast router too? Sometimes the MAC is remembered, especially if the Comcast box is in modem mode.

I'm remote - this is in a branch office. When I called tech support the automated system said they were rebooting the router as a first step. So I believe it was rebooted but I'm not certain.

The tech went on to say that someone had modified the default password. Totally weird since there's no one there to whom that would even occur. She did not that there's a default address on their gear like a 10.1.10.1 but I'm not sure how I'd access that since we're using a public IP. Could it be the case that comcast modems have a 10.1.10.1 internal AND a public IP?

Some can do. Modems will usually have a private IP for management. Routers will have a public and private IP.

When you connected your laptop did you get a public or private IP?

Well they assigned me a private IP with a mask and gateway. So that's what I have configured on the gi 0/0/0 int. I'm connected to something as the interface goes up/up. Just no luck with the arp entry. I haven't tried putting in a private IP. Again DHCP was a no go.