Cisco 1811 behind Comcast Cable DHCP

Unanswered Question
Aug 29th, 2008

Hello all,

I just got Comcast internet with dynamic ip. I am installing cisco 1811 behind the comcast modem. But I cannot send the traffic out. I tried with and w/o BVI option in 1811 but still does not work. Does anyone have any suggestion?

Here are some basical config:

int fas 0

ip add dhcp

ip nat outside

int vlan 1

ip add

ip nat inside

ip nat inside source route-map NoNat interface fas 0 overload

ip access-list extended NATList

deny ip

deny ip

deny ip

permit ip any



route-map nonat permit 10

match ip address NATList

ip route FastEthernet0 name DefaultRouteToComcast

I called Comcast; they said the modem only allows one config and they don't know how to config anything behind the modem other than PCs.

Any help will be greatly apprecated!



I have this problem too.
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mikegrous Fri, 08/29/2008 - 08:09

first off. Is the FAS interface getting a DHCP Address from comcast?

Collin Clark Fri, 08/29/2008 - 08:49

When you plug your PC into the modem, does it get a public or private IP address?

gpan667788 Fri, 08/29/2008 - 09:12

Public Address.

If I use router's fe 0 dhcp, I am getting completed different class of public ip than PC gets.


Collin Clark Fri, 08/29/2008 - 09:27

Once you plug in the router and get an IP, can you ping the default gateway?

gpan667788 Fri, 08/29/2008 - 09:56

Don't know the default gateway if I use router. The ip I am getting is like 24.x.x.x different than the IP I am getting from PC 74.x.x.x. If I hard code the 74.x.x.x and default gateway into router fe interface and setup the default route, then I can ping.


Collin Clark Fri, 08/29/2008 - 13:04

OK, let's move forward and address that later. Can the router ping an internet address like If yes, can the router ping it when you source from the LAN side?

gpan667788 Fri, 08/29/2008 - 13:18

No, cannot even ping anything outside before the extended ping.


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