Unable to get internet on a Catalyst 2960 switch from a 2811 router

[Solomon Samuel]

Hi everyone

I am helping with a network setup and am currently stumped with this issue. I'd be grateful if anybody could shed some light on the situation. The issue is as follows :

I'm changing Comcast modems at the place, and the setup is as follows - The modem goes to a cisco 2811 series router, which then goes to a catalyst 2960 switch from which various devices are connected to. The current setup works fine, but the internet at the switch is knocked out if I swap out the modem. I went to Comcast with the issue, and they told me that a couple of static IP's are enabled on the older modem and they need to be transferred to the new one. We didn't need the static IP's so I enabled DHCP at the FastEthernet 0/0 port on the router that was previously static. No avail, as the internet is still down. Then went to Cisco for help, and they could unfortunately not help either, as the router seems to be an old one that is not supported. 

I have a lot of logs with me as well as router and switch configuration files, and the 2811 Router config file is attached here. Thank you so much if anyone can provide some help with this.

[Kasun Bandara]

@Solomon Samuel hi, as per starting point you can check below steps.

1. is your PC got correct IP from DHCP (or configure as static)

2. can your PC ping gateway IP ? (i suggest to use router as gateway and you need to configure gateway IP at router)

3. Is router configured with WAN side IP at the port which connected to internet line

4. can router ping 8.8.8.8 (google dns) from its cli

5. check NAT configuration on router. (need to enable NAT to allow access internet for internal private network)

[Solomon Samuel]

Hi @Kasun Bandara 

Thank you so much for your suggestions. For additional information, I have attached the router configuration settings as a file with the original post for help with the troubleshooting. I have to apologize as I am not a network professional, so please do feel free to correct me if I go wrong somewhere. These are my observations when trying out your hints -

1. is your PC got correct IP from DHCP (or configure as static)

Yes. As an added observation, when I connect to the old modem, I get an IPV4 address as 169.254.xxx.xxx and when I connect to the new modem, the IPV4 address is 10.1.xx.xxx

2. can your PC ping gateway IP ? (i suggest to use router as gateway and you need to configure gateway IP at router)

I am assuming that the gateway IP for the new modem is the one that is shown from running a ipconfig when I connect my PC to the modem. It was 10.1.xx.1 and I was able to ping it.

3. Is router configured with WAN side IP at the port which connected to internet line

In the config file, I noticed a line saying - 'ip address 50.XXX.XXX.229 255.XXX.XXX.248', underneath the FastEthernet0/0 interface, which should mean that the router is indeed configured with a WAN side IP at the port. Please correct me if this is wrong.

4. can router ping 8.8.8.8 (google dns) from its cli

When I connected to the old modem, yes, I could ping google DNS from the cli. But the request timed out from the new modem.

5. check NAT configuration on router. (need to enable NAT to allow access internet for internal private network)

NAT is enabled in the config file, as I see mention of it. Since the IP address is different across both modems, I am theorizing that this might be the issue, though I cannot understand the nat range mentioned. Feel like this might be the problem, so any suggestions on this would be much welcome. I am reading about NAT in the meantime.

Thank you so much for your inputs and future suggestions.

[M02@rt37]

Hello @Solomon Samuel,

I don't understand when you say that your PC got IP address like 10.1.x.x.

IP dhcp is configured on your 2811 router and should offer IP on that subnet, assuming your PC is in the vlan Data, id 10:

network 192.XXX.XX.0 255.XXX.XXX.192

Does your PC in Vlan "Data" regarding the switch configuration ?

You should have on the 2960 Switch Port where your PC is connected:

switchport mode access

switchport access vlan 10

Also, from the 2811 router do you ping 50.XXX.XXX.230 ?

Please provide us output of these commands:

#sh ip int brief

#sh ip dhcp binding

#sh ip nat translations

[Solomon Samuel]

Hi M02@rt37 

What I was referring to when I said my PC had an IP address like 10.1.x.x. was the IP address of my laptop when I connected directly to the second modem, when DHCP was enabled on the laptop.

Regarding the VLAN data, I could not quite understand it, but one thing to note is that even with the old modem, the internet does not work directly from the router, or in the 2960 switch connected to the router. It works only off a secondary 2960 that is connected to the 2960 from the router, through fiber.

Yes, from the 2811 router, I can ping 50.XXX.XXX.230

This is the response to the above 3 commands. The last two returned empty responses.

#sh ip int brief
Interface IP-Address OK? Method Status Protocol
Vlan1 192.168.X.X YES NVRAM administratively down down
Vlan10 192.168.XX.X YES NVRAM up up
Vlan20 unassigned YES unset up up
FastEthernet0 unassigned YES NVRAM administratively down down
GigabitEthernet1/0/1 unassigned YES unset up up
GigabitEthernet1/0/2 unassigned YES unset down down
GigabitEthernet1/0/3 unassigned YES unset up up
GigabitEthernet1/0/4 unassigned YES unset down down
GigabitEthernet1/0/5 unassigned YES unset down down
GigabitEthernet1/0/6 unassigned YES unset down down
GigabitEthernet1/0/7 unassigned YES unset down down
GigabitEthernet1/0/8 unassigned YES unset down down
GigabitEthernet1/0/9 unassigned YES unset down down
GigabitEthernet1/0/10 unassigned YES unset down down
GigabitEthernet1/0/11 unassigned YES unset down down
GigabitEthernet1/0/12 unassigned YES unset down down
GigabitEthernet1/0/13 unassigned YES unset down down
GigabitEthernet1/0/14 unassigned YES unset down down
GigabitEthernet1/0/15 unassigned YES unset down down
GigabitEthernet1/0/16 unassigned YES unset down down
GigabitEthernet1/0/17 unassigned YES unset down down
GigabitEthernet1/0/18 unassigned YES unset down down
GigabitEthernet1/0/19 unassigned YES unset down down
GigabitEthernet1/0/20 unassigned YES unset up up
GigabitEthernet1/0/21 unassigned YES unset down down
GigabitEthernet1/0/22 unassigned YES unset up up
GigabitEthernet1/0/23 unassigned YES unset down down
GigabitEthernet1/0/24 unassigned YES unset down down
GigabitEthernet1/0/25 unassigned YES unset down down
GigabitEthernet1/0/26 unassigned YES unset down down
GigabitEthernet1/0/27 unassigned YES unset down down
GigabitEthernet1/0/28 unassigned YES unset up up
Port-channel1 unassigned YES unset up up
#
#
#sh ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
#
#sh ip nat translations
#
#

[M02@rt37]

Hello @Solomon Samuel,

Please provide 2811 configuration.

Thanks.

[Solomon Samuel]

Hi M02@rt37 

I have attached the 2811 config file with the post as you had asked for. Thank you !!

[Joseph W. Doherty]

The Comcast modem MAC is locked to, authoritized for, an account location.  If you change replace the modem you need to have replacement modem MAC registered to the account location, by Comcast.

The modem will also remember your device's MAC.  If you swap device using modem, you need to power cycle modem.

The forgoing applies to typical home modem usage, there might be other considerations if using advanced services, like a static Comcast IP, etc.

[Solomon Samuel]

Hi @Joseph W. Doherty 

Thank you for this suggestion. Comcast has reported that there are no issues at their end and I have power cycled as well.