Showing results for 
Search instead for 
Did you mean: 

Cisco 2851 connection to comcast cable modem

Level 1
Level 1

Hi everyone,

I have a Comcast wireless gateway, a Cisco 2851 router, HP DL380 (used for VMware and UC applications), and a 3750 switch.  I want to be able to run Call Manager and other voice applications, but I think my Comcast wireless gateway is limiting me from doing so, there is no way (that I know of) to access the routing table on the Comcast wireless gateway.  This is preventing the traffic from my lab from getting to the internet (I believe).  I need to access the internet because Call Manager and other UC applications have to access it for an NTP reference.  My setup is as follows:

HP DL 380 ----> 3750 switch ----> 2851 router ----> Comcast wireless gateway ----> Internet

Here are the details of each:

[ESXi] IP = (connects to switch and is in both voice and data vlan)

[3750 Switch]

Data vlan = 110

Voice Vlan = 11

fa2/0/48 connection to 2851.  Trunk capable of carrying all vlan data with 110 being the native vlan.

[2851 Router]

Gi0/0 = /24

Gi0/1 = no ip

Gi0/1.11 = /24

Gi0/1.110 = /24

Lo0 = /32

[Comcast Gateway]

IP = /24


I have tried creating a default route on the 2851 router to the comcast gateway  I have tried pretty much everything that I know.  Please let me know if you need more details or have any recommendations.

P.S. If you have encountered this and think it would be easier for me to just get a cable modem and hook the 2851 router directly into that I would be willing to do that if this setup isn't possible.  My question then is will the 2851 put out 110mbps?  Because that is the speed I pay for now.  

5 Replies 5


do you have any statics routes define on the router for return traffic from the Comsat?



Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards

If you mean the 2851 router, then no.  I will try that and let you know.  I'm not able to modify or access any routing table on the Comcast router.


Thank you!

Also, I can ping external IP's from inside the 2851, but cannot ping external IP's from anything that is connected into the switch.  I can ping all internal IP's from the switch though.

As it stands, the only subnet that the Comcast Router knows about is /24 so this is the only one which will be able to access the Internet.

The 2851 has a default route to (the Comcast) so it will be routing packets from all its known subnets to the Comast.

However, the only subnet the Comcast knows about is /24 so it has no way to return packets for the other subnets like /24 and /24. 

Ignore the fact the Comcast is not a Cisco device, think of it like any other Router. In order to forward packets to a destination, it needs to have a route to that destination in its routing table. If traffic is going Towards the internet, it will usually be ok with a default route to an upstream device but the return traffic needs a routing table entry for each destination.

So lets assume you are trying to ping from a PC with an IP address of /24. The first ICMP packet will be forwarded to the devices default gateway which is (Gi0/1.110 on the 2851). The 2851 does not have in its routing table (obviously) so it forwards the packet to its default route ( The Comcast performs NAT so the source IP becomes the Public IP address on the Comcast. However the Comcast does not have an entry for 10.25.110.x in its routing table so has no way of returning traffic.

Hence your issue.

Hopefully that makes sense!

Effectively you need to ensure the Comcast has static routes for all your LAN subnets OR configure NAT on the 2851 so the Comcast only sees one Private address. 

Since it's directly connected wouldn't that cover the return traffic? 

This is from the 2851 router.  The IP of the Comcast gateway is /24.  The interface connecting to the gateway is /24.

Here's what I have:

S* [1/0] via is variably subnetted, 7 subnets, 2 masks
C is directly connected, Loopback0
C is directly connected, GigabitEthernet0/1.11
L is directly connected, GigabitEthernet0/1.11
C is directly connected, GigabitEthernet0/0
L is directly connected, GigabitEthernet0/0
C is directly connected, GigabitEthernet0/1.110
L is directly connected, GigabitEthernet0/1.110

Review Cisco Networking for a $25 gift card