Name resolution problems ever since cisco router install

Posted on 2004-08-11
Last Modified: 2010-04-14
I have had internal and external name resolution problems ever since deploying 2 cisco routers (2501s) in my LAN consisting mostly of windows boxes..  Schematic here:

All my hosts (on the network and network) can ping each other fine. They can also ping any WAN address fine (so long as you use the IP, not the host name of the WAN address).  All my hosts can ping my gateway fine as well (cable router).  

However, when it comes to pinging my ISP's DNS server.....none of my hosts can ping it.  You are only able to ping my ISP's DNS server if you are connected directly to my cable router.

Any idea as to why local name resolution is not occuring? I know netbios broadcasts to resolve host names locally. Why isnt it working?  I know routers dont forward broadcasts by default, but that shouldnt make a different for the hosts all on the same subnet?


Question by:dissolved
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LVL 14

Expert Comment

ID: 11777202
The link to your network diagram isn't working.

Please post the output of "ipconfig /all" and "route print" from a PC that is having these problems. Also post the IP address(es) of your ISP's DNS server and your local DNS server (I assume this IP will be in your network.jpg once the link is fixed).

Author Comment

ID: 11777392
Sorry here is the address of the schematic
I will get the route print to you soon, as I am at work.

My ISPs dns server is
My internal dns server is

I have all my hosts using either the 2501a cisco router as their gateway. Or the 2501b router as their gateway. When the clients plug directly into the cable router, everything works fine.
LVL 14

Expert Comment

ID: 11777727
I will be able to troubleshoot a bit more when I see the "ipconfig /all" and "route print" output... It might actually be helpful to get that output from a client machine on each subnet, 192.168.1. and 192.168.2.

However, based on your network diagram, I have some concerns right off the bat:

1 - Why so many routers? A single Cisco router can easily act as a gateway between a WAN (internet) link and 2 or more private networks.
2 - Why include a DSL / Cable router in this situation? Those are intended for home users who do not have a more expensive, complicated solution (like your Cisco 2500s) available. I can't say this for certain yet, but I think part of your problem may be due to the fact that the cable router is not managable beyond a certain degree... ie, it has no idea that 192.168.0.* and 192.168.2.* are valid networks on its INTERNAL ports. Traffic bound for those addresses in some situations may be send back out through its default interface (the port connected to your cable modem and ISP).

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Author Comment

ID: 11778360
This is my home LAN, just testing things out trying to learn Cisco.  
I have a static route placed in my cable router, so it knows how to find the network. When this wasnt in there, the network couldnt ping WAN

I just added an internal DNS server and DHCP server ( It's gateway is and it CAN ping the WAN. All of my clients can now resolve names locally and remotely. However, I still cannot ping my ISP's DNS server ( when I use the ciscos as the gateway. Pinging everything else on the WAN works.

Here is ipconfig all of the  client:

Windows IP Configuration

        Host Name . . . . . . . . . . . . : dissolved

        Primary Dns Suffix  . . . . . . . :

        Node Type . . . . . . . . . . . . : Unknown

        IP Routing Enabled. . . . . . . . : No

        WINS Proxy Enabled. . . . . . . . : No

Ethernet adapter Local Area Connection 4:

        Connection-specific DNS Suffix  . :

        Description . . . . . . . . . . . : 3Com EtherLink XL 10/100 PCI For Complete PC Management NIC (3C905C-TX)

        Physical Address. . . . . . . . . : 00-04-75-86-10-B3

        Dhcp Enabled. . . . . . . . . . . : Yes

        Autoconfiguration Enabled . . . . : Yes

        IP Address. . . . . . . . . . . . :

        Subnet Mask . . . . . . . . . . . :

        Default Gateway . . . . . . . . . :

        DHCP Server . . . . . . . . . . . :

        DNS Servers . . . . . . . . . . . :

        Lease Obtained. . . . . . . . . . : Wednesday, August 11, 2004 4:54:05 PM

        Lease Expires . . . . . . . . . . : Monday, May 07, 2007 4:54:05 PM

Here is the route

Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 04 75 86 10 b3 ...... 3Com EtherLink XL 10/100 PCI For Complete PC Management NIC (3C905C-TX) - Packet Scheduler Miniport
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
        20        1        1        1        20        20        20        1        1        1        20        1
Default Gateway:
Persistent Routes:
LVL 14

Expert Comment

ID: 11778629
Hmm, your cable router must have a lot more configuration options than mine, since you were able to add that static route for :) That's a good thing in this situation, though.

With that in mind, your clients on the 192.168.1. subnet should be using as their default gateway, and the cable router will know that any traffic bound for 192.168.2.* will need to go through the router. That's a small design change, really. You'll notice how your local routing table has several entries for public Internet IPs, and the gateway listed for those IPs is - this happens because the OS / IP drivers are smart enough to figure out at some point that is not the router that should be handling those packets, and it dynamically adjusts your local routing table so future traffic to those IPs will take the most efficient route.
I don't know a great deal about how these communication protocols work, just the concepts and the result.

I am a bit stumped as to why you can't ping the provider's DNS server though, when all you changed was the gateway of your clients. Your name resolution is probably working correctly only due to the fact that your Server is configured with the gateway, and all forwarded DNS queries pass through the server (not directly from the client workstations).
LVL 14

Accepted Solution

dlwyatt82 earned 125 total points
ID: 11778654
One thing to note - we have no idea exactly where the ping is failing... your packets may have travelled all the way to the provider's DNS server, it may have sent a reply, and somewhere on the return trip, something could have gone wrong with the routing. I don't see anything in your setup that would cause that to happen (perhaps someone else will notice an error),. Normally I would run a packet sniffer to see exactly what's happening, but I doubt you'll be able to sniff your ISP's network beyond your cable modem.

Author Comment

ID: 11778689
Found the problem. Router issue.
Thanks man!

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