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Router causing DNS Issues?

I have a customer that is running a Cisco 871 router.  This has been in place for abotu 6 months.  Last week they started to have some Internet problems.  I have narrowws the problem to a DNS issues, SSH, RDP, etc work while browsing does not.  Browsing goes down for a few minutes at a time multiple times a day.  I have changed DNS servers, and we still have the same issue.  The ISP says its the Cisco, and that is really the only thing I have not removed from the loop. Any ideas?
1 Solution
Unless you have applied any upgrades or patches to the cisco around the same time this started happening I don't see how this could be your router.  

When the browsing goes down you need to try pinging your dns server and check the response times.  You can also run a tracert to your dns server and make sure there aren't too many hops between you and the dns server.  You want your DNS server to be as few as possible hops away for better response.

I have had customers with a virus or worm on just one workstations also cause the same problem.  The worm would intermittently kick in causing the delay in the browsing.  

Can you post Cisco config and IOS version here please. Do you use provider's DNS or internal one? Can you try to use different DNS server?
i had a similar issue with dns stopping browsing, except im running a pix.  what the problem was though is that the pix couldnt handle a larger dns request that came through.  since then we have switched to an asa appliance which shows the correction in the config like this:
 inspect dns maximum-length 4096

standard size was 2000-something by default.  perhaps your router needs a similar line.
rshooper76Author Commented:
The ISP found a problem with their DSLAM.

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