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[Video] Cisco IP DNS Configuration Command and How it Can Improve Your Performance

Posted by Tony Fortunato on 3/12/15 7:00 AM

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TonysVideoI love testing out configuration myths. 

OK, to be fair they are  probably not myths - but many of these suggestions work in very specific situations. I try to caution my friends and clients that blindly following a configuration suggestion can lead to more problems. In other words, your mileage will vary.

When I see a tip or trick that will improve performance, I try to understand what the issue was for the author, what impact the change had and what was their testing methodology.

Ideally I would like to reproduce the test and results to prove that this either worked or that this change made no difference. The last scenario would be if this caused more issues and to back out, but that should be an obvious conclusion and procedure.

In this example I remember reading about Cisco’s ip dns configuration command which turns your Cisco router into a DNS proxy or sorts. This is helpful if you do not have a DNS server locally and if your DNS lookups take a considerable amount of time (i.e. 30ms and higher).


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Here’s how a sample configuration looks:

Router1# configure terminal
Router1(config)# ip dns server
Router1(config)# ip domain-lookup
Router1(config)# ip name-server

Now the fun part: I captured packets of the following scenarios:

  1. Using my router as a DNS server without this command to see what error message – if any that I would receive
  2. Using my router as a DNS server with the ip dns command to measure my DNS response time which should be the same as before
  3. Using my router as a DNS server with the ip dns command to measure my DNS response time which should be quicker now that the DNS name was already in cache

From my Windows computer, I executed the following commands:

  • ipcondig /flushdns – to clear my DNS cache
  • ping gogole.com – forces a DNS lookup and then pings the host

Watch the video here:




Tony Fortunato, CCNA, CFNI, CWI
Sr Network Performance Specialist
The Technology Firm
Getting things to work better - bit by bit-
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