DNS config help - DHCP pool on Cisco switches

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May 31st, 2012
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     Hi ,

I need configuration help when we configure DHCP on cisco 3650 switch . We have configured DHCP pools for various VLANs on our core switch 3650, and we have two internet links connected to router which is connected to core switch.

We mentioned DNS ip details provided by ISP under DHCP pool statements on core switch which is working fine for ISP 1.  But we have taken second ISP link for redundancy ,  when we give DNS ip addresses provided by ISP 2 under DHCP config on core switch,  it is not working when failover occurs.

Internet is not working.  How can we configure the device in such way that , if there is failure on ISP 1 link,  DHCP settings should take DNS parameters of ISP 2.



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John Blakley Thu, 05/31/2012 - 05:24
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I'm not 100% sure that dhcp is your issue. Do you nat on these devices? When failover occurs can you ping an external address? But in answer to your original question, you'll probably need to provide DNS servers for both ISPs on the primary dhcp server. The reason for this is that if you only provide one, for ISP1, then the client will pull that address and may not have to renew the lease should the link fail. So you'd have something like:

ip dhcp pool ISP1


dns-server (1.50=ISP1 DNS and 2.50=ISP2 DNS)

The above would be handed over to the client and the client would be responsible for rolling over to the other DNS server when doing a query.




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