Introduction of Bogus DHCP server to Network

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Sep 20th, 2007
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We run an all cisco network wth a 6500 core switch and 3500 access layer switches.

In one instance a customer had a VOIP gateway device plugged in improperly and as a result all clients on that specific VLAN looses their regularly assigned IP address and picks up on from the VOIP Gateway which is setup to be a DHCP server.

Is there anything that can be doneto prevent unauthorized DHCP servers from causing outages such as these?

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Martin Parry Thu, 09/20/2007 - 07:59
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The newer switches have a feature called dhcp snooping which prevents unauthorised DHCP servers taking out your network. Unfortunately I don't believe that your legacy 3500 switches will have support for this feature.

Details on dhcp snooping can be found here:

paul.matthews Thu, 09/20/2007 - 08:05
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TBH I don't think there is much you can do from where you are. As mentioned, DHCP snooping may help, but another aspect to consider - How big are your subnets? Smaller subnets would mean fewer users would be affected by an incident like this.

rossdmcco Thu, 09/20/2007 - 08:46
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Thanks to both of you for your responses.

I realize this may be the case however I wanted to be certain.

I will read up on DHCP snooping and find out what I would need to put in place to mitigate incidences as these.

Thanks again.


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