Verify that router is not allowing DHCP packets

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Feb 27th, 2008
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Hi all. I have an 1811 router that connects two dissimilar networks.

My question is, how can I verify that the 1811 is not allowing dhcp packets to pass through it? I realize that routers stop broadcast, hence dhcp, but how can I verify that if a non-cisco person wants proof that dhcp packets are being discarded at the ingress interface?

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ciscograyaw Wed, 02/27/2008 - 12:11
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DHCP operates using two mechanisms: The initial request for an address is indeed broadcast, however subsequent transactions are unicast. Short of using ip helpers to forward the broadcast requests to a DHCP server, stopping the broadcast will stop that initial discovery.

If you want to make 100% sure DHCP doesn't cross the networks (this could happen in the case a mobile user already thinks it has an address, moves to the other network, and tries to renew by unicast), a simple ACL against UDP port 67 will stop that.


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