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How do I block DHCP requests and replies from different segments of my network?

I have a wide area network with five T1 feeds coming in from different entry points. I have all manageable switches and would like to block DHCP requests and replies from different segments of my network. I blocked the DHCP server's mac address from the switch using the Unicast filter, but it still blows right through. Any ideas on how to block just DHCP requests and replies to force the clients to use the closer IP sources?

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DHCP request are broadcast, so blocking DHCP's mac address will not help. Instead you need to block port 67 for UDP so that DHCP requests are blocked. Also use IP helper address <server ip address> to force the clients to the nearest server.

This was first published in August 2004

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