DHCP Snooping

Hello Manuel

Rene describes what option 82 does in this post very well.

As to why you should disable option 82, when DHCP snooping is enabled on a switch, option 82 is enabled by default. That is, any DHCP request packets that are sent from a host will enter the switch and will have option 82 information ADDED to the request before it is sent on to the DHCP server. Specifically, enabling DHCP snooping on the switch adds the giaddr value of in the DHCP packet. However, the DHCP server is expecting this field to be set to that of the relay agent (a non zero value), since option 82 is used for DHCP relay. (If you’re not sure what I mean, review the post I linked to above).

The problem is that there are some DHCP servers (including Cisco IOS DHCP servers) that will reject any DHCP packet that retains the giaddr value of in the DHCP packet.

So this is why you should use the no ip dhcp-snooping information option described by Rene. This command tells the switch NOT to add option 82 and thus, any DHCP server receiving the DHCP request will process it normally. Another solution would be to use the commandip dhcp relay information trusted on the interface of the DHCP server which will tell it to trust any relay information, that is option 82 information, provided by the packet.

I hope this has been helpful!


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