DHCP Snooping

Turn on DHCP Snooping globally first

  • ip dhcp snooping
  • ip dhcp snooping vlan xxx

Enable the trusted interface to the DHCP server

  • int Gix/x
  • ip dhcp snooping trust

If Option 82 is enabled, the DHCP server may complain with the following message shown in DHCP server debug:

DHCPD: inconsistent relay information
DHCPD: relay information option exists, but giaddr is zero

This is where Option 82 is enabled which is normally associated with DHCP relay but the relay address field in the packet is all zeros.

It can be fixed by configuring the DHCP server (router in this case) to trust all DHCP packets with a giaddr value of zero:

  • ip dhcp relay information trust-all

In a situation where more than one switch sits between the DHCP client and DHCP server, a similar error message may be generated by switches closer to the DHCP server. This again is caused by Option 82 which can be disabled on the switches nearest the client using the following command:

  • no ip dhcp snooping information option

Source:

 

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