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Haukur,
Thanks very much ! That was it…try to find that using debugging…this saved my day. Indeed, it appears from the moment that you enable snooping on the L2 Access level, this switch inserts Option82 into the DHCP packet. When the L3 switch sees this DHCP broadcast WITH Option82 inserted, he doesn’t trust it (even if it is received on a “dhcp trusted” interface) and drops the packet. You need to expliciclt put “ip dhcp relay information trusted” on the interface, or globally “ip dhcp relay information trust-all” on the L3 switch.
Man, Cisco documentation isn’t very “complete” on this point. (it has documentation,but it is scattered on several places i have noticed).
regards, Geert CCIE#17329
________________________________________ From: Haukur ^srparson [haukur@sensa.is] Sent: 23 March 2008 23:00 To: Geert Nijs; ccielab@groupstudy.com Subject: RE: DHCP snooping and ip helper interaction
Hi Geert, If you add the “ip dhcp relay information trusted” on the interface on the distribution switch the DHCP service should start working again, this command allows the L3 interface to forward packets that include DHCP Option 82 informations.
R. Haukur CCIE#18026
—–Original Message—– From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Geert Nijs Sent: 22. mars 2008 15:33 To: ccielab@groupstudy.com Subject: DHCP snooping and ip helper interaction
All,
Does a DHCP snooping configuration NEED an ip helper address defined on the same switch ?
In my lab setup, DHCP breaks when i configure dhcp snooping without ip helper address. Setup:
Access = C3750 , 12.2(25)SE2 Distribution= C6500, sup720, 12.2(18)SXF8
ACCESS int vlan 100 ip address 1.1.1.250 255.255.255.0