[c-nsp] dhcp snooping clarification

[William Jackson]

Hi

 

I have a dhcp snooping setup on a 3550 switch.

 

I have downstream some other access devices that insert option82
information to the DHCP requests.

 

On my switch I have:

 

ip dhcp smart-relay

ip dhcp relay information option

ip dhcp relay information policy keep

!

ip dhcp snooping information option allow-untrusted

ip dhcp snooping

 

and on the vlan interface

 

interface VlanXXX

 ip address X.X.X.X 

 ip helper-address X.Y.Z.X

 ip dhcp relay information trusted

 

My understanding is that the downstream interface has to be set as
trusted otherwise it will discard any DHCP packets arriving to it with
the option82 already set.

Due to this my command:

 

s3550 #sh ip dhcp binding 

IP address       Hardware address        Lease expiration        Type

s3550#

 

is empty even though the DHCP requests are successfully relayed through.

 

Is there a way to allow the relayed option82 packets through but to also
enforce the snooping bindings so that any traffic that hasn't done a
DHCP request ( ie they have set a manual IP ) will be discarded?

 

cheers