On Wednesday, October 30, 2013 03:56:22 PM Yehuda Katz 
wrote:

> I know some Cisco switches have the option to block DHCP
> replies on ports not marked as trusted (DHCP Snooping).
> I have never seen one where I had access to the
> configuration and the setting was on, so I am not sure
> what to expect, but it might explain why you don't see
> the reply in a mirror.

Enabling DHCP Snooping on a Cisco switch is very simple:

        # conf t
        #  ip dhcp snooping vlan Y-Z
        #!
        #  ip dhcp snooping
        #!
        #  interface GigabitEthernet0/40
        #   ip dhcp snooping trust
        #!

A violate would be logged like so:

%DHCP_SNOOPING-5-DHCP_SNOOPING_MATCH_MAC_FAIL: DHCP_SNOOPING 
drop message because the chaddr doesn't match source mac, 
message type: DHCPDISCOVER, chaddr: <mac.ad.d.re.ss>, MAC 
sa: <mac.ad.dre.ss>

If this helps...

Cheers,

Mark.

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