Fwd: DHCP Snooping not working

 Dear Farrukh,

When configuring a Cisco L2 switch with DHCP snooping feature the problem
will be that the giadd field will be set to 0.0.0.0 as there is no
forwarding address which goes to the DHCP server. If a Cisco IOS device is
configured as a DHCP server it will by default refuse this request and not
respond. To solve this issue on the router you should configure the command
ip dhcp relay information trust all.
However if the switch is a L3 switch it shall successfully relay the
information towards a DHCP server (using the giadd of the interface vlan
which is configured for helper address) and it will unicast the packet with
iself as the source.

Hope that this help with the problem..

Best Regards,

Karim Jamali

On Mon, Jan 18, 2010 at 2:22 PM, Farrukh Haroon <farrukhharoon_at_gmail.com>wrote:

> Dear All
>
> We are facing a weird issue while trying to configure DHCP snooping.
> Users are unable to get/renew IP Addresses after enabling DHCP snooping.
> The DHCP Snooping binding table is always empty.
>
> The configuration is pretty simple
>
> ip dhcp snooping vlan 101,104
> no ip dhcp snooping information option
> ip dhcp snooping
>
> All ports connected to DHCP servers and uplinks set as trusted.
>
> Switch Version: c3560-ipservices-mz.122-35.SE5
>
> I tried the same configuration with another 3560 Switch running an older
> version with no issues at all.
>
> This is the error we see on all the trusted ports, any ideas why this is
> happenning:
>
> Dec 27 08:56:43 KSA: DHCPSNOOP(hlfm_set_if_input): Setting if_input to
> Gi0/49 fo
> r pak. Was not set
> Dec 27 08:56:43 KSA: DHCPSNOOP(hlfm_set_if_input): *Clearing if_input for
> pak. W
> as Gi0/49*
> Dec 27 08:56:43 KSA: DHCPSNOOP(hlfm_set_if_input):* Setting if_input to
> Gi0/49 fo
> r pak. Was not set*
>
> Regards
>
> Farrukh
>
>
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-- 
KJ
-- 
KJ
Blogs and organic groups at http://www.ccie.net