Disable option 82 impacts

[prasad salvi]

We have an issue where we've enabled DHCP snooping for vlan 20,30,40,50 but out of which only vlan 50 is not working(clients are not getting IPs)

Vlan 50 = DHCP server is on directly connected router...rest are with local DHCP servers (working properly)
What might be the issue here?

[marce1000]

 - Try to disable the DHCP snooping (everywhere , if needed) ; if it then works there must have been a configuration error somewhere , 

 M.

[Reza Sharifi]

Can you post DHCP config from the router and the switch?

HTH

[Flavio Miranda]

@prasad salvi 

 Do you have interface vlan on the switch?  If does, vlan 50 have ip helper-address?

If server is connected to the router, you probably need routing and ip helper-adddress to make the request get to the server.

[prasad salvi]

I have L2 vlan on switch and L3 vlan on router..and DHCP scope is configured on router itself no other DHCP server as such for this vlan 50

[Cristian Matei]

Hi,

   When you enable DHCP snooping, switch will insert option 82 information within DHCP client Discovery packet; when this is received by DHCP server (your case), or DHCP relay agent, per the RFC, packet is dropped because it seems that packet was relayed but GIADDR value is still zero, which is not possible per DHCP RFC, as at that point in time, there was no snooping functionality, so no layer 2 relay functionality.

   You have two fixes for this: disable insertion of option 82 on the switch via global command "no ip dhcp snooping information option" or allow DHCP server/relay on the directly connected broadcast domain to allow receipt of DHCP Discovery messages with relay option 82 via interface level command "ip dhcp relay information trusted". In both cases, switch port facing DHCP server needs to be configured as trusted via interface level command "ip dhcp snooping trust", as switch only forwards DHCP client message out on trusted ports, where it's expected to have DHCP servers.

Best,

Cristian.

[MHM Cisco World]

You run dhcp snooping and dhcp for vlan50 is in router direct connect to SW.

Then there are two things to check

1- the port connect SW to router must config as trust 

If above not work 

2- Disable op82 in SW 

MHM