i was wondering if my scenario problem is due to gns3 bug(version 2.2.5) or i am missing something in my configs...if i want to ask briefly: can i have dhcp snooping enabled on L3switch which is dhcp relay agent also(i know it is not suitable to enable dhcp snooping on distribution or core layer but i am trying to test some stuff in my LAB) cause when i enable snooping feature the MLS wont forward dhcp packets to legitimate dhcp server,and by disabling(only on MLS and leave it enabled on 2 access layer switches) it everything works fine!!
so here is my scenario in detailed information:
so my MLS is VTP server containing vlans10,100 and uplinks are 802.1Q Trunk ...MLS is also doing the Intervlan routing with
2 SVI (interface vlan 10 & interface vlan 100) and there is a default route for DHCP Server to the interface vlan 100 on MLS...i have created suitable dhcp pool for vlan10 clients on DHCP server and finally i defined helper-address for interface vlan 10 pointing to DHCP Server IP address... at this moment everything works fine and client can recieve ip address(and other stuff like default-router,dns ,etc...) by dhcp server which make sense!!
I did not enable snooping feature yet ...so in order to do this:
ip dhcp snooping
ip dhcp snooping vlan 10,100
i entered this 2 commands on each of access layer switches and MLS ...now as you know it`s time to make interfaces Trusted according to the scenario:
ALS1 --->interface gig 0/1 : ip dhcp snooping trust
ALS 2 --->interface gig 0/0 : ip dhcp snooping trust
MLS --->interface gig0/0 : ip dhcp snooping trust
BUT we are not done yet!!! now we should handle Option 82 :
i tried 2 different ways none of them works:
1-disabling option 82 on every switch(including MLS) by using:
no ip dhcp snooping information option
2-MLS --> ip dhcp snooping information option allow-untrusted and also
ALS1--> ip dhcp snooping information option allow-untrusted
none of these worked ! so i decided to make all my interfaces Trusted for dhcp snooping except the one which is connected
to the dhcp client and unfortunately it didn`t worked neither.
i can not figure it out ...why it is not working ...the thing that i understood by capturing traffic is that by enabling dhcp snooping on MLS it will not unicast the packets to dhcp server using ip-helper(and if i enable dhcp snooping on ALS1 and ALS2 and disable it on MLS everything works fine as i mentioned earlier)
