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DHCP Snooping and ASA

Posted on 2011-02-24
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Last Modified: 2012-05-11
Hello. We are implementing DHCP Snooping, and everything works fine except for the vlan thar are behind a Firewall. Disabling DHCP Snooping, all host on differents VLAN receive IP from DHCP Server, but with this feature enabled VLAN behind the firewall does not recieve IP from DHCP Server

Another question will be (according to the graph) wich ports must be set as "Trusted"? The por 1 in SW1 in wich the DHCP Server is connected for sure, but in SW2 for example, should be the Trunk Port1 trusted too?

SW: Catalyst 3750 (12.2(35)SE5)
FW: ASA 5520 8.0(4)

Thanks in advance and best regards

 DHCP Smooping
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Question by:gan_nazer
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Expert Comment

by:Istvan Kalmar
ID: 34976786
Hi,

YOu need to enable all vlan DHCP snnoping, and you need to enable on all DHCP servers_
so:

PORT1
PORT3
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Expert Comment

by:Istvan Kalmar
ID: 34976815
And you need to add heper address on the firewall VLAN2 interface to DHCP server!
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Author Comment

by:gan_nazer
ID: 34978959
Thanks ikalmar, I'll try "trusting" in Port3 that was the only that we didn't. What about SW2 Port1, should be trusted to for assign IPs to host in that switch?
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by:Istvan Kalmar
ID: 34979786
yep on SW1 you need to config to trusted port!
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by:gan_nazer
ID: 34981212
It doesn't work behind the firewall, even for vlan 1 when i change the SW1 port 1 (DHCP Server) as untrusted, the host does not recieve IP (I think that is ok), but VLAN 2 (behind firewall) still receiving IP, that does not make sense for me...
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Istvan Kalmar earned 500 total points
ID: 34982583
you need to configure trusted for:
sw1: Port1
sw2: Port1,Port3

Did you configured helper address n the firewall?

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Author Comment

by:gan_nazer
ID: 35001130
We will focused just in SW1 for the moment, I configured DHCP Relay in FW intrface connected to VLAN2 with the IP of DHCP Server, it works without DHCP Snooping feature enabled
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Assisted Solution

by:Istvan Kalmar
Istvan Kalmar earned 500 total points
ID: 35001240
you need to enable DHCP snooping for VLAN1 and VLAN2, and you need to give trusted port the Port1, if it isn't working you need to add Port3
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Expert Comment

by:Qlemo
ID: 35275240
This question has been classified as abandoned and is being closed as part of the Cleanup Program. See my comment at the end of the question for more details.
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Network Monitoring is essential to ensure that computer systems and network devices are running. Use PRTG to monitor LANs, servers, websites, applications and devices, bandwidth, virtual environments, remote systems, IoT, and many more. PRTG is easy to set up & use.

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