Hi @BNgala
I agree with @PhoneBoy a network diagram would be very helpful.
I don't understand the question 100%.
1) Add three interfaces in GAIA
a) external (for example eth1) > internet
b) internal lan (for example eth2.123) > your network
c) management (for example eth2.10) > your management vlan
(set this interface to management)
2) On Cisco switch allow only the used vlan's on the trunk (for example vlan 10 and vlan 123)
3) Add the default route in GAIA to internet
4) In the SmartConsole
a) In the gateway object get the interface topology
b) Set IP spoofing (external to external interface (for example eth1) , internal lan (for example eth2.123) to > network topology) and management (for example eth2.10) to > network topology)
c) On the gateway objekt enable hide NAT
d) Add access rules
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