I found this article stated that I can use the 2nd NIC which not required to connect to LAN.
let say I have bge0 for my ext_if and bge1 for my int_if
bge1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=9b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>...