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Strange Problem

I have a Cisco 1811 router with 2 Ethernet Ports and 1-9 FastEhernet ports( 1-9 act like a switch).  On the Ethernet port I have trunked to my ISP for my private network. And on the FastEthernet1 is the firewall , FastEthernet3  is the Cisco Concentrator and FastEthernet6 ISP internet connection is connect. There is default vlan 1 on the FastEthernet 1-9 .  When I reboot the firewall the FastEthernet1 goes down  as per normal but the LINE PROTOCOL  where the Concentrator is connect goes down as well and does not come up until I log on to the concentrator and ping out from that port.

               ISP ROUTER-----------(FE6)CISCO ROUTER 1811-------(E1)PRIVATE NETWORK (WAN)                                                                                                                                                                                    | (FE1)          | (FE3)

                                                     Concentrator      Firewall                                                                                                                                                  


The Firewall has a public IP address and the Concentrator has a public IP address but they are on different subnets. The ISP router has the firewall subnet as primary and the Concentrator as secondary.

Any clues?