вообще, кого волнует какой там IP адрес у гейтвея? достаточно узнать его MAC ;-)
gw(config)#interface FastEthernet0/0.2
gw(config-subif)#ip address 10.0.0.1 255.255.255.252 sec
gw(config-subif)#exi
gw(config)#arp 10.0.0.2 0011.0a5f.5f5f arpa
gw(config)#ip route 8.8.8.0 255.255.255.0 10.0.0.2
#ping 8.8.8.8 source fastEthernet 0/0.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
Packet sent with a source address of 85.23.126.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 140/140/140 ms