Just tried it, the interface never shows running:
root@obms2-com-indianapolis-in:/root# ifconfig bge1 plumb up
root@obms2-com-indianapolis-in:/root# ifconfig -a
lo0: flags=1000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
bge0: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 2
inet 172.20.10.22 netmask ffffff00 broadcast 172.20.10.255
ether 0:14:4f:56:8f:7d
bge1: flags=1000803<UP,BROADCAST,MULTICAST,IPv4> mtu 1500 index 15
inet 0.0.0.0 netmask ff000000
ether 0:14:4f:56:8f:7e
So I went ahead and tried it on another Solaris 9 box in the same cluster with an interface that is not connected to anything. The first time I 'ifconfig bge2 plumb up' it worked but then after 'ifconfig bge2 down' the interface stopped running:
root@obms3-com-indianapolis-in:/root# ifconfig bge2 plumb up
root@obms3-com-indianapolis-in:/root# ifconfig -a
lo0: flags=1000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
bge0: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 2
inet 172.20.10.23 netmask ffffff00 broadcast 172.20.10.255
ether 0:14:4f:55:a4:2d
bge1: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 3
inet 10.14.5.70 netmask ffffff00 broadcast 10.14.5.255
ether 0:14:4f:55:a4:2e
bge2: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 4
inet 0.0.0.0 netmask ff000000
ether 0:14:4f:55:a4:2f
root@obms3-com-indianapolis-in:/root# ifconfig bge2 down
root@obms3-com-indianapolis-in:/root# ifconfig -a
lo0: flags=1000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
bge0: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 2
inet 172.20.10.23 netmask ffffff00 broadcast 172.20.10.255
ether 0:14:4f:55:a4:2d
bge1: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 3
inet 10.14.5.70 netmask ffffff00 broadcast 10.14.5.255
ether 0:14:4f:55:a4:2e
bge2: flags=1000802<BROADCAST,MULTICAST,IPv4> mtu 1500 index 4
inet 0.0.0.0 netmask ff000000
ether 0:14:4f:55:a4:2f
root@obms3-com-indianapolis-in:/root# ifconfig bge2 up
root@obms3-com-indianapolis-in:/root# ifconfig -a
lo0: flags=1000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
bge0: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 2
inet 172.20.10.23 netmask ffffff00 broadcast 172.20.10.255
ether 0:14:4f:55:a4:2d
bge1: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 3
inet 10.14.5.70 netmask ffffff00 broadcast 10.14.5.255
ether 0:14:4f:55:a4:2e
bge2: flags=1000803<UP,BROADCAST,MULTICAST,IPv4> mtu 1500 index 4
inet 0.0.0.0 netmask ff000000
ether 0:14:4f:55:a4:2f
After I 'ifconfig bge2 down' on the other box I can no longer get the interface to go into a running state after unplumbing it with the origional command 'ifconfig bge2 plumb up' where it worked the first time. So now this box is in the same state as my other one. Weird huh?