Im assuming that the default route on the filer is being used to send out traffic destined for 151.142.223.0/24.
Not on a good day it won't! :-) The filer's networking code should be more than sharp enough to see that it has an interface directly connected to your 151.142.223.0 network and route packets targeted to this network to said interface accordingly. In fact, a "netstat -r" on the console should show you an explicit zero-metric route for each network, directing traffic to the interface on which that network is to be found (look for gateway targets that look like "link#1", "link#2" in the "Gateway" column).
What should I do to make the e1a interface active to the 151.142.223.0/24 network?
I read your e-mail twice and can't spot a configuration mistake. Everything looked fine in fact. Is it possible that there is a more fundamental problem? A bad cable connecting the filer to the switch? A VLAN configuration within the switch that is preventing the filer's port from communicating with the Sun ports?
A complete examination of the routing tables on filer, Suns and Cisco might reveal something peculiar. As an act of desperation, bringing the filer up without the FDDI interface configured (such that it is no more than a single Ethernet interface system connected via a switch to three other single Ethernet interface systems) might also be an experiment worthy of a shot.
Keith