I have several network cards on my filer, each with its own subnet - If I am on a subnet that the cards are not defined for, how does the filer know which subnet/card to send the information out on ?
This decision is made by the routing table ... The filer will look at the routing table to decide the first hop for the reply.
Not usually true, at least for filers. Normally, the filer simply takes the source and destination addresses (both IP and MAC) from the request and reverses them for the reply, sending the reply via the same interface on which the request was received. The only cases in which use of the routing table is required are when the filer initiates an action on its own, such as sending autosupport e-mail, an SNMP trap, a ping from the console, etc.
The routing table is built primarily in 3 ways. Static routes (like the default route and interface routes), ICMP redirects, and if used routing protocols like RIP.
Filers have a very limited implementation of RIP -- they listen to it only to determine availability of routers identified as candidates for use as defaults. A filer won't learn a new default route via RIP, nor will it pay any attention to non-default routes.
I'm not sure about ICMP redirects, but I wouldn't be surprised if they are ignored.
Finally, static routes in the normal sense aren't really supported. The static routes which are added as a side-effect of defining interfaces are there, but not an explictly defined static route via an arbitrary router. You can define a default route, but it's not quite static -- if there's also an /etc/dgateways file, the router may chance the "static" default route to something else listed in that file, depending on info gleaned from RIP.
The section on Routing in the "Network Administration" chapter of the System Administrator's Guide might be interesting supplemental reading.
-- Karl Swartz Network Appliance Engineering Work: kls@netapp.com http://www.netapp.com/ Home: kls@chicago.com http://www.chicago.com/~kls/