Solaris

If I have 2 interfaces in a server on the same subnet/network does Solaris automatically choose to route packets destined for this network out the first interface. I.e if ce0 and ce1 were on same network ce0 would be chosen as it's first interface?

Are you trying to make a bridge - machine that connects to two subnets?

This is unsupported. Each active physical interface must be bound to a distinct network (more precisely, to different link-layer broadcast domains). Should you want to have more than one interface on the same network for redundancy and/or load balancing reasons, you need to configure them to use IPMP.

If I use an IPMP group and place both interfaces in it, will all outbound traffic be spread over the 2 interfaces in a round robin fashion?

Also regarding the routing, if I have ce0 and ce1 in an ipmp group on one network, and another network on ce3, will the box know to route packets for the network on ce0/1 out the right interface without having to specify multiple gateways?

It sounds like you might want to setup link aggregation (ce0 and ce1) will be bound as aggr0. Look up the command dladm. If you choose to setup link aggregation, the corresponding ports on the switch will have to be configured as well.

Yes, each network needs it's own default gateway, look at /etc/gateways.

Overall outgoing traffic will be spread over both interfaces but not necessarily on a round-robin fashion. Especially if they have different speed.
I think so as you do not need to specify the interface to use when configuring either the default or a specific gateway.

---------- Post updated at 11:27 ---------- Previous update was at 11:25 ----------

A bridge works at the link layer, not at the IP one. What you describe is a router / gateway.