My clients router has the Fe0/0 configured with an ip address and 3 secondary addresses. The Fe0/0 is pluggeded into a switch that doesn't support VLANS. (Older non-cisco switch)
The 4 networks (flat on the same wire in the switches) are then moved out with static routes. Until the client gets funded and is able to purchase more modern hardware I would like to move these networks to sub-interfaces and add a routing protocol because they are having experiencing issues. (Surprise?) When I try to configure the sub-interfaces it tells me that the trunk port needs to be configured. I have the ability to migrate them to a 3560 Switch as well. Thanks in advance.
If you have 4 logical subnets in 1 flat network then secondary addressing is the only alternative that works for you if the switch does not support VLANs.
Trying to do subinterfaces on the router interface requires trunking to be enabled. And Jon is quite right that if the existing switch does not support VLANs then it would not support trunking.
If you are able to migrate to a 3560 switch (as you indicated in the original post) it would provide a solution for this situation. The 3560 does support VLANs and trunking. So you could configure the 4 VLANs on the 3560 and do inter-vlan routing on the 3560. If you wanted you could use the old switch as an access switch in a single access VLAN and connect it to an access port on the 3560. You would have a choice of how to connect the 3560 to the router. You could make the connection between the 3560 and the router a trunk and extend each VLAN to the router. Or you could make the link between the 3560 and the router a routed link with its own subnet. Of these options I would prefer the routed subnet option (I see little reason to extend the VLANs to the router if the 3560 is doing inter-vlan routing). But it would be a decision that you make as seems best to you.