We are upgrading our main router from a 3725 to a 3825. We've added a GBE HWIC to have 2 inside connections to our core switch stack (2 x 3750G). We are currently running 'router on a stick' with 2 vlan subinterfaces on the 3725 - one for voice and one for data.
In moving to 2 inside interfaces, I'm not sure how to configure the redundant ports.
Do they need to be set up as etherchannel?
Do vlan subinterfaces need to go on both inside ports?
"it's my understanding that with 2 connections from the 3750's to the 3825, most routing protocols will automatically load balance connections with the same administrative distance, is that correct?"
Yes, the 3750's will see 2 equal cost paths to all destinations beyond the 3825 router and will use both paths, and the 3825 will see 2 equal cost paths to all the vlans on the 3750's and will use both.
I recommend using a dynamic routing protocol if possible but it does depend on your feature set on the 3750's. If you only have the IP Base image then EIGRP/OSFP are not supported but you can just add 2 static routes on each device and you will still use both links.