I'm currently working on a design which contains gigabit ethernet. Due to some weird size constraints, I can't use a magjack or one of those transformer ICs, so I'm using discrete magnetics. I've managed to pick a transformer (actually three... would be nice if they stayed in stock long enough to lay out the board), but am not exactly sure how to choose a common mode choke. In this marketing copy from Bourns, the SRF2012A-801Y is mentioned as a CMC for gigabit ethernet. But I don't know why (or even if!) this is a good choice. Is it the impedance (seems kinda high to me)? DC resistance/max current? Will any old common mode choke work for gigabit ethernet, or is there something to it? What about wire wound vs multilayer chip ones like this?
If you are plannig to go a test laboratory with your finished product, I strongly suggest you to buy an off-the-shelf common mode filter suited for Gigabit ethernet.
The common mode you mentioned, Panasonic EXC14CG/EXC14CE, might not work because the datasheet doesn't say anything about Gigabit ethernet. Yet, it's too little for filtering ethernet currents.
Stick with what datasheets state. If that particular feature is not expressively mentioned in the datasheet, than that product is not meant for what you got in mind.