When I'm planning an 820C 1+1 link, why doesn't the N000082L027, PTP 820C Act.Key - 2nd Core Activation, show up in the BOM?
The 820's 1+1 feature provides hardware redundancy. When hardware at one end of the link fails, the other hardware at that end takes over, and the link stays up until the failed hardware can be replaced.
When you plan a 1+1 S, C, or G link, LINKPlanner puts into the BOM two S, two C, or two RFU-C radios, respectively, for each end of the link.
(To see this, plan a link in LINKPlanner and count the number of S, C, or RFU-C radios appear in the BOM.)
Even though the C has two radios, there is no 1+1 functionality in the C to enable the second core to take over when the first core has failed.
You could use the 820C's 2+0 functionality (with MC-ABC, internal LAG, or external LAG) to provide redundancy--if you "undersubscribed" a 2+0 link, and one of the radios failed, the link would continue to carry traffic.
However, to fix the link, you would have to take it down completely while you replaced the failed radio.
If you used a 1+1 solution, you wouldn't have to take the link down completely while you replace the failed radio. (You remove the failed radio from the coupler and replace it with a good radio.)
When you plan an 820C 1+1 link, you're really planning a 1+1 link that can be remotely upgrade to a 2+2 link later. (At that time, you will need the appropriate activation keys to enable the second core at each end of the link, enable XPIC (if needed), additional GigE port, etc.)
Remote upgrade without a tower climb is one of the features that makes the 820 compelling.
I hope this helps!
Dave