LX4 and LRM optics are not compatible

Here is something I ran into and is certainly worth noting down and remembering:

LX4 and LRM optics are not compatible

This perhaps should be obvious as they are different standards but it wasn't until I looked into it and now I know for certain.

LX4 is standard IEEE 802.3ae and LRM is standard IEEE 802.3aq, you can look into these further on Wikipedia or another resource but the gist of it seems to be that the standard work in different ways making them incompatible. There is a good support forum link in the resources below.

This all came around because a customer wanted to buy more X2-10GB-LX4 optics but these are EOS, the recommended replacement is the X2-10GB-LRM optic which is incompatible. The options here then are to upgrade the existing line cards and replace the old LX4 optics with LRM optics or try to purchase any existing LX4 optics which may be in stock somewhere, but this is only a temporary fix as Cisco are not making them any more.

So a work of caution, don't take the Cisco recommended replacements completely at face value.

As a side note, optics of different form factors are compatible, so a X2 LRM and Xenpak LRM module would be fine, in the same way a LC SFP module can connect to a SC GBIC module if they are both using the SX standard.

Resources
Cisco Support Forum post:
https://supportforums.cisco.com/discussion/11270096/x2-10gb-lx4-sfp-10g-lrm-compatibility