I hope you can help, I have 2 data centers connected via a L3 10gb (dark fiber) now I have a few more fiber strands available between the 2 data centers; so for Disaster Recovery and server clustering (requiring same subnet) does it make sense to extend certain vlans across using these extra fiber strands or is it best practice to keep the layer 3 separation, thanks in advance!
It does make sense in terms of clustering. Not sure exactly what you mean in terms of disaster recovery, that really depends on your topology/addressing.
Basically i would route where you can and extend L2 when you have to. Be aware you are extending L2 between data centres and tha brings STP issues. Obvioulsy make sure you only allow the vlans you need on this link and route all else.
There are other ways to extend a L2 vlan across a L3 link - L2TPv3 springs to mind.