LTM Act/Stdby config

Hi Gurus

 

I am a newbie to F5 world. I have pair of F5 LTMs and I would like to have Act/Stdby config between these LTMs. Active LTM will be in Primary DC and Stdby will be in Secondary DC. Could you please let me know what would be basic criteria to span the cluster across the DC via DWDM link? like..

 

1. Max allowed distance between the two box to establish the HA
2. can this HA happen on routed link or do they need spanned L2 link across DC.

Thanks Mahendra