yes, the backup option exist with the CSM
If you server1 active and server2 backup, here is an example
gdufour-cat6k-2(config-module-csm)#vserver www
gdufour-cat6k-2(config-slb-vserver)#serverfarm linux1 backup ?
WORD backup server farm name
gdufour-cat6k-2(config-slb-vserver)#serverfarm linux1 backup linux2
Gilles.