You do this with pools and priority groups.
For example:
ltm pool some-https-pool {
members {
10.1.1.1:443 {
address 10.1.1.1
session monitor-enabled
state up
}
10.2.2.2:443 {
address 10.2.2.2
priority-group 100
session monitor-enabled
state up
}
}
min-active-members 1
monitor some-https-monitor
service-down-action reselect
}
In the above case, traffic won't go to 10.2.2.2 unless 10.1.1.1 is unavailable; the equivalent of how backup vservers work (but it's much more readable, IMNSHO). You can have as many members in any given priority group as you require, and have as many priority groups as you want (in case you have, say, primary, backup, tertiary). Highest priority wins.
So if you have 200, 100, 0 (3 priority groups) with min-active-members 1, traffic will go to group 200 until no members are available, then will send traffic to 100, until no members are available, then to 0.