A pool using Priority Groups should do the job, like Surgeon said.
- Enable Priority Groups in the pool by setting the Priority Group Activation to be Less than 1 Available Member(s).
- Under the Pool Member for group A, set the Priorty Group to be 20
- Under the Pool Member for group B, set the Priorty Group to be 10
Example of what your pool should look like:
pool my_pool {
lb_mode fastest
min active members 1
member ServerA:82801 priority 20
member ServerB:82801 priority 20
member ServerA:83801 priority 10
member ServerB:83801 priority 10
}
Now if both pool members ServerA and ServerB on port 82301 as disabled or the monitors fail, all traffic will be load balanced to ServerA and ServerB on port 83801.