with the scale that the a9k supports (256k today, 512k tomorrow) having a keepalive value of 1 second will drive the cpu insane.
for safety reasons, the minimum value is set to 30 seconds.
In IOS for at that time the high scaling platforms like c10k, we always recommended interval of 60 seconds to ease the cpu.
If that takes too long for your taste you can set the retries lower, to say 3, but I wouldnt go any lower then that to prevent "false positives" and creating too much session churn.
XR 51 has the ability to set it to 10 seconds minimum but I would NOT recommend that:
RP/0/RSP0/CPU0:A9K-BNG(config)#dynamic-template type ppp TEST
<10-180> Enable keepalives with this interval (default is 60 seconds for RP S
ubscriber, 120 seconds for LC Subscriber)