HP 2510 radius server fallback / shift time

[Asim_Ali]

Hello

Facing some issue here need help in HP 2510 switches with radius server.

Scenario

have HP 2510 switches

have 2 radius servers (Microsoft NAP)

have defined 

aaa authentication port-access eap-radius
radius-server host <192.168.1.xxx> key abc123
radius-server host <192.168.1.yyy> key abc123

Problem

Want users to shift from radius server 1 to radius server 2. But

When shutdown radius server 1 machine, then users / clients are not getting authenticated, they are not getting proper IP address / network conectivity. 

Tried

Removed radius-server host <192.168.1.xxx> key abc123 entry then clients get authenticated by radius server 2.

What can be the issue here ?

is that fallback / shifting time ? what is that time to shift when one radius server is not available.

Read HP Access security Guide, chapter # 5, 

ftp://ftp.hp.com/pub/networking/software/2510-Security-July2007-59914763.pdf

page 5-19

If access to a RADIUS server fails during a session, but after the client has been authenticated, the switch continues to assume the server is available to receive accounting data. Thus, if server access fails during a session, it will not receive accounting data transmitted from the switch.

How switch can shift the session, when ?

Thanks