To configure exponential backoffs of RADIUS retransmits over an extended period of time on a global basis and per RADIUS host, perform the following steps:

To configure exponential backoffs of RADIUS retransmits over an extended period of time per RADIUS server group, perform the following steps:

To verify feature functionality, use any of the following EXEC commands:

The following example shows how to configure your router for exponential backoff retransmit of accounting requests. In this example, an exponential backoff is configured globally (via the radius-server backoff exponential command) and for the RADIUS server host "172.107.164.206" (via the radius-server host command).

aaa new-model
aaa authentication login default group radius
aaa authentication ppp default group radius
aaa authorization exec default group radius 
aaa authorization network default group radius 
aaa accounting send stop-record authentication failure 
aaa accounting update periodic 1
aaa accounting network default start-stop group radius
!
radius-server host 172.107.164.206 auth-port 1645 acct-port 1646 backoff exponential max-delay 60 backoff-retry 32 
radius-server backoff exponential max-delay 60 backoff-retry 32 
radius-server retransmit 3
radius-server key rad123
end

The following example shows how to enable exponential backoff retransmits on a per-server basis. In this example, assume that the retransmit is configured for 3 retries and the timeout is configured for 5 seconds; that is, the RADIUS request will be transmitted 3 times with a delay of 5 seconds. Thereafter, the router will continue to retransmit RADIUS requests with a delayed interval that doubles each time until 32 retries have been achieved. The router will stop doubling the retransmit intervals after the interval surpasses the configured 60 minutes; it will transmit every 60 minutes.

radius-server host foo.xyz.com backoff exponential max-delay 60 backoff-retry 32

After enabling this command, the retransmits will be sent as follows ("t" equals seconds):

t = 0 req sent 
t = 5 retrans 1 
t = 10 retrans 2 
t = 15 retrans 3 
t = 25 retrans 4 
t = 45 retrans 5 
t = 85 retrans 6 
t = 165 retrans 7 
t = 325 retrans 8 
t = 645 retrans 9 
t = 1285 retrans 10 
t= 2565 retrans 11 
t = 5125 retrans 12 
t = 8725 retrans 13 (The interval has stabilized to 60 minutes here). 
t = 12325 retrans 14 till retransmit 35 

After all the retransmits are sent, the RADIUS request follows the same path that it would when all the normal retransmits are done.