Introduction
This document describes the configuration of Call Admission Control (CAC) for Point-to-Point Protocol (PPP)/Virtual Private Dial-up Network (VPDN) and the algorithm used by IOS to calculate the maximum value after which the router starts to drop the incoming session requests. CAC is a deterministic and informed decision that is made before a network session is established and is based on whether the required network resources are available to provide suitable service for a new session. The CAC function can be used as a best practice, especially on a router that terminates multiple sites.
Prerequisites
Requirements
Cisco recommends that you have knowledge of PPP/VPDN session establishment, that is the control plane packets exchanged to form a PPP/VPDN session.
Components Used
This document is not restricted to specific software and hardware versions.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Configure
Call Admission control is a method to restrict the number of control plane packets a router responds to, in a given timeframe. The restriction can be based on CPU utilization, Total session limit or Calls Per Second.
To enable CAC, Call admission new-model command must always be configured.
CPU Utilization
In order to prevent high CPU load conditions, the router stops accepting new sessions when the router CPU utilization is above a specific value (in %).
This is set with Call admission cpu-limit 80 command.
In this example, the cpu-limit of 80 means incoming calls will be dropped when the measured 5-second CPU utilization is 80% or higher which can be determined by the show process cpu or show process cpu sorted command.
Session Limit
In order to set a cap for the maximum number of PPP/VPDN sessions that can be established with the router, you define the call admission session limit on the router, this can be set using call admission session limit 10000 command.
After the numbers of PPP/VPDN sessions reach 10000, the incoming session requests will be dropped until the number of sessions reduces below 10000.
Session Charge Limit
In order to set a cap for the number of session requests the router should respond to (per second) from various clients, the session charge is defined on the router through call admission control. The session charge limit is set by using call admission limit 1000 command.
Along with the session charge limit, the session charge per time-frame (example: per second) for PPP/VPDN is also define. You might also define the session charge per time-frame for both PPP and VPDN simultaneously. The command used to define the session charge per time-frame is :
call admission pppoe 10 1
call admission vpdn 10 1
These values are used to calculate the call admission control attribute Calls Per Second.
Calls Per Second Calculation
The formula takes these parameters:
call admission limit <A>
call admission pppoe/pppoa/vpdn <B> <C>
A: Total session charge the router will accept before dropping incoming control packets of PPPoE/PPPoA/VPDN.
B: session charge per time-frame (example in 1 second)
C: life-time Charge.
The Call rate is equal to: [ <A> / {<B> * ( <C> + 1)}]
The "+1" is automatically added by the ASR1k to calculate the call rate. Hence if <C> = 1, then ASR will add 1 to <C>, making it 2.
For example:
call admission limit 1000
call admission pppoe 10 1
CPS = [1000/ {10 * (1 + 1)}] = 50 CPS
Related Outputs
"Show call admission statistics detailed"
Important values to be checked:
- Total calls rejected
- Total calls accepted
- Current hardware CAC status is
Router#show call admission statistics detailed
CAC New Model (SRSM) is ACTIVE
CAC statistics duration: 4294967(seconds) Total duration for which the CAC is Active
Total calls rejected 11388090, accepted 877611899 Total number of rejected and accepted calls
Current hardware CAC status is: Not Dropping Indicates if the CAC is dropping calls
Total call Session charges: 350, limit 1000 Current Session Charge and Limit set by CAC, If session charge is greater than Limit, CAC status will be dropping
CPU utilization: Five Sec Average CPU Load, Current actual CPU: 22%, Limit: 70%
CAC Events:
Reject reason Times of activation Duration of activation (secs) Rejected calls
CPU-limit: 2027 687 1927 Drops due to CPU utilization
SessionCharges: 11386163 17488881 11386163 Drops due to admission limit command
LowPlatformResource: 0 0 0 Drops due to low resources
Session Limit: 0 0 0 Drops due to session-limit command
Total dropped FSOL packets at data plane: 847838073
IOSD_CPU_OVERLIMIT_DROPS: 239184
CPS_OVERLIMIT_DROPS: 847598889
The command to show a brief of this command is show call admission statistics. Example of the command:
Router#show call admission statistics
CAC New Model (SRSM) is ACTIVE
CAC statistics duration: 4294967(seconds)
Total calls rejected 13798084, accepted 863223739
Current hardware CAC status is: Dropping
Recommended values for ASR1000
RP1:
call admission new-model
call admission limit 600
call admission cpu-limit 65
call admission pppoe 10 1
call admission pppoa 10 1
call admission vpdn 10 1
Calls Per Second (CPS) = 30CPS
RP2:
call admission new-model
call admission limit 1000
call admission cpu-limit 80
call admission pppoe 10 1
call admission pppoa 10 1
call admission vpdn 10 1
Calls Per Second (CPS) = 50CPS
Verify
There is currently no verification procedure available for this configuration.
Troubleshoot
There is currently no specific troubleshooting information available for this configuration.
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