Call Admission Control

Call Admission Control (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 quality of service (QoS) for the new session.

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Overview of Call Admission Control for IP Sessions

In Service Provider WiFi deployments, the mobility of subscribers contributes to more dynamics when compared to wireline broadband deployments. This mobility translates into a high-average session churn rate and potential session count peaks during busy periods. For the Intelligent Wireless Access Gateway (iWAG) to function effectively under such conditions, the Call Admission Control (CAC) feature must be configured to help control IPoE session establishment.


Note

In this document, a Forwarding Processor (FP) corresponds to the Embedded Services Processor (ESP) component on the ASR 1000 Series Aggregation Services Routers.

The following CAC features are implemented from Cisco IOS XE Release 3.11S onwards:

  • A threshold is set for the maximum number of authenticated subscribers on the router, including the Point-to-Point Protocol over Ethernet (PPPoE) and IP over Ethernet (IPoE) sessions, but not the walk-by sessions.

  • To prevent low-memory conditions, the router stops accepting a session when the committed memory from a route processor (RP) or a FP is above a specific percentage value. This also enables you to upgrade the hardware and memory with ease.

  • To prevent low-memory conditions from occurring, the router stops accepting a session when the used memory from the Cisco Quatum Flow Processor (QFP) is above a specific percentage value.

  • To prevent high Buffer, Queue, and Scheduler (BQS) active queue usage, the router stops accepting new sessions when the BQS active queue usage is above a specific percentage value.

  • To prevent high CPU load conditions, the router stops accepting new sessions when the per-core CPU utilization per active FP or RP is above a specific percentage value.


Note
After the configured session limit under CAC is attained, single-stack sessions cannot be converted into dual-stack sessions.

Call Admission Control-Supported IP Session Initiators on a Data Plane

The following IP session initiators are supported by the Call Admission Control (CAC) on a data plane:

  • Unclassified MAC address IPv4 initiator

  • Unclassified IPv4 initiator

  • Dynamic Host Configuration Protocol v4 (DHCPv4) initiator

  • Unclassified MAC address IPv6 initiator types:

    • IPv6 Neighbor Discovery (ND) packets (RS/NS/NA) that are dropped by the control plane (i.e. RP)
    • IPv6 data packets that are dropped at the data plane (i.e. FP)

Platform System Resource Monitor

A platform system resource monitor process is created when any of the CAC features listed in the following table are configured on the Cisco ASR 1000 Aggregated Services Router platform using the corresponding command:
Table 1 CAC Features and their Platform System Resource Monitor Commands
CAC Feature Command

CPU

platform subscriber cac cpu

MEM

platform subscriber cac mem

BQS

platform subscriber cac bqs

The platform resource monitor process is disabled when all of the above CAC features are disabled on the platform.

The platform resource monitor process periodically monitors system resources such as CPU, memory, and Buffer, Queue, and Scheduler (BQS). You can set the periodic monitoring interval using the following command: 

platform subscriber cac timer interval
The timer value specifies the frequency at which the CAC process is polled to check platform resources. The valid interval value is in the range of 1 to 10 seconds with a granularity of 1 second. The default value is 5 seconds.

Note
Committed Memory is used for monitoring subscriber CAC memory resource utilization. It provides an estimate of the percentage of RAM that is needed to ensure a 99.99 percent guarantee that there is never an Out Of Memory (OOM) condition during worst-case conditions. Normally, the kernel will over commit memory by sharing the memory between several processes. A problem with the actual memory availability can be seen only when the memory is being used. While the Committed Memory is a good indicator of the memory status, the value is less dynamic. That is, if you use the platform subscriber cac mem command as part of CAC, a session may stay rejected for a long period of time if it reaches the memory limit. While this may be a good indicator of the decreasing memory resources, it may not be useful in production environments. We recommend that if you are using the platform subscriber cac mem command, ensure that the values are at least at the 95 percent threshold to enable additional troubleshooting.
Example: Configuring CAC to monitor platform specific resources, such as forwarding processor (FP) CPU, FP memory, and Quantum Flow Processor (QFP) memory. 
platform subscriber cac timer 5      #configures the frequency at which the CAC process polls or monitors the PD resources
platform subscriber cac bqs active-queues 95
platform subscriber cac mem rp 95
platform subscriber cac mem fp 95
platform subscriber cac mem cc 95
platform subscriber cac mem qfp 95
platform subscriber cac cpu rp 95
Example: Configuring CAC to monitor IOS CPU load, maximum number of sessions, and session charges (or calls per second). 
call admission new-model
call admission limit 1580
call admission cpu-limit 90
call admission session-limit 128000
call admission ip 10 1

CAC is applicable to any initiator for IP sessions such as unclassified MAC or DHCP-initiated sessions. To enable CAC, the call admission new-model command must always be configured.

The call admission limit command specifies the total session charge the system will accept before it starts rejecting incoming calls. In the above example, the cpu-limit of 90 means incoming calls will be dropped when the measured 5-second CPU utilization is 90% or higher.

Depending on which condition occurs first, cpu-limit or call admission limit (charge limit), the IP sessions will be rejected. The call admission ip command specifies the charge for a single IP session. In the example given above, the charge for a single session is 10 is and lifetime of the fixed charge (2s) is 1.

In the example above, CAC will accept 79 CPS based on the following calculation: 
Approximate CPS = (Call admission limit)/(Single session charge * Charge lifetime)
                = 1580/(10 * (1*2))
                = 79 CPS

Examples

Enabling the Call Admission Control Feature

The following examples show how to enable the Call Admission Control (CAC) feature, and how to display the CAC statistics.

The platform subscriber cac cpu rp value command enables the CAC feature based on the per-core average CPU load on the active RP: 

Router# config t
Router#(config)# platform subscriber cac cpu rp 95
Router#(config)# end

No new IP session will be allowed in the system if the CPU on the active RP is greater than 95 percent; also, the FSOL packet is immediately dropped at the data plane layer.

Displaying the Call Admission Control Feature Statistics in Detail

The show call admission statistics detailed command displays the CAC statistics in detail. It also provides packet drop statistics for each CAC type.


Note
The show call admission statistics detailed command only displays the details of platform resources, which are already configured. For example, the detail for the MEM_RP field is displayed in the output, only if it has been configured. Otherwise, it is not displayed in the output. 
Router# show call admission statistics detailed 

CAC New Model (SRSM) is ACTIVE
CAC statistics duration:  1873(seconds)
Total calls rejected 29, accepted 1749
Current hardware CAC status is: Not Dropping
Total call Session charges: 0, limit 0

CPU utilization: Five Sec Average CPU Load, Current actual CPU: 1%, Limit: 2%
Total count of session 1659, Limit: 128000

CAC Events:
        Reject reason          Times of activation   Duration of activation(secs)     Rejected calls
        CPU-limit:                       9                      42                         9
        SessionCharges:                  18                     42                         18
        LowPlatformResource:             8                      832                        1
        Session Limit:                   1                      47                         1

Total dropped FSOL packets at data plane: 4581  # total packets dropped at PD is 4581
  IOSD_CPU_OVERLIMIT_DROPS:     2381    # 2381 packets dropped due to IOS CPU overload
  CPS_OVERLIMIT_DROPS:          1892    # 1892 packets dropped due to CPS over limit
  TOTAL_SESSION_OVERLIMIT_DROPS:189     # 189 packets dropped due to total session limit
  CPU_RP_OVERLIMIT_DROPS:       20      # 20 packets dropped due to RP CPU overload
  CPU_FP_OVERLIMIT_DROPS:       20      # 20 packets dropped due to FP CPU overload
  MEM_RP_OVERLIMIT_DROPS:       20      # 20 packets dropped due to RP memory over limit
  MEM_FP_OVERLIMIT_DROPS:       20      # 20 packets dropped due to FP memory over limit   
  MEM_QFP_OVERLIMIT_DROPS:      20      # 20 packets dropped due to QFP memory over limit
  MEM_CC_OVERLIMIT_DROPS:       19      # 19 packets dropped due to CC memory over limit

platform resource low: FALSE
platform resource polling interval: 5 seconds
    BQS_QUEUE       : current:  0%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 0
    MEM_RP          : current: 67%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 251
    MEM_FP          : current:  8%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 494
    MEM_CC          : current: 52%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 829
    MEM_QFP         : current: 11%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 778
    CPU_RP          : current:  7%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 383
    CPU_FP          : current: 11%,  limit: 95%,  overlimit: FALSE, overlimit_seconds: 697

Reference

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

iWAG commands

Cisco IOS Intelligent Wireless Access Gateway Command Reference

MIBs

MIB

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Feature Information for Call Admission Control

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.
Table 2 Feature Information for Call Admission Control

Feature Name

Releases

Feature Information

Call Admission Control

Cisco IOS XE Release 3.11

Call Admission Control (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 quality of service (QoS) for the new session.