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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the features documented in this module, and to see a list of the releases in
which each feature is supported, see the feature information table.
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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.
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.
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.
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
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
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.
configured session limit under CAC is attained, single-stack sessions cannot be
converted into dual-stack sessions.
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
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)
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
platform subscriber cac
platform subscriber cac
platform subscriber cac bqs
resource monitor process is disabled when all of the above CAC features are
disabled on 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 timerinterval
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
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.
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
Configuring CAC to monitor IOS CPU load, maximum number of sessions, and
session charges (or calls per second).
CAC is applicable to
any initiator for IP sessions such as unclassified MAC or DHCP-initiated
sessions. To enable CAC, the
new-model command must always be configured.
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
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:
examples show how to enable the Call Admission Control (CAC) feature, and how
to display the CAC statistics.
platform subscriber cac cpu
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
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.
Call Admission Control Feature Statistics in Detail
show call admission
command displays the CAC statistics in detail. It also
provides packet drop statistics for each CAC type.
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
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
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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.
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Table 2 Feature Information
for Call Admission Control
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.