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H.323 Gateways and Resource Allocation Indication (RAI)

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RAI Concept:

Resource Allocation Indication allows gatekeepers to make informed call routing decisions based on a predefined threshold levels for DS0 channels and DSP channels.  The gateway will report its utilization statistics via H.225 RAI to the H.323 gatekeeeper.  When a monitored resource falls below the configured threshold, the gateway will inform the gatekeeper that it is almost out of resources.

H.323 version 2 and 3 support yes/no resource availability information.  H.323 version 4 introduced call capacity.

How RAI Works:

Resource reporting thresholds are configured by using the resource threshold command under the gateway CLI. The upper and lower thresholds are separately configurable to prevent the gateway from operating sporadically due to the availability or lack of resources.  The default for “high” and “low” values is 90.

resource threshold [all] [high percentage-value] [low percentage-value]

Utilization is calculated taking with the following formula:
Accessible channels = Inuse + Free
Utilization = Inuse / Acceptable

For example, assume you have four T1 PRIs: two PRIs for incoming calls, two for outgoing. You have busied out 44 timeslots of the outgoing timeslots and you have one call on one of the outgoing timeslots. You will have:

  • Total = 92 (23 B channels x 4 T1 PRIs)
  • Addressable = 46 (23 B channels x 2 T1 PRIs)
  • Disabled = 44
  • Inuse = 1
  • Free = 1

The utilization = 1/(1+1)= 50 %.   Therefore, if the configured high threshold is 90%, the Gateway still accepts calls.

Additional Insights:

  • The above calculations only took the DS0 resources into consideration. However, the DSP resources are monitored and calculated in the same way. Also, depending on which resource (DSP or DS0) reaches the low or high threshold first, the gateway sends the RAI messages.
  • No configuration is needed on the gatekeeper to activate the RAI.
  • An RAI message, like any other RAS message, is UDP. Once the gateway sends an RAI message to the gatekeeper, it starts a three second timer. If the timer expires before it receives the RAC, the gateway tries to send the RAI again nine more times. Then, it gives up until the resource availability status changes again.
  • RAI is useful if you want to give priority to a certain gateway. Also, once the threshold is reached, then you route the traffic to other gateways.
  • With the command lrq reject-resource-low, the gatekeeper rejects the inter-zone call if all gateways in that zone are marked as almost-out-of-resources. This command is integrated in Cisco IOS Software Release 12.1(3a)XI6. If you do not use this command, the gatekeeper does not reject any calls from other zones when all gateways in that zone are marked as out of resources.

H.323 Gateway Configuration

interface Ethernet0
ip address 30
h323-gateway voip interface
h323-gateway voip id cisco_2 ipaddr 1718
h323-gateway voip h323-id 5300-3
h323-gateway voip tech-prefix 2#
resource threshold high 70 low 60


show gateway
show pool
show call resource voice stat
show call resource voice threshold
show gatekeeper gw-type-prefix
show gatekeeper endpoint


Turn on debug ras and debug h225 asn1 if you think that the gateway is not sending the proper RAI message or the gatekeeper is not sending the RAC message.

Written by Matthew Berry

February 4th, 2010 at 6:20 am

Posted in Blog

Tagged with , , , ,

One Response to 'H.323 Gateways and Resource Allocation Indication (RAI)'

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  1. Don't forget the 2 best debugs (hidden of course) for troubleshooting gatekeeper issues.

    debug gatekeeper main 10

    debug gatekeeper call 10


    James Key

    4 Feb 10 at 12:22 am

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