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Walkthrough Wednesdays

How to busy out the B-channel on an ISDN PRI in IOS gateways


Core Issue

The local voice busy out feature provides a way to busy out a voice port or DS-0 group (time slot) if a state change is detected in a monitored network interface (or interfaces). When a monitored interface changes to a specified state to out-of-service or in-service the voice port presents a seized/busy out condition to the attached private branch exchange (PBX) or other customer premises equipment (CPE). The PBX or other CPE can then attempt to select an alternate route.


When you troubleshoot ISDN trunks, you can make B-channels busy out. Issue the ds0 busyout X command under controller configuration mode for the channel associated signaling (CAS) trunk.

                    Router(config-controller)#ds0 busyout X

In order to busy out Common Channel Signaling (CCS) or ISDN PRI trunks, use the isdn service b_channel X state 2 command under interface configuration mode.

For T1

          Router(config)#interface serial 0:23

For E1

         Router(config)#interface serial 0:15

         Router(config-if)#isdn service b_channel X state 2

The valid states are 0=Inservice, 1=Maint, 2=Outofservice, and X is the B-channel number in both CCS and CAS configurations.

The show isdn service command can be used in order to find the state of each B-channel.

For more information, refer to TDM Switching of Voice and Data Calls on AS5400, AS5350, and AS5850 Gateways Configuration Example.

For related information, refer How to busy-out B-channels on a Catalyst 6608 module in Cisco CallManager for troubleshooting

Mark VandeVere

I needed to drop a PRI out of service for carrier testing, and needed to not interrupt primetime calls at our hospital. This was the ticket.

William Cooper

Will this command drop current calls ? or will it wait for the call to disconnect before

going to maint or outofservice ?

any reply appreciated.


It can do either if you add an argument.  If you enter "isdn service b_channel X state 2 ?" you'll see you have three options: hard, immediate or soft.  Hard will drop an active call on that channel and set the new service state, soft will wait for the channel to clear before it changes the service state.  I believe immediate will change the service state right away but let an active call finish rather than cutting it off, but you'll want to test that in a test environment to validate.