>Подскажите на каком иосе она есть и бывает ли вообще. А зачем ее использовать?
IOS 12.3(11)T11 - команда есть, с киско-кома:
Release
Modification
11.3(1)MA
This command was introduced on the Cisco MC3810.
12.0(2)T
This command was integrated into Cisco IOS Release 12.0(2)T. The implicit option was added.
12.0(4)T
This command was modified to support ISDNBF PRI QSIG signaling calls.
12.0(7)XK
This command was implemented on the Cisco 2600 series and Cisco 3600 series. The implicit keyword was removed and the extra keyword was added.
12.1(2)T
This command was integrated into Cisco IOS Release 12.1(2)T.