In the context of a SIP call request, the ANI (Automatic Number Identification) refers to the calling party, as determined by the username in the P-Asserted-ID or From header. The DNIS (Dialed Number Identification Service) refers to the called party as determined by the username in the Request URI.  An ANI-DNIS loop refers to a situation when more than one call request shares the same combination of ANI and DNIS values. To avoid excessive looped call requests, the SBC provides configuration options to limit the number of pending call requests that share the same ANI-DNIS combination.

When you enable the ANI-DNIS loop detection feature, the SBC maintains a count of existing requests, per ANI-DNIS combination. When the SBC receives an initial INVITE request, it compares the incoming ANI-DNIS against its list of counters. If the combination is not being tracked, then the SBC adds the combination to the list. If the combination is already being tracked but the number of instances is still below the configured limit, the SBC increments the loop counter and continues to set up the call. If the ANI-DNIS combination is being tracked and the new request reaches the configured limit, the SBC rejects the request with a 482 "Loop Detected" response. Once a call is connected, the SBC decrements the loop count for that ANI-DNIS combination. 

You enable ANI-DNIS loop detection and set the loop limit at the zone level. 

A value of "1" in SIP sub-field 81 of CDR ATTEMPT records indicates that a call was rejected due to ANI-DNIS loop detection. (In START and STOP records the value is set to 0.)