Symptom:
An ISR4451 running IOS XE 3.12S or later may log the following message:

%CPPOSLIB-3-ERROR_NOTIFY:cpp_cp:  cpp_cp encountered an error -Traceback= 1#XXXXXXXX...

Conditions:
In IOS-XE 3.12S, there was a change to IPC (inter-processor communication) code. IPC is a general facility for passing messages between internal router processes. Beginning in this release, the IPC began maintaining a counter for messages pending in a transmit queue. Under certain conditions, the counter could run into a negative value. This behavior in turn raises CPP errors but there is no corresponding impact to services.

This bug does not affect the IPC mechanism used by the ASR1000 product family (i.e. ASR1004, ASR1006, etc).