IOS Versions
Because of the VIP architecture, when a new feature comes out for the core
router platforms, the code needs significant rewriting to port it to the RSP platform.
As a result, a significant amount of time may pass before the feature is available
in distributed mode, if at all, on the RSP platform.An example is RTP
header compression (cRTP). Although this functionality was originally released
on most platforms with IOS version 11.1(15), it was not released as a distributed
feature on the 7500 Series until version 12.1(5)T. RTP header compression is a
very processor-intensive service that would not scale to many connections on the
RSP platform without distributed support.