Cisco Cisco IOS Software Release 12.4(9)T

You should understand the concepts and general configuration procedures for IP header compression. 
For information about IP header compression, see the 

 chapter of in Part 6 the 

, Release 

12.4T.

After a map class containing an IPHC profile is applied to an interface, subsequent attempts (using the 
frame-relay ip rtp header-compression command) to apply Real-time Transport Protocol (RTP) 
header compression for all Frame Relay maps on a physical interface are blocked.

To configure FRF .20 support, you should understand the following concept:

Header compression is a mechanism that compresses the IP header in a packet before the packet is 
transmitted. Header compression reduces network overhead and speeds up the transmission of either 
RTP or Transmission Control Protocol (TCP) packets. With IPHC over Frame Relay, compression 
parameters are negotiated across a DLCI.

One method of configuring header compression on your network is to use an IPHC profile. An IPHC 
profile is a kind of template within which you can configure the type of header compression that you 
want to use, set all of the optional features and parameters for header compression, and then apply the 
profile to an interface, subinterface, or Frame Relay permanent virtual circuit (PVC). 

ECRTP is robust over links that are susceptible to frame loss. Header compression happens early in the 
Cisco Express Forwarding (CEF) switching path before queueing. If an interface or PVC is 
oversubscribed, all the dropped frames are compressed frames, which can impact CRTP compression 
performance. Using ECRTP can minimize the problem, because it provides better error recovery.