Cisco Systems 78-11741-02 manual Enabling Express RTP Header Compression, IPC-433

Configuring IP Multicast Routing

Configuring RTP Header Compression

By default, for PPP or HDLC encapsulation, the software allows 32 RTP header compression connections (16 calls). This default can be increased to a maximum of 1000 RTP header compression connections on an interface.

To change the number of compression connections supported, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# frame-relay ip rtp	Specifies the maximum number of RTP header compression
compression-connections number	connections supported on the Frame Relay interface.
Router(config-if)# ip rtp	Specifies the total number of RTP header compression connections
compression-connections number	supported on the PPP or HDLC interface.

See the “RTP Header Compression Examples” section later in this chapter for an example of how to change the number of header compression connections.

Enabling Express RTP Header Compression

Before Cisco IOS Release 12.0(7)T, if compression of TCP or RTP headers was enabled, compression was performed in the process-switching path, which meant that packets traversing interfaces that had TCP or RTP header compression enabled were queued and passed up to the process to be switched. This procedure slowed down transmission of the packet, and therefore some users preferred to fast switch uncompressed TCP and RTP packets.

With Release 12.1 and later releases, if TCP or RTP header compression is enabled, it occurs by default in the fast-switched path or the Cisco Express Forwarding-switched (CEF-switched) path, depending on which switching method is enabled on the interface.

If neither fast switching nor CEF switching is enabled, if enabled, RTP header compression will occur in the process-switched path as before.

For examples of RTP header compression, see the sections “Express RTP Header Compression with PPP Encapsulation Example” and “Express RTP Header Compression with Frame Relay Encapsulation Example.”

The Express RTP and TCP Header Compression feature has the following benefits:

•It reduces network overhead.

•It speeds up transmission of TCP and RTP packets. The faster speed provides a greater benefit on slower links than faster links.

One restriction affects Multilink PPP (MLP) interfaces that have link fragment and interleave (LFI). In this case, if RTP header compression is configured, RTP packets originating on or destined to the router will be process switched. Transit traffic will be fast switched.

The CEF and fast-switching aspects of this feature are related to these documents:

•Cisco IOS Switching Services Configuration Guide

•Cisco IOS Switching Services Command Reference

Cisco IOS IP Configuration Guide

IPC-433