Cisco 3825 - Manual (Page 174)

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Cisco 3825 Mobile Wireless Edge Router Software Configuration Guide
OL-15667-03
Appendix A Cisco 3825 Mobile Wireless Edge Router RAN-O Command Reference
ip rtp header-compression
The passive Keyword
By default, the ip rtp header-compression command compresses outgoing RTP traffic. If you specify
the passive keyword, outgoing RTP traffic is compressed only if incoming RTP traffic on the same
interface is compressed. If you do not specify the passive keyword, all outgoing RTP traffic is
compressed.
The passive keyword is ignored on PPP interfaces. PPP interfaces negotiate the use of
header-compression, regardless of whether the passive keyword is specified. Therefore, on PPP
interfaces, the passive keyword is replaced by the IPHC format, the default format for PPP interfaces.
The iphc-format Keyword
The iphc-format keyword indicates that the IPHC format of header compression that will be used. For
PPP and HDLC interfaces, when the iphc-format keyword is specified, TCP header compression is also
enabled. For this reason, the ip tcp header-compression command appears in the output of the show
running-config command. Since both RTP header compression and TCP header compression are
enabled, both UDP packets and TCP packets are compressed.
The iphc-format keyword includes checking whether the destination port number is even and is in the
ranges of 16,385 to 32,767 (for Cisco audio) or 49,152 to 65,535 (for Cisco video). Valid RTP packets
that meet the criteria (that is, the port number is even and is within the specified range) are compressed
using the compressed RTP packet format. Otherwise, packets are compressed using the less-efficient
compressed non-TCP packet format.
The iphc-format keyword is not available for interfaces that use Frame Relay encapsulation.
Note The header compression format (in this case, IPHC) must be the same at both ends of the network. That
is, if you specify the iphc-format keyword on the local router, you must also specify the iphc-format
keyword on the remote router.
The ietf-format Keyword
The ietf-format keyword indicates that the IETF format of header compression will be used. For HDLC
interfaces, the ietf-format keyword compresses only UDP packets. For PPP interfaces, when the
ietf-format keyword is specified, TCP header compression is also enabled. For this reason, the ip tcp
header-compression command appears in the output of the show running-config command. Since both
RTP header compression and TCP header compression are enabled, both UDP packets and TCP packets
are compressed.
With the ietf-format keyword, any even destination port number higher than 1024 can be used. Valid
RTP packets that meet the criteria (that is, the port number is even and is higher than 1024) are
compressed using the compressed RTP packet format. Otherwise, packets are compressed using the
less-efficient compressed non-TCP packet format.
The ietf-format keyword is not available for interfaces that use Frame Relay encapsulation.
Note The header compression format (in this case, IETF) must be the same at both ends of the network. That
is, if you specify the ietf-format keyword on the local router, you must also specify the ietf-format
keyword on the remote router.
Support for Serial Lines
RTP header compression is supported on serial lines using Frame Relay, HDLC, or PPP encapsulation.
You must enable compression on both ends of a serial connection.