Cisco RTP Payload Types

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Jun 11, 2009 1:07 AM
Jun 11th, 2009

Cisco uses RTP payload types from the values specified as dynamic and unassigned by RFC 3551 for signaling and also for designating RTP packets with certain types of data. This table is useful when you see unknown RTP payload types in a packet capture or in a voice gateway debug. The values shown here are the default ones but you should be aware that IOS commands exist to alter most of these values.

Default Dynamic and Unassigned RTP Payload Types Used by Cisco Voice Products
RTP Payload Value RTP Payload Definition
90Indicates fax/modem passthrough data with RFC 2198 redundancy
96Sent by a voice gateway to signal a switchover to Cisco fax relay.
97Sent by a voice gateway to confirm a successful switchover to Cisco fax relay.
100Designates an NSE (Named Signaling Event packet). NSE packets can signal a variety of different messages so it is helpful to know the Cisco NSE Events.
101Designates an NTE (Named Telephony Event) as defined in RFC 2833.
119Indicates Cisco text relay information.
121Specifies Cisco RTP DTMF Relay.
122Indicates Cisco fax relay data.
123Indicates Cisco CAS information.
125Specifies that the payload contains Cisco clear-channel codec information.
126Indicates G.711ulaw data is being carried and also notifies the other gateway that an upspeed to G.711ulaw has occurred. This is the old method (IOS version 12.1T) for switching over to fax passthrough before NSEs.
127Indicates G.711alaw data is being carried and also notifies the other gateway that an upspeed to G.711alaw has occurred. This is the old method (IOS version 12.1T) for switching over to fax passthrough before NSEs.
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