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
Indicates fax/modem passthrough data with RFC 2198 redundancy
Sent by a voice gateway to signal a switchover to Cisco fax relay.
Sent by a voice gateway to confirm a successful switchover to Cisco fax relay.
Designates 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.
Designates an NTE (Named Telephony Event) as defined in RFC 2833.
Indicates Cisco text relay information.
Specifies Cisco RTP DTMF Relay.
Indicates Cisco fax relay data.
Indicates Cisco CAS information.
Specifies that the payload contains Cisco clear-channel codec information.
Indicates 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.
Indicates 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.